Seismic Interpretation

1

Structural Interpretation Methods 2 Shaw, Connors, and Suppe

1A-1: Defining folds

Dip domains are separated by axial surfaces; imaginary planes which, when viewed in two dimensions, form axial traces. Anticlinal axial surfaces occupy concave-downward fold hinges; synclinal axial surfaces occupy concave-upward fold hinges.

Basic concepts

anticlinal axial surfaces

Folds are bends or flexures of layered rock that form in response to motion along faults, diapirism, compaction, and regional subsidence or uplift. Folds are expressed in seismic reflection profiles as one or more regions of dipping reflections (dip domains) that correspond to inclined stratigraphic contacts.

axial trace

Folds come in three basic types: monoclines

anticlines

synclines angular hinge

multiple angular hinges

curved hinge

synclinal axial surfaces

Folds are composed of one or more dip domains, and may have angular or curved fold shapes: anticlines fold limbs

crest single hinge

multiple angular hinges

curved hinge

Axial surfaces often occur in pairs that bound fold limbs, which are also called kink bands: single hinge

multiple hinges

two sets of paired axial surfaces

curved hinge

paired axial surfaces

synclines

single hinge

multiple hinges

curved hinge

kink band kink bands 3

Part 1: Structural Interpretation Methods

Seismic Interpretation of Contractional Fault-Related Folds

Folds in seismic sections

Monocline, San Joaquin Valley, California, U.S.A.

Single Hinge Anticline, Niger Delta, Nigeria

Multiple Hinge Anticline, Permian Basin, Texas, U.S.A.

Syncline, Santa Barbara Channel, California, U.S.A. 4 Shaw, Connors, and Suppe


Folds and bedding thickness Folds are classified based on whether or not the thickness of stratigraphic layers changes in dip domains or across axial surfaces. Parallel folds preserve layer thickness, and are common in strata that deformed predominantly by flexural slip (see inset at right). Axial surfaces bisect inter-limb angles in parallel folds.

Parallel fold model

Parallel fold, synclinal axial surface

Parallel folds commonly form by a deformation mechanism called flexural slip, where folding is accommodated by motions on minor faults that occur along some mechanical layering — usually bedding. Flexural-slip surfaces, which can be observed in core or outcrop, may vary in spacing from a few millimeters to several tens of meters in spacing.

slip surfaces

Layer thickness is conserved: Bed thickness T1 equals bed thickness T2. Bisecting axial surfaces: Interlimb angle γ1 equals interlimb angle γ2. Various types of folds exhibit non-parallel behavior, where the thickness of stratigraphic layers changes gradually in dip domains or abruptly across axial surfaces. These thickness changes may be caused by various deformation mechanisms, including ductile flow within incompetent beds. Alternatively, thickness changes may be depositional in origin. Axial surfaces do not bisect interlimb angles in non-parallel folds. Rather, axial surface orientations are governed by the magnitude of the change in bed thickness.

Non-Parallel fold, anticlinal axial surface

The amount of offset on flexural-slip faults increases as the fold tightens (note slip increase from models 1 to 2), and is a function of the spacing of slip surfaces.

Non-parallel fold model Slip changes instantaneously across axial surfaces in angular folds (models 1, 2); whereas, slip increases along bedding surfaces through the hinge in curved-hinge folds (model 3). 5 Part 1: Structural Interpretation Methods


Shortcomings in seismic images of folds

Locating axial surfaces in seismic sections

Folds can be distorted or only partially imaged in seismic sections. Two common shortcomings are: Overlapping reflections in non-migrated or under-migrated sections; and poor imaging of steeply dipping fold limbs.

Migration moves dipping reflections upward and laterally to properly image the fold geometry, but reflections on non-migrated or under-migrated sections do not accurately represent fold shape. However, axial surfaces can be inferred on these sections by mapping the truncations of horizontal reflections. Model

Balanced model

Stacked section (synthetic) Step 1: Pinpoint truncations of horizontal reflections as they enter the poorly imaged zone. Note that diffractions, dipping toward the fold, may emanate from these truncations.

Synthetic seismic

reflection truncations

Overlapping reflections occur in synclines (1) on this stacked section; similar patterns persist in under-migrated sections. The steep limb is not imaged and diffractions are present (2).

diffractions

Stacked section Migrated section (synthetic)

Stacked section (synthetic) Step 2: Fit an axial surface that best matches the aligned truncations. Note that the interpreted axial surface matches closely with the axial surface defined in the migrated section (left).

Proper migration removes overlapping reflections and collapses diffractions, but the steep limb remains un-imaged.

Migrated section

axial surface

6 Shaw, Connors, and Suppe


Interpreting folds in poorly imaged zones

B: Method for interpreting parallel folds in poorly imaged zones

Poorly imaged zones on folds are commonly caused by, and interpreted as, faults or steep limbs. Both solutions are often permissible and should be evaluated. Here, we describe a method of interpreting parallel folds in poorly imaged zones.

Step 1: Pinpoint truncations of reflections as they enter the poorly imaged zone.

A: Is the poorly imaged zone a fault or steep fold limb? fault

Step 2: Fit parallel axial surStep 3: Define the dip of beds faces that best match the in the kink band by making γ2 aligned truncations. Measure equal to γ1. the average dip outside of the fold limb and measure γ1.

water-bottom multiples

fold

reflection truncations

2-D, post-stack time migration displayed in depth Data courtesy of Texaco, Inc.

D: Confirmation of fold geometry with dipmeter log and 3-D seismic image

C: Interpretation using the parallel fold method

3-D, post-stack time migration displayed in depth Data courtesy of Texaco, Inc.

In this example, 3-D seismic data and a dipmeter log confirm the presence of steeply dipping beds in the poorly imaged zone. The primary test of the fold interpretation, however, is whether or not the horizons correlate properly across the poorly imaged zone. If they do, a parallel fold interpretation is permissible. If they do not, a non-parallel fold or fault likely occupies the poorly imaged zone. 7



1A-2: Recognizing thrust and reverse faults Faults are identified in seismic reflection profiles through: fault cutoffs — terminations of reflections or abrupt changes of reflection attributes (e.g., amplitude, polarity) at fault surfaces;

Recognizing and interpreting faults in seismic section fault cutoffs inferred fault position

terminations of fold limbs or kink bands; and

cutoff

direct fault-plane reflections, produced by changes in velocity and density across or within fault zones. cutoff

Cutoffs and fault plane reflections (criteria 1 and 3) directly constrain fault positions. Thrust faults and their cutoffs, however, are generally difficult to image and identify, and thus the recognition of kink-band terminations (criterion 2) is a vital component of interpreting these faults. In this section, we describe how these criteria can be used together to identify and interpret thrust and reverse faults in seismic sections.

Data courtesy of Texaco, Inc.

Fault cutoffs and kink-band terminations balanced model Incipient fault with markers along fault surface.

Abrupt terminations (cutoffs) and duplications of prominent reflections constrain the position of a gently dipping thrust fault. (2-D seismic data, Permian basin, Texas, U.S.A.)

kink-band terminations

Fault with offset markers and cutoffs. Note that hanging wall kink bands terminate downward into the fault surface. hanging wall cutoffs

Thrust faults and bed-parallel detachments can be identified by the abrupt, downward terminations of kink bands. Terminations are generally marked by regions of dipping reflections above horizontal or more gently dipping reflections, and may contain fault cutoffs. Dipping reflections in kink bands represent strata folded in the hanging wall of a thrust/reverse fault or detachment; whereas, horizontal or more gently dipping reflections represent footwall strata below the fault or detachment. Thus faults and/or detachments should be interpreted at the transition between these two dip domains.

footwall cutoffs

in outcrop Fault cutoffs in outcrop, Mississippian Joana limestone, Nevada, U.S.A.

inferred detachment

in synthetic seismic Seismic forward model showing fault cutoffs (1) and downward terminating kink-bands (2).

fault-plane reflection inferred fault

Data courtesy of Texaco, Inc.

fault

Downward terminating kink band (2) defines the position of a gently dipping thrust. (3-D seismic data, Permian basin, Texas, U.S.A.).

Data courtesy of Mabone, Ltd.

Downward terminating kink band (2) and fault-plane reflection (3) define the position of a thrust fault that shallows to an upper detachment. (3-D seismic data/Niger Delta).



Interpreting thrust ramps on seismic sections Combinations of the three fault recognition criteria are employed to interpret thrust faults on the seismic section presented here.

Seismic Example: Peruvian Andes

This section images structures that involve two large thrust faults, which can be interpreted using the fault recognition criteria. The top panel is an uninterpreted section across a fold and thrust belt in the Andean foothills, Ucayali basin, Peru. Faults in the lower section are interpreted using: Cutoffs (1), kink-band terminations (2), and fault-plane reflections (3). Note how a series of cutoffs and kink-band terminations can corroborate, and be used to extrapolate beyond, the fault-plane reflections. (2-D seismic data, reprinted from Shaw et al., 1999, and published courtesy of Perupetro).

dipping over horizontal reflections

dipping over horizontal reflections

interpreted faults

VE of 1:1

9 Part 1: Structural Interpretation Methods


Recognizing detachments

Seismic Example: Sichuan basin, China

Detachments are faults that run along bedding or other stratigraphic horizons, and thus generally are horizontal or dip at low angles. In fold and thrust belts, detachments are commonly referred to as decollements. Detachments are generally not imaged directly on seismic sections, but rather are interpreted at the base and/or top of thrust ramps. Basal detachments can be located in seismic sections by defining the downward terminations of kink bands, as described on the preceding pages.

These two seismic sections have prominent detachments. In the section at right, the detachment is located at the base of a singlethrust thrust ramp. The fold in the hanging wall of the thrust is produced by slip across the fault bend that is formed at the connection of the thrust ramp and detachment. This class of faultbend fold is described in section 1B-1. In the section below, a regional detachment forms the base of several thrust ramps.

ramp

detachment

Seismic Example: Nankai Trough, Japan

ramp

detachment



1A-3: Recognizing growth strata Growth or syntectonic strata are stratigraphic intervals that were deposited during deformation. The ages of growth strata therefore define the timing of deformations. In contractional fault-related folds, growth strata typically thin across fold limbs toward structural highs. The geometries of growth structures are controlled primarily by the folding mechanism and the relative rates of sedimentation and uplift. Thus, growth fold patterns imaged in seismic data are often considered diagnostic of folding mechanism and sediment-touplift ratio. In this section, we describe common patterns of growth strata in fault-related folds that are imaged in seismic reflection data.

Growth strata in seismic section: Uplift exceeds sedimentation

Growth strata in seismic section: Sedimentation exceeds uplift

onlapping growth strata


pre-growth strata

growth strata

pre-growth strata

In cases where the sedimentation rate exceeds the uplift rate, growth strata are typically characterized as sequences, bounded by two or more seismic reflections, that thin toward the structural high. Growth strata are generally folded in one or more limbs of the structure. In this seismic section, growth strata thin onto the fold crest, with the lowermost growth units exhibiting the greatest thickness changes. (2-D seismic data, reprinted from Shaw et al., 1997).

In cases where the uplift rate exceeds the sedimentation rate, growth strata typically thin toward, and onlap, the structural high. Growth strata are generally not present above the fold crest, but are folded in one or more limbs of the structure. In this seismic section, growth strata onlap the backlimb and forelimb of a fault-related fold. The growth strata are overlain by post-tectonic strata, which are described later in this section. (This structure is interpreted more completely in sections 1B-1 and 1B-4).



Growth strata as records of fold kinematics Two folding mechanisms — kink-band migration and limb rotation — are commonly ascribed to contractional fault-related folds. These folding mechanisms typically yield distinctive patterns of deformed growth strata above fold limbs. Thus, seismic images of growth folds can be used to identify the folding mechanisms, which in turn can dictate the kinematic theory (e.g., fault-bend folding or detachment folding) that is most appropriate to guide the structural interpretation of the seismic data.

Folding by kink-band migration pre-growth strata only

Folding by progressive limb rotation sedimentation > uplift

sedimentation < uplift

pre-growth strata only

sedimentation > uplift

sedimentation < uplift

growth strata

growth strata

growth triangle

inactive axial surface

inactive axial surface fold scarps

active axial surface

growth axial surface

fanning of limb dips growth axial surface onlaps

pre-growth strata

pre-growth strata

In fault-related folds that develop purely by kink-band migration, fold limbs widen through time while maintaining a fixed dip (Suppe et al., 1992), as illustrated in the sequential model involving pre-growth strata only (above left). Material is incorporated into the fold limb by passing through an active axial surface, which at depth is generally pinned to a bend or tip of a fault (Suppe, 1983; Suppe and Medwedeff, 1990). The fold limb in growth strata is bounded by the active axial surface and the growth axial surface, an inactive axial surface that defines the locus of particles originally deposited along the active axial surface. In these sequential models, the synclinal axial surface is active, and the anticlinal axial surface is inactive. In the case where sedimentation rate exceeds uplift rate (above center), strata are folded through the synclinal axis and incorporated into the widening fold limb. The dip of folded growth strata is equal to dip of the fold limb in pre-growth strata. The width of the dip panel for each growth horizon corresponds to the amount of fold growth that occurred subsequent to the deposition of that marker. As a result, younger horizons have narrower fold limbs than do older horizons, forming narrowing upward fold limbs or kink bands in growth strata (growth triangles). In the case where uplift rate exceeds sedimentation rate (above right), each increment of folding produces a discrete fold scarp located where the active axial surface projects to the Earth’s surface (Shaw et al., 2004). Subsequent deposits onlap the fold scarp, producing stratigraphic pinchouts above the fold limb. Fold scarps and stratigraphic pinch-outs are displaced laterally and folded as they are incorporated into widening limbs.

onlaps

In fault-related folds that develop purely by limb rotation with fixed hinges (i.e., inactive axial surfaces), the dip of the fold limb increases with each increment of folding as illustrated in the sequential model involving pre-growth strata only (left). In the case where sedimentation rate exceeds uplift rate (center), strata are progressively rotated with each increment of folding. Thus, older growth horizons dip more steeply than do younger horizons, yielding a pronounced fanning of limb dips in growth strata. Fold limb width, however, remains constant. In the case where uplift rate exceeds sedimentation rate, growth strata also exhibit a fanning of limb dips. However, growth strata typically onlap the fold limb. Contractional fault-related folding theories that exclusively invoke limb rotation include certain classes of detachment folds (Dahlstrom, 1990; Hardy and Poblet, 1994).

Contractional fault-related folding theories that exclusively invoke kink-band migration include fault-bend folding (Suppe, 1983), constant-thickness and fixed axis fault-propagation folding (Suppe and Medwedeff, 1983), and basement-involved (triple junction) folding (Narr and Suppe, 1994). 12 Shaw, Connors, and Suppe


Growth structures in seismic data

Folding by progressive limb rotation

Growth structures imaged in seismic sections commonly exhibit patterns that are similar to the kink-band migration or limb-rotation models that were described on the previous page. In other cases, folds may develop by a combination of kink-band migration and limb rotation, resulting in hybrid patterns of growth structure. This section presents three seismic profiles as examples of kink-band migration, limb rotation, and hybrid growth structures.

Seismic Example: offshore Angola fanning of limb dips

Folding by kink-band migration Seismic Example: Santa Barbara Channel, California, U.S.A.

growth triangle

salt mound limb rotation model growth strata

detachment

kink-band migration model growth strata

Folding by both kink-band migration and limb rotation Seismic Example: San Joaquin basin, California, U.S.A. fanning of limb dips

(top) The seismic section above shows a narrowing upward fold limb, or growth triangle, where bed dips within the fold limb generally do not shallow upward, consistent with folding by kink-band migration. Dipmeter data in the wells corroborates the reflector dips. (upper right) In this section, a fanning and upward shallowing of limb dips within growth strata are consistent with folding by progressive limb rotation. The core of the anticline is filled with salt, which presumably thickened during deformation, leading to progressive rotation of the overlying fold limbs. (lower right) The growth structure in this section contains both a growth triangle and a fanning of limb dips, suggesting folding by a combination of kink-band migration and limb rotation mechanisms. Kinematic theories that employ hybrid folding mechanisms include shear fault-bend folds (Suppe et al., 2004; see section 1A-4), certain classes of detachment folds (Dahlstrom, 1990; Hardy and Poblet, 1994; see section 1B-3), and trishear fault-propagation folds (Erslev, 1991; Hardy and Ford, 1997; Allmendinger, 1998; see section 1B-2).

hybrid model growth strata

growth triangle



Distinguishing drape from growth strata Sedimentary drape sequences are stratigraphic intervals that were deposited above a structure after deformation ceased, yet they are warped due to primary sedimentary dip and/or compaction. Drape sequences exhibit a wide range of patterns depending on the sedimentary environment and facies. In some cases, drape sequences have patterns that are similar to those of growth strata deformed by limb rotation. In this section, we illustrate the potential similarity of drape and growth patterns, and show an example of a drape sequence in a seismic section.

Drape folding Kinematic models Drape sequence

Seismic Example: offshore California Borderlands, U.S.A. axial surfaces dips away from crest

drape

Growth fold

axial surfaces dips toward crest

growth strata

The top model shows a post-tectonic drape sequence above a rigid basement high. The drape sequence thins toward the crest of the structure, with younger strata having less relief than older units. The lower model shows growth strata above a fold developed by progressive limb rotation. The two stratigraphic patterns are similar, and in some cases difficult to distinguish. Incorrect interpretations of drape and growth sequences can lead to flawed estimates of structural timing and kinematics. Thus, care should be taken in trying to distinguish between drape and growth sequences. One common difference between drape and growth sequences is the orientation of axial surfaces. Axial surfaces in drape sequences often are vertical or dip away from the structural crest, reflecting a state of tension and due, in some cases, to compaction (Laubach et al., 2000). In contrast, axial surfaces in contractional folds generally dip toward the structural crest, reflecting a state of compression. Thus, careful interpretation of axial surfaces, along with consideration of regional tectonic history, can, in some cases, help to distinguish between drape and growth sequences.

drape

basement

This seismic section images a siliciclastic drape sequence that onlaps and overlies a ridge of metamorphic basement rocks.



1B-1: Fault-bend folds Basic concept

Synclinal fault-bend folds

Fault-bend folds form as hanging wall-rocks move over bends in an underlying fault. This section describes the geometry and kinematics of fault-bend folding after Suppe (1983) and introduces basic techniques for interpreting these structures in seismic data.

Synclinal fault-bend folds form at concave-upward fault bends. Synclinal axial surfaces are pinned to the fault bend and are generally active; whereas anticlinal axial surfaces are inactive and move with the hanging wall block. Figures below show a kinematic model, a field example, and a seismic example of synclinal fault-bend folds.

To describe the basic concept of fault-bend folding, we will consider the hypothetical case of a fault in cross section with one bend joining upper and lower segments. Rigid-block translation of the hanging wall parallel to the upper fault segment produces a void between the fault blocks; whereas, translation of the hanging wall parallel to the lower fault segment produces an “overlap.” Both of these cases are unreasonable.

Field Example

axial surface

Kinematic Model Rigid-Block Translation

fault

Seismic Example: Argentina axial surface

In contrast, folding of the hanging wall block through the development of a kink band accommodates fault slip without generating an unreasonable overlap or void. This fault-bend folding (Suppe, 1983) is localized along an active axial surface, which is fixed with respect to the fault bend. After strata are folded at the active axial surface, they are translated above the upper fault segment. The inactive axial surface marks the locus of particles that were located along the active axial surface at the initiation of fault slip. The inactive axial surface moves away from the active axial surface with progressive fault slip, and thus the width of the intervening kink band is proportional to the amount of fault slip.

Fault-Bend Folding

fault



Anticlinal fault-bend folds

Seismic Example: Niger Delta

Anticlinal fault-bend folds form at concave-downward fault bends. Anticlinal axial surfaces are pinned to the fault bend and are generally active; whereas, synclinal axial surfaces are inactive and move with the hanging wall block. Figures below show a kinematic model, a field example, and seismic examples of anticlinal fault-bend folds. axial surface

Kinematic Model

fault

Seismic Example: Permian basin, U.S.A. Field Example axial surface axial surface

fault

fault



Quantitative fault-bend folding Based on assumptions of conservation of bed length and thickness during folding, the shape of a fault-bend fold is related to the shape of the fault by:

Where θ is the hanging wall cutoff angle before the fault bend; φ is the change in fault dip; β is the hanging wall cutoff after the fault bend, and; γ is one half of the interlimb angle, such that the axial surfaces bisect the interlimb angles and bed thicknesses are preserved. If two of these values are known, the remaining two values can be determined. The fault-bend fold relations are displayed in the graph below. The left side of the graph describes anticlinal fault-bend folds, where the fold is concave toward the fault; the right side of the graph describes synclinal fault-bend folds, where the fold is convex toward the fault.


When interpreting seismic sections, typically the interlimb angle (γ) can be observed (see section 1A-2) and one of the hanging wall cutoff angles (θ or β) can be specified. Using the graph, the change in fault dip (φ) and the remaining cutoff angle can be determined. For anticlinal fault-bend folds there are two fold solutions for each θ and φ value; first mode solutions produce open folds that have been shown to effectively describe many observed fold geometries; whereas, second mode solutions are geometrically valid but have not been shown to effectively describe natural fold shapes.




In cases where the initial cut-off angle (θ) equals zero, then R equals one (R=1). When the initial cut-off angle (θ) does not equal The magnitude of fault slip can change across fault bends, as slip zero, R can be determined if any two of the four geometric paramis consumed or produced by fault-bend folding. In cases where eters (θ, φ, β, γ) are specified using fault-bend fold theory (Suppe, the initial cutoff angle is not equal to zero (θ ⫽ 0), anticlinal fault- 1983). The graph below plots R as a function of initial cut-off angle bend folds consume fault slip and synclinal fault-bend folds pro- (θ), interlimb angle (γ), and change in fault dip (φ), and is of the duce fault slip. The change in fault slip is described by the same format used to describe fault-bend fold geometry. parameter R, which is the ratio of slip magnitude beyond (S1) R varies greatly as a function of the tightness of the fold, which and before (S0) the fault bend. is reflected in part by the interlimb angle (γ). Tight (perhaps with

Fault slip and fault-bend folds


steep limbs) anticlinal folds generally consume larger amounts of slip (hence they have lower R values) than do gentle anticlinal folds. Tight synclinal folds generally produce larger amounts of slip (hence they have higher R values) than do gentle anticlinal folds. In both synclinal and anticlinal fault-bend folds with a single fault bend, the width of the fold limb measured along the fault equals the slip (S1).




Seismic interpretation of a synclinal fault-bend fold

2. Synclinal fault-bend fold graph

This section describes the interpretation of a synclinal fault-bend fold imaged in seismic reflection data. The lower portion of the fault and the syncline are well imaged, and fault-bend folding theory is used to predict the orientation of the upper portion of the fault. In Figure 1, fault-plane reflections define the position of a thrust ramp located beneath a syncline. Based on the imaged fold shape and fault ramp, the initial cutoff angle (θ) and interlimb angle (γ) can be measured as:

θ = 15°; γ = 82° Using the synclinal fault bend fold graph (Figure 2), γ and θ are used to determine the change in fault dip (φ) and the hanging wall cutoff after the fault bend (β):

φ = 15°; β = 14° φ and β values are used to model the structure in Figure 3. Note that the predicted upper fault segment agrees closely with the fault position as constrained by reflection terminations and potential fault-plane reflections.

Synclinal fault-bend fold, Argentina 1. Observations/Initial Interpretation

3. Prediction



Seismic interpretation of an anticlinal fault-bend fold

2. Anticlinal fault-bend fold graph

This section describes the interpretation of an anticlinal fault-bend fold imaged in seismic reflection data. In Figure 1, fault-plane reflections and reflection truncations define the position of a thrust ramp located beneath an anticline. Based on the imaged fold shape and fault ramp, the initial cut-off angle (θ) and interlimb angle (γ) can be defined as: θ = 24°; γ = 80° Using the anticlinal fault bend fold graph (Figure 2), γ and θ are used to determine the change in fault dip (φ) and the hanging wall cutoff after the fault bend (β): φ = 16°; β = 28 φ and β values are used to model the structure in Figure 3. Note that the predicted upper fault segment agrees closely with the fault position as constrained by reflection terminations and the downward termination of the forelimb. In this example, slip below the fault bend (S0) is also interpreted based on offset reflections. Based on the slip ratio R predicted for this fault-bend fold (obtained using the graph presented in the previous section), the slip above the fault bend (S1) is calculated as follows: R = (S0/S1) = 0.87; given S0 = 1.7 km, then S1 = 1.5 km

Anticlinal fault-bend fold, Niger Delta 1. Observations / Initial Interpretation

3. Prediction



Composite fault-bend folds: Ramp anticlines The most common representation of a fault-bend fold involves deformation above a thrust ramp connecting upper and lower detachments, often referred to as a ramp anticline. In fact, this structure consists of two fault-bend folds — one related to each fault bend — and thus is part of a class of “composite” fault-bend folds. This section describes the kinematic evolution of a simple ramp anticline after Suppe (1983), the geometry of which is governed by the quantitative fault-bend folding theories described in the preceding pages.

Kinematic development of a composite fault-bend fold

Seismic Example: Pitas Point, Santa Barbara Channel, California, U.S.A. Uninterpreted section

0: An incipient thrust fault and axial surfaces in undeformed strata.

1: Fault slip causes folding of the hanging wall block along active axial surfaces A and B that are pinned to the two fault bends. Inactive axial surfaces A⬘ and B⬘ form at fault bends and are passively translated away from active axial surfaces by slip. Kink-band width A-A⬘ or B-B⬘ measured along bedding equals slip on the underlying fault segment. The difference in kink-band width between back and front limbs reflects slip consumed in folding. 2: Progressive fault slip widens both kink bands. Models 1 and 2 are in the crestal uplift stage because the fold crest elevates with increasing fault slip.

Interpreted section

3: When the axial surface A⬘ reaches the upper fault bend, material from the back limb is refolded onto the crest and the front limb kink-band B-B⬘ is translated along the upper detachment. In model 3, A and A⬘⬘ are active axial surfaces; B and B⬘ are inactive axial surfaces. Model 3 is in the crestal broadening stage because the fold crest widens without producing additional structural relief with increasing fault slip. In the crestal broadening stage, slip exceeds the width of the fold limbs, and is equal to the distance between axial surfaces A-A⬘ measured along the fault. 21 Part 1: Structural Interpretation Methods


“Multi-bend” fault-bend folds In addition to simple ramp anticlines, composite structures include multi-bend fault bend folds (Medwedeff and Suppe, 1997), which contain two or more bends of similar concavity or convexity. Initially, slip across each bend produces a distinct kink band; however, with progressive fault slip, kink bands merge and interact. These interactions can be highly complex, spawning many new axial surfaces and dip domains. Thus, multi-bend fault-bend folds are generally characterized by the presence of multiple dip domains in backlimbs and forelimbs. Figures below show kinematic models of multibend fault-bend folds and a seismic example.

Kinematic development of multi-bend fault-bend folds Convex upward (anticlinal) bends

Concave upward (synclinal) bends

Seismic Example: Niger Delta Multi-bend fault

bends

fault

Interpreted section

refolded

refolded

axial surface axial surface

0: Incipient fault with two convex upward bends. 1: Slip yields two kink bands associated with the two fault bends. 2: Kink bands widen with progressive slip. 3: Portions of kink bands are refolded, yielding a steeply dipping fold panel.

0: Incipient fault with two concave upward bends. 1: Slip yields two kink bands associated with the two fault bends. 2: Kink bands widen with progressive slip. 3: A portion of the lower kink band is refolded as it moves onto the upper fault ramp.

fault



Modeling curved fold hinges Folds generally exhibit some curvature in their hinges. Most fault-related fold analysis techniques approximate these curved hinge zones as perfectly angular folds or as multi-bend folds composed of two or more planar hinge segments (Medwedeff and Suppe, 1997). In many cases, these approximations adequately describe large folds, with small zones of hinge curvature separating long, planar fold limbs of the scale typically imaged in seismic data. Moreover, these approximations are useful because they allow for rigorous area and line length balancing. In some cases, however, it may be necessary to more accurately describe curved hinge zones. Here we introduce a curved-hinge fault-bend fold model after Suppe et al. (1997), which obeys fault-bend folding relations but imparts fault curvature on the fold shape using the concept of entry and exit axial surfaces. Other techniques of modeling curved fold hinges (e.g., trishear folding — Erslev, 1991) are described in later sections.

Seismic Example: Sichuan basin, China

Synclinal fault-bend folds Angular Hinge

Multibend Hinge

incipient axial surface

active axial surface inactive axial surface

Uninterpreted section

Curved Hinge incipient axial surface

active axial surfaces inactive axial surfaces

incipient entry axial surface incipient exit axial surface

entry active axial surface exit active axial surface

Interpreted section

Sequential models of a syncli- Sequential models of a multi- Sequential models of a curved hinge synnal fault-bend fold with an bend synclinal fault-bend fold clinal fault-bend fold. 0: Two incipient acangular hinge. with two fault ramp segments. tive axial surfaces bound the zone of curvature on the fault. 1: Slip causes folding of the hanging wall rocks. Folding begins as rocks pass through the entry active axial surface (A), and ceases as rocks pass through the exit active axial surface (B). 2: Progressive slip widens the kink band, as inactive axial surfaces (A⬘ and B⬘) are passively translated up the fault ramp. 23 Part 1: Structural Interpretation Methods


Growth fault-bend folds — high sedimentation rates

Seismic Example: Santa Barbara Channel, California, U.S.A.

Fault-bend folds develop by kink-band migration, where fold limbs maintain a constant dip but generally widen as fault slip increases. When sedimentation rate exceeds uplift rate, folds that develop by kink-band migration have syntectonic (growth) strata that form narrowing upward dip domains, or growth triangles, above fold limbs (see section 1A-3). Below, we use kinematic models to describe how these growth structures develop in a composite fault-bend fold, and show examples of growth structures in seismic sections.

Active synclinal axial surface — backlimb FBF

Fault-bend fold with growth strata growth pre-growth forelimb

crestal uplift stage

Seismic Example: Los Angeles basin, California, U.S.A. crestal uplift stage

Active anticlinal axial surface — forelimb FBF

crestal broadening stage

Sequential model of a growth fault-bend fold (Suppe et al., 1992; Shaw et al., 1996) with sedimentation rate > uplift rate. Model 1 consists of a composite fault-bend fold developed above a ramp between detachments. The fold is in the crestal uplift stage of growth (Shaw et al., 1994b), as fault slip is less than ramp width. In Model 2, additional slip widens the kink bands, which narrow upward in the growth section (see section 1A-4). In Model 3, fault slip is greater than ramp width. Thus, strata are refolded from the back limb (A-A⬘⬘) onto the crest of the structure, which widens with fault slip (crestal broadening stage, Shaw et al., 1994b). Growth strata are also folded above the crest, as they pass through active axial surface A⬘⬘. Forelimb axial surfaces (B-B⬘) are released from the fault bend and passively translated above the upper detachment, and thus do not deform young growth strata. 24 Shaw, Connors, and Suppe


Growth Fault-Bend Folds — low sedimentation rates In cases where sedimentation rate is less than or equal to the uplift rate, fault-bend folds develop patterns in growth strata that are distinct from growth triangles (see section 1A-3). In limbs with active synclinal axial surfaces, growth strata are folded concordantly with the underlying kink band; whereas, in limbs with inactive synclinal axial surfaces growth strata simply onlap kink bands. Below we describe how these growth patterns are expressed in a composite fault-bend fold after Medwedeff (1989) and Suppe et al. (1992).

Fault-bend fold with growth strata

growth

pre-growth forelimb

backlimb

Seismic Example: San Joaquin basin, California, U.S.A. Composite Fault-Bend Fold with Growth Strata



folded growth strata folded growth strata onlapping growth strata


time transgressive angular unconformity


Sequential model of a growth fault-bend fold (Medwedeff, 1989; Suppe et al., 1992) with a sedimentation rate equal to the uplift rate. Model 1 consists of a composite fault-bend fold developed above a ramp between detachments. Growth strata in the backlimb are folded concordantly with the underlying kink band. In contrast, undeformed growth strata onlap the forelimb. In Model 2, additional slip widens kink bands and the growth pattern is maintained. In Model 3, fault slip is greater than ramp width. Thus, strata are refolded from the back limb (A-A⬘⬘) onto the crest of the structure, which widens with fault slip. Growth strata are also re-folded above the crest, as they pass through active axial surface A⬘⬘. Formerly inclined growth strata from the backlimb become horizontal. Coeval deposition above the fold crest forms a time trangressive angular unconformity. In Model 3, the sedimentation rate is held constant and equal to the uplift rate of particles within the back limb.

Seismic reflection profile across the Western San Joaquin basin (Lost Hills anticline) showing contrasting patterns of growth strata between backlimb (west) and forelimb (east) that are consistent with fault-bend folding where sedimentation rate is less than or equal to uplift rate (see model 2, left). The fanning of limb dips above the front limb may be due to sedimentary drape and compaction, or may reflect a component of limb rotation in fold growth (see section 1A-3).



1B-2: Fault-propagation folds Basic concept

Examples

Fault-propagation folds form at the tips of faults and consume slip. These folds are generally asymmetric, with forelimbs that are much steeper and narrower than their corresponding backlimbs. Several modes of folding at fault tips have been described to explain these structures, including: constant thickness and fixed axis fault propagation folding (Suppe and Medwedeff, 1990); trishear folding (Erslev, 1991; Hardy and Ford, 1997; Allmendinger, 1998); and basement-involved (triple junction) folding (Narr and Suppe, 1994). In this section, we describe these kinematic theories, emphasizing their common characteristics, and introduce basic techniques for interpreting fault-propagation folds in seismic data.

Fault-propagation folds are common in outcrop and at scales typically imaged by seismic reflection data. This field example (right) has several characteristics of fault-propagation folds, including asymmetry, the presence of a narrow, steeply dipping forelimb, and the downward increasing tightness of the fold.

Schematic fault-propagation fold model

The seismic example is a fault-propagation fold at the southern margin of the Tanan Uplift in the southern Tarim basin. In this example, a thrust ramp delineated by fault-plane reflections terminates upward into the forelimb of an asymmetric fault-propagation fold.

To describe the basic concept of faultpropagation folding, we will consider the hypothetical case of a fault ramp in cross section that propagates upward from a detachment (note that fault-propagation folds may originate from faults with or without detachments). As the fault ramp propagates upward in sequential models 0 to 2, an asymmetric fold develops in the hanging wall with vergence in the transport direction. The fold consumes slip on the ramp, with slip being greatest at the ramp base and zero at the fault tip. As slip increases, the fault tip advances and the fold grows larger while maintaining the same basic geometry.

Field Example

fault

Professor Bill Brown highlighting a fault-propagation fold in Cambrian Fort Sills limestone, Arbuckle Mountains, OK, U.S.A. (S.C. Hook)

Seismic Example: Tarim basin, China

fault tip

Common characteristics Although fault-propagation folds exhibit a wide range of geometries, several characteristics are common to most structures, including: 1) folds are asymmetric, with forelimbs that are generally much steeper and more narrow than their corresponding backlimbs;

fault

2) synclines are pinned to the fault tips; 3) folds tighten with depth; and 4) slip on the fault decreases upward, terminating within the fold.

purple arrows denote slip on the base and top of the green unit



Constant thickness fault-propagation folds Suppe and Medwedeff (1990) present a general relationship between fold shape and fault shape for parallel (constant thickness) fault propagation folds assuming angular fold hinges and conservation of bed length. This section describes the kinematic development of a constant-thickness fault-propagation fold, and the quantitative relations that can be used to model or interpret these structures. These graphs show the relationships between fault shape (θ2) and fold shape (γ and γ*) for constant thickness fault-propagations folds. The special case of ramping from a detachment is shown as the lines θ2 = φ. These relations will be used to interpret a fault-propagation fold imaged in a seismic profile later in this section.

Kinematic Model Constant thickness fault-propagation folds develop as a fault propagates upward from a bend. An active, synclinal axial surface is pinned to the fault tip. As strata pass through this axial surface, they are folded into the forelimb. Depending on the fault geometry, strata may also pass through the anticlinal axial surface into the forelimb, or from the forelimb onto the fold crest. The backlimb develops much like a fault-bend fold, although the limb width is typically greater than fault slip. Fault-propagation folds have several geometric relations that are useful in constructing models and interpreting structures, including: 1) The distance between the fault bend and the point where the anticlinal axial surface meets the fault equals the fault dip-slip at the bend. 2) The bifurcation point of the anticlinal axial surface occurs along the same bedding horizon as the fault tip.

FPF terminology The following terms are used in the derivation and graphs that describe fault-propagation folds. θ1 = hanging wall cut-off (lower fault segment) θ2 = footwall cut-off (upper fault segment) φ = change in fault dip γ = forelimb syncline interlimb angle γ* = anticlinal interlimb angle ␦b = backlimb dip ␦f = forelimb dip 27 Part 1: Structural Interpretation Methods


Fixed-axis fault-propagation folds Suppe and Medwedeff (1990) present a second, general relationship between fold shape and fault shape called fixed-axis fault-propagation fold theory. This theory is similar to the constant thickness theory, except that it allows for bed thinning or thickening in the forelimb (see also Jamison, 1987). These thickness changes are induced because the forelimb anticlinal axial surface is fixed, meaning that material does not pass through it. The style and magnitude of bed thickness changes are dictated by the initial fault shape and cut-off angles. This section describes the kinematic development of a fixed-axis fault-propagation fold, and the quantitative relations that can be used to model and interpret these structures. These sequential models (0–2) illustrate that fixed-axis fault propagation folds develop in a similar manner to constant-thickness faultpropagation folds. However, the anticlinal axial surfaces are fixed (inactive), causing forelimb thickening or thinning. Folds with low cutoff angles generally exhibit forelimb thickening, whereas, folds with high cutoff angles generally exhibit forelimb thinning.

Kinematic Models

with forelimb thinning

with forelimb thickening

fixed axial surface fixed axial surface

forelimb thickens

forelimb thins

FPF terminology Fixed-axis theory redefines the axial angles (γ values) associated with a fault-propagation fold. The remaining parameters (θ, φ, δb, and δf) are the same as in constant thickness faultpropagation folds. γe = forelimb syncline exterior axial angle γi = forelimb syncline interior axial angle γe*= anticlinal exterior axial angle γi* = anticlinal exterior axial angle

These graphs show the relationships between fault shape (θ2) and fold shape (γe, γe*, γi, and γi*) for fixed-axis fault-propagations folds. The special case of ramping from a detachment is shown on the two graphs at left as the lines θ2 = φ. Note that separate graphs must be used to define the interior (γi, and γi*) and exterior (γe and γe*) axial angles.



Seismic interpretation using fault-propagation fold theory This section presents an interpretation of a structure imaged in seismic reflection data as a fault-propagation fold as described by Suppe and Medwedeff (1990). The seismic profile shows a highly asymmetric fold, with a poorly imaged forelimb, which are characteristics of many seismic images of fault-propagation folds. The seismic section shown below is interpreted in five steps on this and the following page. To help distinguish between the two alternative theories, the graph below (from Suppe and Medwedeff, 1990) shows the relationship of forelimb to backlimb dips for both constant thickness and fixed axis fault propagation folds. Pairs of limb dips that plot along the “Fixed-Axis Theory” curve indicate that the structure may be interpreted using this theory. Limb dips that plot along, or to the left of, the φ = θ2 curve may be interpreted using constant-thickness theory. The two theories are coincident along the portion of the “Fixed-Axis Theory” curve that lies on, or to the left of, the φ = θ2 curve.

Limb dips in fault-propagation folds Limb dips estimated from seismic profile

Step 1: Limb dips are estimated in the seismic profile by interpretation of the reflector dips on the backlimb, and by correlation of horizons 1 and 2 across the poorly imaged forelimb.

Step 2: Based on the forelimb (δf = 58°) and backlimb (δb = 11°) dips estimated on the seismic profile, the fold is inconsistent with fixed-axis theory. However, the structure may be interpreted as a constant thickness fault-propagation fold with a change in fault dip (φ) of 7° and an initial cutoff angle (θ1) of 42°. On the following page, these values are used to predict the fold shape (γ and γ*) and cutoff (θ2) angles, and to generate an interpretation of the structure. 29



Step 3: To interpret the structure using constant-thickness fault propagation fold theory, the upper portion of the fold is interpreted using the kink method, where axial surfaces bisect the interlimb angles (see section 1A-1). This interpretation yields a forelimb interlimb angle (γ) of 61°.

Initial Interpretation

Complete Interpretation

The tip of the fault is located by projecting the axial surfaces that bound the fold crest to their point of intersection. From this intersection point, follow bedding along the forelimb (as defined by δf) until it intersects the forelimb synclinal axial surface. This intersection defines the tip of the fault.

Step 4: The remaining fault-propagation fold parameters (θ2 and γ*) are then determined from one of the two constant thickness faultpropagation fold graphs. Given a γ value of 61° and a change in fault dip (φ) of 7° (from preceding page), the theory predicts an interlimb angle (γ*) of 55.5° and a cut-off angle (θ2) of 49°. These values are used to complete the interpretation.

30

Step 5: The interpretation is completed by extending the fault down from its tip at an angle of 49° (based on θ2) to the point where it intersects the backlimb synclinal axial surface. At this point, the fault shallows by 7° (based on φ) to a dip of 42°. The interior anticlinal axial surface bisects the interlimb angle between the forelimb and backlimb, and extends down to the fault. The distance between the point where this axial surface intersects the fault and the fault bend equals the fault slip at the bend. In summary, this model-based interpretation provides an internally consistent, area balanced description of the structure that honors the seismic data. In general, constant-thickness and fixed-axis fault-propagation fold theories are most applicable to structures with pairs of discrete, parallel axial surfaces bounding fold limbs with roughly constant bed dips. Bed thickness changes in the forelimb, relative to other parts of the structure, are best explained with fixedaxis theory. Comparisons of the forelimb and backlimb dips can also be used to distinguish between these two alternative theories. On the following pages, we describe other modes of folding that may better describe structures with broadly curved fold hinges, variable forelimb dips, non-parallel axial surfaces, and/or substantial footwall deformation. Shaw, Connors, and Suppe


Trishear fault-propagation folds

Seismic section

Erslev (1991) proposed an another mode of fault-propagation folding, known as trishear. Trishear folds form by distributed shear within a triangular (trishear) zone that expands outward from a fault tip. Folds develop in the trishear zone and cross sectional area, but not bed thickness or length, and are preserved through deformation. The displacement field, and thus fold shape, is straightforward to calculate. However, it must be done iteratively. Hence, the method cannot be applied graphically or analytically (Allmendinger, 1998). Here, we describe some of the basic characteristics of trishear folds, and use the theory as implemented by Hardy and Ford (1997) and Allmendinger (1998) to model and interpret these structures.

Theory

Kinematic model

Trishear interpretation

The trishear zone (a-b-c) is bound by two surfaces that define an intervening apical angle. The surfaces may or may not be symmetric with respect to the fault (Zehnder and Allmendinger, 2000). To preserve cross sectional area (a-b-c = a-a⬘-b-c) during deformation, there must be a component of displacement toward the footwall, as reflected by the velocity vectors. To model a trishear fold, the apical angle, the fault dip, and the propagation to slip ratio (P/S) of the fault are specified. (after Erslev, 1991; and Allmendinger, 1998).

This sequential model (0 - 2) shows the development of a trishear fault-propagation fold at the tip of a thrust ramp that steps upward from a detachment. The backlimb of the structure is a simple fault-bend fold. The geometry of the forelimb is a function of the apical angle, the fault dip, and the P/S ratio. Small apical angles generally yield tight, highly strained forelimbs, whereas large apical angles generally yield broad, gently strained forelimbs. At a given apical angle, the steepness of the forelimb increases with progressive slip. The steepness of the forelimb also increases downward. This pattern is characteristic of trishear folds, and contrasts with the constant forelimb dips exhibited by constant-thickness and fixed-axis fault-propagation folds.

Propagation to slip ratio The fault-propagation fold in this seismic section has a broadening upward zone of folding and a fanning of forelimb dips (1). These patterns are forward modeled using trishear, based on parameters derived through an inversion method (Allmendinger, 1998). The best fitting model is displayed on the seismic section in the lower panel. Fault propagation to slip ratio (P/S) has an important influence on fold shape. Low P/S ratios generally yield steep, tightly folded forelimbs with pronounced bed thickening. High P/S ratios generally yield shallow, gently folded forelimbs with less bed thickening (from Allmendinger, 1998).

In summary, trishear folds are easily distinguished from constant-thickness and fixed-axis fault-propagation folds, in that they display an upward-widening, curved fold limb ahead of the fault tip, which leads to an upward decrease in limb dip. 31



Basement-involved (drape) folding with migrating triple junctions Fault-propagation folds that involve basement (crystalline) rock are commonplace, and tend to have shapes that differ from those described by constant-thickness and fixed axis fault-propagation fold models (Suppe and Medwedeff, 1990). Several geometric and kinematic theories have been developed to explain these structures, including models with forelimb shear distributed in triangular zones, such as the trishear model described in the preceding section (Erslev, 1991; Mitra and Mount, 1998). This section describes another kinematic theory proposed by Narr and Suppe (1994), in which fold growth is governed by the migration of a fault-fault-fold triple junction. The theory is then applied to interpret a fault-propagation fold in seismic data.

Kinematic model

In the Narr and Suppe (1994) basementinvolved model, folding is driven by the migration of a fault-fault-fold (axial surface) triple junction. The triple junction moves upward with progressive fault slip, causing shear of the footwall that forms a monocline. Uplift of the hanging wall also induces folding of the sedimentary cover, producing a forelimb with bed dips that are parallel to the dip of the upper fault segment. Stages 0–2 show progress development of a migrating triple junction fold model.

Fold and fault shape

Seismic Example: Orito Field, Putamayo basin, Colombia These graphs describe relations among the five parameters that describe triple-junction folds. Each graph is for a specific ε value. When modeling structures imaged in seismic sections, ε is generally selected by interpreting the forelimb dip value (δf). The dip of the footwall monocline is also commonly resolved on seismic sections, leaving one additional parameter to be determined (φ or θ) before a unique solution can be obtained. From Narr and Suppe (1994).

Triple junction fold terminology Five parameters describe basement-involved triple junction folds, three of which must be specified to derive the remaining two values: θ1 = hanging wall cutoff of lower fault segment ε = dip of upper fault segment (generally = 180°- δf) β = dip of footwall monocline φ = dip of footwall shear orientation ψ = footwall angular shear

Seismic profile of a basement-involved fault propagation. The footwall monocline and steep (poorly imaged) forelimb are characteristic of triple junction fault-propagation fold models. In the interpretation, the shear orientation (φ) and angle (ψ) were estimated from the graph at left using: 1) the maximum forelimb dip value (δf), estimated from oriented well core and surface dips, to define ε (120°); 2) the reflection truncations to estimate the fault dip (θ = 60°), and; 3) the dip of the footwall monocline (β = 9°). The interpreted section involves additional deformation induced by a breakthrough of the main fault, a process which is described later in this section, but nevertheless the structure maintains the basic geometry described by the migrating triple junction theory of Narr and Suppe (1994).



Growth fault-propagation folding

Seismic Example: Bermejo anticline, Argentina

Syntectonic (growth) strata are folded in distinctive patterns above fault-propagation folds. Forelimb growth structures, in particular, vary among the different fault-propagation fold models and thus can be diagnostic of the folding mechanism. In this section, we contrast growth patterns developed above fault propagation folds as described by Suppe and Medwedeff (1990) and trishear folds (Erslev, 1993), using kinematic models and examples imaged in seismic sections. Growth fault-propagation fold

Growth trishear fold

Kinematic models Growth axial surface

Fault-propagation folds of Suppe and Medwedeff, (1990) grow by kink-band migration, with two active axial surfaces bounding the backlimb, and one or two active axial surfaces bounding the forelimb. Syntectonic strata above the fold limbs form growth triangles. When sedimentation rate exceeds uplift rate, as in this model, two growth triangles develop on the backlimb. Fixed-axis fault-propagation folds have a single forelimb growth triangle, whereas, constant thickness fault-propagation folds may have one or two forelimb growth triangles depending on the fault geometry. This sequential model (0–2), with a 29° fault ramp and a decollement, is a case where both constant-thickness and fixed axis theory converge to yield the same geometry.

Trishear folds (Erslev, 1993) generally develop by a combination of kink-band migration and limb rotation mechanisms, and these fold kinematics are reflected in growth strata. Progressive forelimb rotation during the formation of trishear folds generally yields an upward shallowing of bed dips in growth strata. This fanning of limb dips in trishear growth folds contrasts markedly with the growth triangles predicted by the constant-thickness and fixed axis theories. This sequential model (0–2) (after Hardy and Ford, 1997) has a sedimentation rate that slightly exceeds the uplift rate. The backlimb of this model forms by fault-bend folding, yielding a single backlimb growth triangle.

Time transgressive unconformity

Seismic example of a forelimb growth truncations triangle in a faultpropagation fold from the Bermejo foreland basin, central Argentina from Active axial surface Zapata and Allmendinger (1996). Reproduced courtesy of the American Geophysical Union. Seismic Example: Tarim basin, China

Effects of low sedimentation rates

Sedimentation rate relative to uplift rate can have a pronounced impact on resultant growth geometries. These three examples (a-c) show the effects of local non-deposition and erosion on growth structures in fault-propagation folds (after Suppe et al., 1992).

Seismic example of fanning forelimb dips in growth strata from the Tanan Uplift, Tarim basin, China. Section is overlain by a modeled trishear fold, described in the trishear folding section, that includes modeled growth horizons (yellow). 33



Breakthrough fault-propagation folds At any stage of fold growth, faults may cut through fault-propagation folds, altering the geometries of these structures. The shapes of these “breakthrough” structures are influenced by the path of the fault, which often breaks through the forelimb or shallows to an upper detachment, as well as the folding mechanism. In cases where the slip on the breakthrough fault is substantial and/or structures are deeply eroded, only remnants of the original fault-propagation fold geometries may remain. In this section, we use several kinematic models to describe styles of breakthrough fault propagation folding, and show an example of this type of structure in a seismic section. Triple junction fold breakthrough Trishear fold breakthrough Forelimb breakthrough

Kinematic models

This sequential model (1–2) shows a constant-thickness fault propagation fold (1) where the fault breaks through the middle of the forelimb (2). The fault modifies the original fold geometry by offsetting the hanging wall portion of the forelimb, and producing an additional kink band within the backlimb that develops by fault-bend folding. Breakthrough styles

Faults in trishear and triple-junction fault-propagation folds may also breakthrough at any stage of fold growth. These models are examples of synclinal fault breakthroughs in: a) trishear fold after Allmendinger (1998); and b) a triple junction model after Narr and Suppe (1994). The geometries of breakthrough structures in all classes of fault-propagation folds vary substantially based on the fault path and, if the fault is non-planar, on folding kinematics after breakthrough. Seismic Example: Argentina

Models showing possible types of breakthrough structures after Suppe and Medwedeff (1990). a and b) decollement breakthroughs; c) synclinal breakthrough; d) anticlinal breakthrough; e) high-angle (forelimb) breakthrough; and f) low-angle breakthrough.

This seismic section illustrates a common forelimb breakthrough pattern. Although the forelimb is poorly imaged, reflection truncations and the hanging wall and footwall positions of the correlated horizon suggest that the fault extends through the structure. Nevertheless, the basic geometry of the fold is consistent with a fault-propagation folding mechanism, implying that this is a breakthrough structure.



Examples

1B-3: Detachment folds Basic Concept Detachment folds form as displacement along a bedding-parallel fault is transferred into folding of the hanging wall layers. Although detachment folds may share some geometric similarities with fault-bend and fault-propagation folds, they differ from these structures because they are not directly related to thrust ramps but rather to distributed deformation above detachments. In this section, we describe basic aspects of the geometry and kinematics of detachment folds. These insights are used to guide the interpretation detachment folds in seismic images.

Styles of Detachment Folds Detachment folds form at a variety of scales, as isolated structures or in long fold trains, and many names are used to describe them. The term detachment fold is commonly applied to symmetric or asymmetric folds that develop above a relatively thick ductile unit and basal detachment. If folds are symmetric, have steep limbs, and develop above a relatively thin ductile unit, they are often called pop-up or lift-off folds (Mitra and Namson, 1989; Mount, 1990). Lift-off folds develop by isoclinal folding of the detachment in the core of the anticline, and when they have flat crests they are referred to as box folds.

Detachment folds are common in outcrop and at scales typically imaged by seismic reflection data. They have been documented in the foreland of fold and thrust belts such as the Jura, Appalachian Plateau (Wiltschko and Chapple, 1977), and Tian Shan (Ferrari et al., section 2-14, this volume). Detachment folds are also common in passive margin fold belts, such as the Mississippi Fan (Rowan, 1997) and Perdido Fold Belts (Carmilo et al., section 2-24, this volume) in the Gulf of Mexico, and in the Campos Basin, Brazil, (Demercian et al., 1993), and the Niger Delta (Bilotti et al., section 2-12, this volume).

Field Example: Canadian Rockies

The field and seismic examples shown here have many of the common characteristics of detachment folds described at lower left.

Kinematic models of detachment folds Seismic Example: Gulf of Mexico

Common characteristics Detachment folds generally share the following characteristics: 1) An incompetent, ductile basal unit thickened in core of fold, with no visible thrust ramp. 2) A detachment that defines the downward termination of the fold. 3) Competent pregrowth units that, if present, generally maintain layer thickness. 4) Growth units, if present, that thin onto the fold crest and exhibit a fanning of limb dips. 35 Part 1: Structural Interpretation Methods


Geometry and kinematics of detachment folds

Kinematic models of detachment folds

There is no unique, quantitative relationship between fold shape and underlying fault shape for detachment folds, due in part to the ductile thicknening occurring in the fold core that generally does not preserve bed length or thickness. Thus, it is often difficult to uniquely constrain the geometry of these structures unless they are completely imaged. Nevertheless, several geometric and kinematic models have been developed (Dahlstrom, 1990; Ephard and Groshong, 1995; Homza and Wallace, 1995; Poblet and McClay, 1996) that can serve as guides for interpreting detachment folds in seismic images. In this section, we present a geometric and kinematic model of detachment folding developed by Poblet and McClay (1996) that is particularly useful when analyzing growth strata associated with detachment folding that involves a competent unit. These authors propose three distinct mechanisms by which a fold can develop above a propagating detachment. In each of the models, it is the geometry and kinematics of folding in the competent layer (in particular, limb lengths and limb dips) that controls the folding. The incompetent, or ductile layer, is able to flow into, or out of, the fold core as deformation progresses. Layer thickness, line length, and area are conserved in the competent layers. If the detachment level is allowed to change, or if differential shortening occurs in the incompetent unit, then area is conserved in the ductile layer as well. Poblet and McClay (1996) present three modes of detachment fold growth that are illustrated in the figure to the upper right (models 1–3), and differentiated based on their folding mechanisms as follows: 1) Primarily Limb Rotation. In this model, the limb lengths remain constant but the limbs rotate to accommodate shortening. A small amount of material must move through the axial surfaces, inducing a minor component of kink-band migration, as folding progresses. The incompetent unit is area balanced only if the detachment level varies or differential shortening occurs in the incompetent unit.

Detachment fold terminology

2) Kink-band Migration. In this model, limb dips remain constant, but their lengths increase to accommodate shortening. Material moves through the synclinal axial surfaces as folding progresses. The incompetent unit is area balanced only if the detachment level varies or differential shortening occurs in the incompetent unit.

S = Slip

Lf = Front limb length Lb = Back Limb length ϑf = Front limb dip ϑb = Back limb dip u = Uplift

3) Limb Rotation and Kink-band Migration. In this model, limb dips vary, as do limb lengths, but the ratio of the limb lengths remains the same. Strata moves through axial surfaces (primarily the synclinal surfaces), and rotate to accommodate shortening. The incompetent unit area is balanced. Two fundamental equations relate the shortening and uplift to the limb lengths and limb dips of these detachment folds: (equations) S = Lb (1 - cos ϑb) + Lf (1 - cos ϑf + Lt sin ϑf) u = Lb (sin ϑb) = Lf (sin ϑf) based on the detachment fold terminology defined in the figure to the lower right. 36 Shaw, Connors, and Suppe


Growth strata associated with detachment folds

Kinematic models of growth detachment folds

It is usually not possible to determine the folding mechanism of a detachment anticline from the geometry of pregrowth strata alone. For example, the three models on the previous page have identical final geometries, but the paths they took to get there (i.e., the fold kinematics), and the folding mechanisms, were quite different. Growth strata are, however, typically diagnostic of folding mechanism because they record the kinematic history of fold growth (see section 1A-3). Thus, growth strata can be used to distinguish between the modes of detachment folding described by Poblet and McClay (1996). As illustrated in section 1A-3, kink-band migration causes growth strata to form narrowingupward kink bands, or growth triangles, with bed dips that are parallel to those of the underlying pregrowth strata. Growth triangles are bounded by at least one active axial surface. In contrast, limb rotation causes progressive changes in limb dips that result in a fanning of limb dips in growth strata. In limb rotation structures, a minor amount of material may still move through axial surfaces that are continuously changing orientation, resulting in a minor amount of kinkband migration. Poblet and McClay (1996) refer to these as “limited-activity axial surfaces.” These models define the activity of axial surfaces that are involved in the three types of detachment folds defined by Poblet and McClay (1996):

Axial Surface Activity

Seismic Example: Gulf of Mexico

Based on these fold kinematics, growth strata have distinctive patterns in each type of detachment folds that are shown in the models (1–3) at upper right, which are described as follows: 1) Primarily Limb Rotation. In this model, growth strata predominantly display fanning of dips, recording the progressive rotation of the fold limbs. Small growth triangles form that define growth strata which migrated through the limited-activity axial surfaces. 2) Kink-band Migration. In this model, growth strata form growth triangles because strata have migrated through the active synclinal axial surfaces. 3) Limb Rotation and Kink-band Migration. In this model, growth strata display some fanning of dip due to rotation of the fold limbs as well as growth triangles that record the migration of strata through the active synclinal axial surfaces.

This seismic line images a detachment anticline with patterns of growth strata that reflect folding by both limb rotation and kink-band migration, suggesting that the structure is compatible with model 3 shown above. 37



Seismic interpretation of a detachment fold: Angola continental slope In this section, we describe the interpretation of a detachment fold imaged in a seismic reflection profile based on the fold models presented on the preceding pages. Initial Observations. The fold from offshore west Africa shown at right is symmetric, with units that conserve layer thickness (3) and other units that clearly do not (1, 4). There is no obvious thrust ramp present, although reflectors underlying the fold are essentially flat suggesting the presence of a detachment (2).

Initial observations

Structural Interpretation. Based on the initial observations, this structure is interpreted as a detachment fold in the section at lower right. The detachment is interpreted to separate folded layers above from undeformed strata below. Above the detachment, a poorly imaged stratigraphic interval is thickened in the core of the fold (1). This incompetent unit represents an Aptian salt bed. The units directly above the salt broadly conserve layer thickness (3), indicating these strata have acted competently during deformation, probably deforming by flexural slip (see section 1A-2). The constant thickness of the units also indicates that they were deposited prior to folding. Above these units, layers that thin onto the crest of the fold (4) are growth strata. The growth strata generally fan above the fold limbs, with only small panels in the limbs having the same stratigraphic thickness that they do in the synclines. Thus, the fold grew mostly by limb rotation with only minor kink-band migration, similar to the model 1 detachment fold of Poblet and McClay (1996).

Calculating detachment depth: Why doesn’t it always work? Several techniques (e.g., Chamberlin, 1910; Epard and Groshong, 1993; Homza and Wallace, 1995) have been developed to determine the depth-to-detachment beneath anticlines based upon balancing the area uplifted in the fold with the displaced area as shown below in model A. In cases where the detachment depth is know independently, several authors have pointed out that the predicted and observed detachment depths do not always match (Wiltschko and Chapple, 1977; Jones, 1987; Dahlstrom, 1990; Groshong and Epard, 1994; Homza and Wallace, 1995; Poblet and Hardy, 1995). (In the case of the Angolan detachment fold interpreted in this section, the predicted depth-to-detachment is greater than 15 km!). These discrepancies arise because balancing the uplifted area with displaced area has two implicit assumptions, namely that: 1) The thickness of the ductile unit outside of the fold is maintained, and; 2) All of the material in the thickened

zone comes from within the plane of the section. One or both of these assumptions may be invalid for detachment anticlines as well as other types of fault-related folds, as shown below in model B. In particular, detachment folds with highly ductile cores involving salt or over-pressured muds often show withdrawal of material in the synclines (and away from fold), causing local thinning of the ductile interval and subsidence of overlying strata. Withdrawn material is presumably moved into the core of the fold. Alternatively, or in addition, material in the thickened core of the fold may be derived from out of the plane of section. Both processes invalidate the assumptions of classic depth-to-detachment calculations, leading to predicted detachment depths that are generally far too deep. Thus, care should be taken to avoid applying these methods of calculating depth-to-detachment in detachment folds with ductile cores.

Structural interpretation

Depth-to-detachment calculations

Model A shows the classic method of calculating the depth to detachment, based on the assumption that the uplift area is equal to the displaced area. The shortening, which is typically determined by unfolding a layer while conserving line length, and the uplift area are used to calculate the detachment depth by: depth-to-detachment = displaced area / shortening Model B shows a typical detachment fold where the uplift area greatly exceeds the displaced area. In these cases, standard depth-to-detachment calculations inaccurately predict detachment depths.



1B-4: Shear fault-bend folds Basic concept

Seismic Example: Niger Delta

Shear fault-bend folding produces ramp anticlines with very distinctive shapes that reflect a significant non-flexural-slip component to the deformation. The structural style typically shows long back-limbs that dip less than the fault ramp, in contrast with classical fault-bend folding. This section describes the geometry and kinematics of shear fault-bend folding after Suppe, Connors, and Zhang (2004) and introduces basic techniques for recognizing and interpreting these structures in seismic images.

Recognizing the structural style The typical structural style for ramp anticlines produced by shear fault-bend folding has back limbs that dip less — in many cases very much less — than the fault-ramp (1). If a significant stratigraphic section is deposited over the back limb during fold growth it typically shows evidence of limb rotation (2). These ramp anticlines also commonly show front limbs (3) that are quite narrow relative to their long back limbs.

Seismic Example: Cascadia Canada



Shear is the essence

Shear in a seismic example: Cascadia Canada

Classical fault-bend folds (section 1B-1) deform by flexural slip of the beds as they slide over fault bends (A), conserving layer thickness. In contrast, shear fault-bend folds undergo additional distortion of the hanging wall or footwall, that is they undergo additional shear. This additional shear usually is concentrated in a weak detachment interval such as shale or evaporite that deforms by bedding-parallel simple shear — like the geometric model below (B). Alternatively, shear may be more distributed as in the analog model from David Elliott (1976) based on sheets of paper (C) or it may involve a bedding-parallel shortening and thickening, which is called pure shear. Shear fault-bend folds can also form by some combination of pure and simple shear or by more heterogeneous deformation as shown below in the distinct-element mechanical simulation by Luther Strayer (D).

Flexural-slip unfolding of a shear fault-bend fold yields a hanging wall shape that doesn’t match the footwall because there has been deformation in addition to flexural slip. In this example from the Cascadia accretionary wedge, offshore western Canada, the hanging-wall fault shape is determined by unfolding the layers while conserving line length. The difference between the unfolded hanging-wall fault shape and the actual fault shape yields the shear profile, showing that there has been layer-parallel simple shear. The shear is concentrated in the yellow and red basal layers.

Interpreted section

Models A: Classic fault-bend fold

B: Shear fault-bend fold

C: Analog model of shear fault-bend fold

Flexural-slip unfolding gives the shear profile

D: Mechanical model of shear fault-bend fold



End-member shear fault-bend folding

Fold types

End-member shear fault-bend folding. We can understand the fundamentals of shear fault-bend folding and quantitatively check our seismic interpretations by using two simple end-member theories, both involving a weak basal decollement layer of thickness h (shown in yellow). In the simpleshear end member, the decollement layer undergoes bedding-parallel simple shear with no actual basal fault, just a distributed zone of shear. In the pure-shear end member, the decollement layer slides above a basal fault and shortens and thickens in a triangular area above the ramp. Mixtures between these end members are possible, as shown at right, but many actual folds are close to the end members. Classical fault-bend folding is also an end member, with a basal layer of zero thickness (h = 0). The shape of the fold shows us which stratigraphic interval is the decollement layer. The anticlinal axial surface terminates at the top of the decollement interval at (A). The synclinal axial surface terminates at the bottom (B). Also, if there is pure shear, the synclinal axial surface (C) doesn’t bisect within the decollement layer because the latter is thickened above the ramp. These properties are useful in seismic interpretation.

Simple-shear end-member

Pure-shear end member Graphs of end-member theory. These end-member shear fault-bend fold graphs give the balanced relationship between ramp dip θ, back limb dip δβ, and shear (αe or α) across the basal layer. The shear is tan d/h, where d is the displacement at the top of the basal layer and h is its thickness. The dip of the back syncline in the basal layer (ψ) is useful in the pureshear and mixed cases. The inset drawing of the simple-shear graph shows a model shear fault-bend fold that corresponds to the yellow square (θ = 23°, δb = 6.5°, and αe = 42°). The drawing of the pure-shear graph corresponds to the angles shown by the red square (θ = 34°, δb = 15.5°, α = 68°, and ψ = 30°). Curiously, these shear fault-bend fold graphs also encompass classical no-shear fault-bend folding, which is reached in the limit of zero thickness h to the basal layer. Shear (d/h) becomes infinite (αe or α = 90°) and the limb dip becomes parallel the fault (θ = δb) (D).



Seismic interpretation of a simple shear fault-bend fold: Cascadia, Canada Initial assessment. The structure imaged in this seismic section from offshore western Canada (Hyndman et al., 1994) shows the characteristics of a shear fault-bend fold, especially the steepness of the fault dip (35-40°) relative to the back limb dip (5-13°). A front limb much narrower than the back (1) is also typical of shear fault-bend folds. Interpreting the ramp geometry. The fault picks (shown below in red) constrain the fault geometry and rule out strongly listric fault interpretations. Also, note that there is a downward dying out of the fault throw (2), with throw going to zero at the base of the ramp (3). This is characteristic of shear fault-bend folds, in contrast with classical fault-bend folds. Significance of synclinal geometry. The back syncline is planar, bisects the inter-limb angle (4), and terminates at the base of the fault ramp (3), indicating a simple-shear rather than a pure-shear fault-bend fold (see models previous page).

Fault picks

Timing of growth. Onlapping shallow reflectors (5) show that 120 m of growth strata have accumulated. Deformation began soon after termination of slip on the shallow hinterland thrust to the east, as defined by a seismic horizon (6) that is folded in the backlimb of the shear faultbend fold but is undeformed above the thrust tip in the hinterland structure. Thickness and dip variations in growth strata record deformation by limb rotation and kink-band migration (5), consistent with shear fault-bend folding. 42 Shaw, Connors, and Suppe


Refining and testing the seismic interpretation: Cascadia, Canada Refining the interpretation. This structure is more complex than the simple models shown previously because the fault ramp is not straight but composed of two segments dipping 35° and 40°. Furthermore the backlimb has two kink bands ab and bc of different dips (1 and 2).

Two segments of the back limb

Testing the interpretation. Let us begin by treating each kink band of the backlimb (1 and 2) separately, predicting two shear amounts from the two limb dips. Then we will compare the predicted shear with the shear determined from unfolding the hanging wall to see if our interpretation is consistent. Applying the simple-shear graph (shown at far right), we find that a backlimb dip δb of 11-12° within the lower kink band ab and a lower ramp dip θ of 35° predict an external simple shear αe of 31-32° (1'). This agrees with the shear αe of 31° determined by unfolding the hanging wall while conserving bed length as shown below (1"). The backlimb dip δb of 5° within the upper kink band bc and an upper ramp dip θ of 40° predict an external simple shear ae of about 8° (2'), which also agrees with shear determined by the unfolding (2"). These quantitative tests give us more confidence that our seismic interpretation of this ramp anticline as a shear fault-bend fold is reasonable.

Interpreted depth section

Two intervals of shear



Evolution of shear fault-bend folds

Relationships of backlimb dip (δb) to shear (αe and a)

Kinematic evolution. Both simple- and pure-shear fault-bend folds develop by combinations of limb lengthening (kink-band migration) and limb rotation. The graphs at right show the relationship between limb dip and shear for both fold types. In the limit of large shear (i.e., displacement), the fold geometry in pregrowth strata approaches the geometry of classical fault-bend folding, with a back-limb dip that approaches the ramp dip (θ approaches δb). However, even in these cases folds will grow with a component of limb rotation, recording their shear fault-bend fold heritage.

Heterogeneous simple shear

Note that in this heterogeneous simple-shear fold that the highest shear interval defines the base of the backlimb panel that most closely approaches the ramp dip.

Growth strata. The combination of limb rotation and limb lengthening that occurs in shear fault-bend folding is recorded by growth strata, as illustrated in the sequential kinematic models (A1-A3) shown below. Fanning of dips recording limb rotation (1) and growth triangles recording kink-band migration (2) (see section 1A-4). Growth strata in the example from the Niger delta at right show evidence of limb rotation. As mentioned above, the fold geometry in pre-growth strata approaches the geometry of classical fault-bend folding, with bed dips (3) approaching the ramp dip, in the limit of large shear (i.e., displacement). The sequential large shear model at right (B1–B2), however, demonstrates that the component of limb rotation is recorded in growth strata (4), and thus can be used to distinguish large shear fault-bend folds from classical fault-bend folds.

Given a constant ramp dip, the backlimb dip (δb) steepens as shear (αe and α) increases. Points A1 to A3 correspond to models presented at lower left.

Large shear (displacement) fault-bend folds

Niger delta limb rotation

Limb rotation plus kink-band migration



Seismic interpretation of pure-shear fault-bend folds: Nankai trough, Japan Initial assessment. This line shows ramp anticlines developed in overpressured Shikoku basin turbidites above the master detachment (D) of the Nankai trough accretionary wedge. Note that the degree of shortening in the structures increases from south to north. Notice the qualitative characteristics of shear fault-bend folds, including backlimb dips that are less than ramp dip (A). Nevertheless these structures are more complex than the end-member models because of superposed low-amplitude detachment folding and secondary deformation, seen in both footwalls and hanging walls (B). This depth-migrated dip line passes through Ocean Drilling Project holes ODP-808 and ODP-1174, which reach to the top of oceanic crust (C) (line NT62-8 Moore et al., 1990, 1991, 2002). The 19-meter-thick master detachment was cored in ODP-808 just above transparent pelagic sediments of the Shikoku basin (D).

Nankai trough, Japan



Refining the seismic interpretation: Nankai trough, Japan Strategy. We can test our qualitative interpretation by comparing the seismic geometry with the end-member theories.

Picking the fault Pure-shear end member

Fault and limb geometry. In the seismic section shown at upper right, the faultramp is located based on reflector terminations shown as red arrows and by core from the ODP-808 hole (1). This gives a remarkably straight ramp, dipping at θ = 35°, which is much greater than the average dip of the irregular backlimb (δb = 11-13°), suggesting that this is a shear fault-bend fold. The back syncline in the strong reflectors (2) is displaced substantially to the hinterland of the base of the ramp, which favors pure-shear or mixed-shear models that we now test. Comparing with the end-member theory. Plotting the backlimb dip δb = 13° and ramp dip θ = 35° on the pure-shear graph at far right (3) predicts a back synclinal dip ψ = 31° in the basal decollement layer, which quantitatively agrees with the seismic image at right. In theory, the location of the top of the decollement layer (in orange) is at the inflection in the back syncline, which agrees with the location indicated independently by the fault cutoff of the back anticline (4) — supporting our pureshear fault-bend fold interpretation. A complete interpretation is shown on the seismic image at lower right (see also Suppe et al., 2004).

Comparing the seismic with an end-member model

Fault slip. The back-dip and ramp angles plotted on the graph (3) also give us the shear α = 69° of the basal layer. From this we can calculate the fault slip d = 390 m, based on a basal layer thickness h of about 230 m (tan α = d/h = 1.7).

Depth section

Interpreted depth section



1B-5: Imbricate fault-bend folds Basic concept

Seismic Example: Alberta Foothills, Canada

Imbricate structures form by the stacking of two or more thrust sheets and are common in fold and thrust belts worldwide. Imbricate structures can form by break-forward propagation of thrust sheets, by break-backward thrusting, or with coeval motion on both deep and shallow faults. In this section, we describe the basic characteristics of imbricate structures, and outline an approach to interpret these structures in seismic profiles using imbricate fault-bend fold theory (Suppe, 1983; Shaw et al., 1999). Break-forward imbricate Break-backward imbricate

Kinematic Models

Seismic Example: Niger Delta, Nigeria

Imbricate structures develop where two or more thrust sheets are stacked vertically. These thrust faults may or may not involve detachments, but imbricate structures are more common in regions with detachments. In the sequential break-forward model (0–2) shown above, slip on the deep thrust fault produces a fault-bend fold that refolds the overlying thrust sheet. In the sequential break-backward model (0–2), a pre-existing fault-bend fold is cut by a shallow, younger thrust ramp.

Common characteristics Imbricate fault-bend folds typically contain: 1) Two or more vertically stacked thrust ramps; 2) Bedding dips that change across thrust ramps; and 3) Fold limbs at high structural levels with multiple dip domains, reflecting refolding caused by multiple ramps. (Note: multiple dip domains may also be produced by multi-bend fault-bend folds, see section 1B-1).

These seismic sections show the three common characteristics described in the model at left, including (1) multiple ramps, (2) changes in bedding dip across ramps, and (3) multiple dip domains in fold limbs 47



Interpreting break-forward imbricate structures using fault-bend fold theory Suppe (1983) presents a strategy for interpreting break-forward imbricate structures based on the fact that each lower imbricate increases the dips in the overlying imbricates by fixed or quantum amounts that are predictable using fault-bend fold theory. Here we assume that bed-length and thickness are conserved and that all faults step up from a detachment at the same initial step-up angle (ramp dip). This section describes how to implement this approach to interpret imbricate structures imaged in seismic sections.

Dip values measured on seismic profile

Theory Imbricate fault-bend fold theory describes the increases in dip order caused by refolding of shallow thrust sheets by younger and deeper faults. In model 0, with a single thrust ramp A, the forelimb and backlimb dip values are first order (-I and +I), because each limb was formed by strata passing over a single fault bend. Incipient thrust B is shown in the footwall of thrust A. In model 1, slip on fault B refolds the shallow thrust sheet, producing second order (-II and +II) dip panels. These second order panels were folded once by thrust A, and again by thrust B. The dips of the forelimb and backlimb panels (-I, +I, -II, and +II) are prescribed by fault-bend fold theory based on the initial cutoff angles (θ).

Two backlimb dip values are observed in this seismic section near the well. The lesser value (-I = 13°) occurs between faults A and B, and in the hanging wall of fault A to the right of the well. The steeper value (-II = 25°) is restricted to the hanging wall of fault A. These two backlimb dip values are compared with the values shown in the table at lower left, to determine if they are consistent with imbricate fault-bend fold theory.

Interpreted section

Forelimb and backlimb dip values are based on the initial cutoff angle (θ) and the number of imbricated thrusts. This table shows the prescribed forelimb and backlimb dips for first- through seventh-order (I-VII) panels based on 8 to 24° fundamental cutoff angles. The order of the dip panel (IVII) generally corresponds to the number of imbricated faults. Dip panels are typically measured on seismic sections, and then compared with rows of prescribed values. If a general match between observed and prescribed dip values is obtained, then the structure can be interpreted using this table. If a match is not obtained, it may suggest that the initial cutoff angles of the ramps are not equal, requiring use of values different that those on this table (see Mount et al., 1990). These more complex situations can be interpreted using the folding vector technique presented on the next page.

The two backlimb dip values (-I = 13° and -II = 25°) correspond to a 13°initial cutoff angle based on the table at left (see row highlighted in yellow). Thus, the geometries of faults A and B can be interpreted as part of a break-forward thrust sequence. The lower fault (B) dips at 13°, corresponding to the prescribed initial cutoff angle. It shallows to upper and lower detachments based on simple fault-bend fold theory (see section 1B-1) with θ = φ = 13°. The upper fault (A) dips at the second-order value (-II = 25°) where it lies above the backlimb kink band formed by fault B. Where fault A extends beyond the underlying backlimb kink band, it dips at -I = 13°, corresponding to the prescribed initial cutoff angle. The geometries prescribed by the table match the reflection patterns closely. Note, however, that other faults in the section further complicate some aspects of the geometry.



Interpreting break-forward imbricate structures using folding vectors Here we describe a method of interpreting break-forward imbricate structures using folding vectors (Shaw et al., 1999). This method can be applied to a wide range of structures, including imbricate systems where initial cutoff angles of faults vary, bed thickness changes occur, or faults do not sole to detachments. Folding vectors describe the relative displacement of bedding or other surfaces, such as faults, across a fold limb or kink band. Thus, folding vectors can be used to describe the refolding of overlying thrust sheets due to imbrication. In this section, we describe how to determine folding vectors and use them to interpret a break-forward imbricate structure imaged in a seismic section.

Using folding vectors To describe how folding vectors are used to interpret break-forward imbricate structures, we will consider the case of a shallow thrust sheet (above fault A) being refolded by a deeper thrust (B). In model 1, slip on the deep thrust B has produced a backlimb kink band that must refold the overlying thrust sheet (A). Hence, the orientation of fault A, and beds in its hanging wall, will change as the thrust sheet passes over the underlying kink band. In model 2, the deflection of bedding across the deep kink band is used to determine the folding vector (U). Folding vectors are measured parallel to axial surface orientations. The deflection of thrust A across the deep kink band is described by vector X, which is equal to the folding vector U. This results in shear, and hence line length, being preserved parallel to the axial surface orientation. The orientation of bedding that is refolded in the hanging wall of fault A can be determined using fault-bend fold theory (see section 1B-1), or by using folding vectors as shown in model 3. However, in this (and perhaps many) cases, the axial surface orientation changes between the footwall and hanging wall of fault A because bed dips change. Thus, the new hanging wall axial surface orientation must be used to measure a new folding vector (Y), which is equal to the deflection of fault A. This folding vector, in turn, equals the deflection of bedding in the hanging wall of fault A that is described by vector Z. This method also applies in cases where axial surfaces do not bisect interlimb angles, and thus bed thickness is not preserved. In all cases, however, proper use of folding vectors results in area-balanced interpretations.

Measuring a folding vector The folding vector method is used to interpret this seismic section, in which fault A is refolded by an underlying kink band bounded by axial surface S⬘. Fault A enters the left side of the kink band at a dip of 22°. The folding vector U is measured as the deflection of a bed* across axial surface S⬘ in the footwall of fault A.

*Note that folding vectors must be measured parallel to, but not necessarily along, axial surfaces. In this case, the paired axial surface corresponding to S⬘ is located off the right side of the section, so the folding vector is measured at an arbitrary point in the direction parallel to axial surface S⬘.

Interpreting a folded thrust

The folding vector U is then used to predict the deflection of the fault (A) across the kink band (U = X). The predicted fault position is consistent with reflection terminations that appear to represent fault cutoffs. Moreover, the folded fault dips about 30°, roughly parallel to beds in the overlying kink band (T-T⬘).

Note: This method can also be used to model the folding of angular unconformities, sedimentary growth wedges, and other cases where bed dips within a kink band are not parallel.



Recognizing break-back thrusting This section describes fault and fold patterns that are common in break-backward imbricate structures, and shows examples in seismic sections.

Patterns of fault cutting older fold limbs

Patterns of break-backward thrusting in seismic data

To describe structural patterns common in break-back imbricate structures, we will consider some simple patterns for a shallow, break-backward thrust ramp (model A1) and detachment (model A2) cutting across a fold limb (S-S’) related to an older and deeper thrust. The shallow thrust ramp may cut across and offset a part of the fold limb without changing fault orientation (model B1). Alternatively, the shallow thrust could change its orientation across the fold limb, offsetting and refolding parts of the structure (model C1). In the case of model C1, note that the deep folding vector (U) need not equal the deflection of the break-backward thrust (X), in contrast to the break-forward example described on the previous page. In the case of the detachment, the shallow fault could follow bedding planes across the fold limb (model B2). Based on fault-bend fold theory (Suppe, 1983), slip on this shallow detachment would not modify the fold shape. Alternatively, the shallow detachment could follow bedding across the fold limb but cut up section beyond the fold (model C2). In this case the shallow fault conforms to one axial surface and offsets the other.

These seismic sections show patterns that reflect thrusting sequence. In section A, axial surface S terminates upward into a thrust that is overlain by gently dipping strata. This pattern is comparable to that shown in model B1 (at left) and reflects breakback thrusting. In sections B and C, axial surfaces S’ are offset by shallow thrust faults. These patterns are comparable to model C2 (at left) and are consistent with breakbackward or coeval, but not break-forward, thrusting.

B: Peruvian Andes A: Permian Basin, Texas, U.S.A.

C: La Puna, Argentina

Patterns in models B1 and C1 are generally diagnostic of break-backward imbricate thrusting. However, patterns in models B2 and C2 are more ambiguous. A detachment that conforms to bedding across a fold, as in model B2, can be either a break-backward fault that followed bedding planes or a folded detachment. Similarly, the pattern shown in model C2 reflects break-backward thrusting only if the offset axial surface is considered active (i.e., it is pinned to a bend or tip of the underlying thrust). In contrast, if the offset axial surface is inactive (S’), then the pattern may reflect either break-backward thrusting or coeval motions on the deep and shallow faults. Thus, some patterns are diagnostic of thrusting sequence while others are not. Care should always be taken in interpreting thrusting sequence based on fault and fold shapes. 50 Shaw, Connors, and Suppe


Determining thrusting sequence using growth strata Growth strata can be used to determine the thrusting sequence in cases where two or more growth structures can be related to separate faults. Associating growth structures with specific faults can be difficult in cases where thrust sheets are everywhere vertically superimposed, but it is straightforward where faults are separated, at least in part, horizontally. This section presents seismic profiles with examples of break-forward and break-back thrust systems interpreted using growth strata.

A: Break-forward thrusting

These seismic sections both image two faults (X and Y) that are separated horizontally at shallow levels, but vertically overlap one another at depth. In section A, the fold associated with fault Y does not deform, and thus pre-dates, the annotated horizon. The fold related to fault X clearly deforms, and thus post-dates this horizon, reflecting a breakforward thrusting sequence. In section B, the fold associated with fault X does not deform, and thus pre-dates, the annotated horizon. The fold related to fault Y clearly deforms, and thus post-dates, this horizon, reflecting a break-backward thrusting sequence. Both seismic images are from the deepwater Niger Delta, Nigeria.

B: Break-back thrusting



1B-6: Structural wedges

Examples

Field Example

Basic concept Structural wedges contain two connected fault segments that bound a triangular, or wedgeshaped fault block. The two fault segments, which typically include two ramps or one ramp and one detachment, merge at the tip of the wedge. Slip on both faults accommodates propagation of the wedge tip and causes folding (Medwedeff, 1989). Wedges occur at a variety of scales. At large scales associated with mountain fronts, wedges are typically referred to as triangle zones (Gordy et al., 1975). In this section, we describe common types of wedges and illustrate how these structures are interpreted in seismic sections.

Conjugate faulting theory

Kinematic Model

Structural wedge in Carboniferous Rundle Formation, Front Ranges of the Canadian Rockies. Note the highly deformed rocks near the wedge tip. Several smaller wedges are contained within the larger wedge structure. (J. H. Shaw and F. Bilotti)

Seismic Example: Alberta Foothills, Canada

(above left) Brittle failure of rocks in compression commonly leads to the development of two conjugate thrust faults that dip in opposite directions (Anderson, 1942). Planes of weakness, such as bedding, can also lead to the development of detachments. In cross section (above right), two conjugate thrusts bound a wedge-shaped fault block and merge at the wedge tip (model 0). Slip on both bounding faults causing propagation of the wedge (model 1). In this case, the wedge propagates along a detachment, and causes folding of the hanging wall block. The lower thrust is commonly referred to as the forethrust or sole thrust, and the upper thrust is called the back thrust or roof thrust (Boyer and Elliot, 1982).

Common characteristics Wedges exhibit a wide range of geometries. However, several characteristics are common to most wedge structure, including: 1) presence of coeval fore- and back-thrusts; 2) folding localized along an active axial surface pinned to the wedge tip; and 3) folds may exist in the footwall of the back thrust that produce structural relief.

This seismic section images a large structural wedge, or triangle zone, at the eastern front of the Canadian Rocky Mountain fold and thrust belt. The common characteristics of structural wedges, (1–3) as described at left, are present in this structure. Note that a second, smaller back thrust is present within the main wedge block. When the back or roof thrust and its hanging wall are gently tilted or warped, but not deformed to the extent exhibited within the wedge block, the term passive roof thrust is sometimes used. Passive roof thrusts are common in triangle zones, as shown in this example.



Wedge models developed using fault-bend fold theory Structural wedges exhibit a variety of shapes and styles that reflect initial fault geometries, propagation direction, and folding mechanisms. In this section, we present a series of kinematic models that describe basic types of structural wedges governed by fault-bend fold theory (Suppe, 1983; Medwedeff, 1989; see section 1B-1). Models A through C involve detachments, whereas model D does not.

A (0–2): Simple wedge with a detachment and back thrust. Propagation of the wedge tip forms a kink band above the back thrust that is bounded by an active axial surface, which is pinned to the wedge tip. Strata in the kink band are parallel to the back thrust (β = 0) because the fault rises from a detachment (θ = 0).

Note: green dashed lines are active axial surfaces, red dashed lines are inactive axial surfaces. See section 1B-1 for description. Part 1: Structural Interpretation Methods

B (0–2): Wedge with a lower forethrust ramp and an upper detachment that acts as the back thrust. With slip, the wedge tip propagates along the detachment surface. Strata within the wedge are folded in an anticlinal fault-bend fold that deforms the detachment or back thrust. A kink band develops above the back thrust with strata that are parallel to the underlying fault and fault-bend fold. The synclinal axial surface pinned to the wedge tip is active, as is the anticlinal axial surface within the wedge block. The anticlinal axial surface above the back thrust, however, is inactive.

C (0–2): Wedge formed by a dipping forethrust and back thrust. With slip, the wedge tip propagates along a detachment surface. Strata within the wedge are folded in an anticlinal fault-bend fold that deforms the back thrust. A kink band develops above the back thrust with strata that are parallel to the underlying fault, but that dip more steeply than the beds within the wedge block. Both the synclinal axial surface pinned to the wedge tip and the anticlinal axial surface pinned to the fault bends are active. The anticlinal axial surface in the hanging wall of the back thrust is active (in contrast to model B) because a small amount of strata is folded from the crest into limb, thus passing through the axial surface. These kinematics facilitate the conservation of bed length. Alternatively, a small amount of shear or bed-parallel extension could accommodate fault slip without moving strata from the fold crest into the limb.


D (0–2): Wedge formed by a dipping forethrust and back thrust. With slip, the wedge tip propagates along the trajectory of the forethrust. Strata within the wedge are not folded, as they do not pass over a fault bend. A kink band develops above the back thrust with strata that dip more steeply than the fault. The geometry of the kink band (θ) is governed by faultbend fold theory (see section 1B-1), with ␾ equal to the acute angle between the back thrust and the propagation direction, and β as the hanging wall cutoff angle relative to the propagation direction. Note that in this wedge the roof thrust locally cuts down the stratigraphic section as it extends upward. This is an unusual relationship for thrust faults, but nevertheless may occur in non-decollement wedges.

53

Seismic examples of structural wedges Here we present examples of structural wedges imaged in seismic reflection data.

B: Santa Barbara basin, CA, U.S.A. Section A images a simple structural wedge that involves a back thrust extending upward from a forethrust ramp. The wedge tip propagation direction is along the path of the forethrust. Note that in this case, the back thrust has very little displacement relative to the forethrust. Section B images a wedge comprised of a gently dipping back thrust that extends from a forethrust ramp. The wedge propagation direction is along the path of the forethrust, which corresponds with an angular unconformity. Folding at the wedge tip is consistent with the pattern displayed in model D on the previous page. Section C images a complex structural wedge comprised of a back thrust extending upward from a folded detachment similar to model A on the previous page. The detachment level is constrained by the discordance of strata and the base of the thrust ramp located east of the wedge tip. The wedge structure, including the detachment, overlies an anticline that is related to a deeper level of faulting.

A: Niger Delta, Nigeria

C: Dashen structure, Sichuan basin, China



Growth structure in wedges This section describes growth structures above wedges that are modeled using fault-bend fold theory, after Medwedeff (1989). Growth structures can be very helpful in distinguishing structural wedges from other types of fault-related folds.

Seismic Example: Sumatra, Indonesia

Kinematic models Wedges

Forelimb fault-bend folds

Interpreted section In wedges that are governed by fault-bend fold theory (see section 1B-1), folds grow by kink-band migration. Folding generally occurs along an active axial surface that is pinned to the propagating wedge tip. In cases where sedimentation rate exceeds uplift rate, syntectonic strata form growth triangles above the wedge tip that are bounded by a planar synclinal (active) axial surface and a curved anticlinal (inactive) axial surface (model W1). In contrast, simple forelimb fault-bend folds have growth triangles bound by a curved synclinal (inactive) axial surface and a planar anticlinal (active) axial surface (model F1). In cases where uplift rate exceeds sedimentation rate, the contrast between wedges and simple fault-bend folds is even more distinct. In a structural wedge, growth strata are folded about an active synclinal axial surface and are parallel to the underlying forelimb dip (model W2). In contrast, syntectonic strata are not folded above the forelimb of a simple fault-bend fold (model F2), because they have not passed through an active axial surface. Growth strata, therefore, are horizontal, or maintain a primary sedimentary dip, and onlap the forelimb. (right) This seismic section images a structure with characteristics of a growth wedge. The structure consists of a forelimb developed above a south-dipping forethrust. Growth strata thin onto the crest of the structure, and are folded above the forelimb. The synclinal axial surface is roughly planar and folds the growth strata. In contrast, the anticlinal axial surface is curved, with an abrupt change in orientation at the contact between pre-growth and growth strata. Based on this growth pattern, which is similar to model W1 above, the structure is interpreted as a wedge. (For more details on this interpretation, see Shaw and Brennan, section 2-23, this volume). 55 Part 1: Structural Interpretation Methods


Shear fault-bend fold wedges

Seismic example: Niger Delta, Nigeria

Structural wedges can form with non-flexural-slip components of deformation, resulting in fold geometries that differ from those presented on the previous pages. Here, we describe a class of these wedges that form by shear fault-bend folding (Suppe et al., 2004; see section 1B-4), and show an example in a seismic section.

Kinematic models

Simple-shear wedges (model A) have shear in the footwall of the back thrust. This shear folds, and induces slip, on the fault, producing a forelimb that is similar to the back-limb fold produced by the forward-thrust, simpleshear fault-bend fold equivalent (see section 1B-4). In this model, growth strata are eroded above the fold crest. Pure-shear wedges (model B) have shear in the hanging wall of the back thrust that occurs as the wedge tip propagates. The back thrust is not folded, and slip produces a forelimb that is similar to the back-limb fold produced by the forward-thrust, pure-shear fault-bend fold equivalent (see section 1B-4). In both shear wedges, the forelimb beds dip less than the underlying back thrust, and the growth structures record folding by a combination of limb rotation and kink-band migration (see section 1A-5). In contrast, classical fault-bend fold wedges (model C) generally have hanging wall beds that are parallel to the back thrust, and growth structures that record folding dominantly by kink-band migration.

The seismic section shown above images two thrust ramps rising from a detachment. The ramp on the left dips in the same direction as the majority of faults in the region, and thus is considered a forethrust. The ramp on the right is a back thrust. Slip on the back thrust produces a hanging wall structure that has the characteristics of a shear fault-bend fold. However, given that this is a back-thrust above a detachment, the structure is a shear wedge. Based on the fault cut-off angle (θ) and back-limb dip (δb), the structure is interpreted as a pure-shear wedge in the section shown at right. Based on shear fault-bend fold theory (Suppe et al., 2004, see section 1B-4), the fault cutoff angle and backlimb dip yield a 27° dip of the synclinal axial surface (ψ) in the basal layer and a shear angle (α) of 67°. Growth strata exhibit a fanning of limb dips that is consistent with the shear wedge interpretation.

Interpreted section



1B-7: Interference structures Basic concept

Seismic Example: Gulf of Mexico, U.S.A.

Interference structures form when two or more monoclinal kink bands intersect, often yielding distinctive patterns in cross section with anticlines perched above synclines. Interference structures have been documented in the field and laboratory (e.g., Dewey, 1965; Paterson and Weiss, 1966; Stewart and Alvarez, 1991), and have been proposed as the origin of structures imaged in seismic profiles (e.g., Mount, 1989; Novoa et al., 1998; Camerlo et al., section 2-24, this volume). In this section, we describe a simple style of interference structure comprised of two kink bands with opposing dips, and present examples of these structures imaged in seismic sections.

Kinematic Models

These models (A and B) illustrate interference structures formed by the intersection of two kink bands (1 and 2) that dip in opposite directions. Model A forms by clockwise shear of the through-going kink band (2), whereas model B forms by counter-clockwise shear of the through-going kink band (1). In both models the through-going kink band separates the other kink band into two pieces that are joined along two shear surfaces that are parallel to bedding. As a result, the shear surfaces connect points where the axial surfaces bifurcate. The axial surfaces in these models bisect the interlimb angles (see section 1A-1), and thus bed length and thickness are preserved. The most distinctive aspect of these structures is that they yield anticlines perched above synclines.

Kink-band interference can result from many different structural configurations, involving various types of fault-related folds (Mount, 1989; Medwedeff and Suppe, 1997; Novoa et al., 1998). These three models (C–E) illustrate general structural configurations that can yield kink-band interference. The interfering kink bands are developed: C) above two bends in the same fault; D) by imbrication of two faults; and E) as forelimbs developed above faults that dip in opposite directions. Note that the shallow fold geometries are identical in each of these models. Thus, the geometries of interference folds are not always diagnostic of the underlying fault configurations. The different structural configurations do, however, involve different patterns of active (green) and inactive (red) axial surfaces, which may, in some cases, be distinguished using growth structures (Novoa et al., 1998; see section 1A-3).

This seismic section images an interference structure from the Perdido fold and thrust belt (after Mount, 1989; Novoa et al., 1998). The structure is comprised of two monoclinal kink bands that intersect at about 5.2 seconds (TWTT). The interfering kink bands produce an anticline that is perched above a syncline, similar to the models shown at left. The sense of shear in the interference structure appears to be counterclockwise, similar to model B. This section is displayed in TWTT, with a V.E. of about 1:1 for a velocity of 2000 m/s, which is representative of the shallow section.



Complex interference structure Interference structures that are faulted and/or involve more than two kink bands may have very complex geometries. In this section, we describe a complex, faulted interference structure imaged in a seismic section. We use a partial restoration of the structure to document its origins as an interference fold.

The seismic profile shown in panel A images a complex fold from the Sichuan basin, China. The structure exhibits the basic pattern of an anticline perched over a syncline that is characteristic of interference structures. The structures differ from the simple models shown on the previous page, however, in that the core of the fold is cut by a thrust. A narrow monocline appears to be offset by this fault.

A: Uninterpreted section

C: Geologic section

B: Interpreted section

D: Partially restored section

In panel B, the section is interpreted with a simple interference fold below the main thrust. Folds in the hanging wall of the thrust are interpreted to be displaced elements of the interference fold that, in part, are refolded by a steepening upward splay of the fault. Panel C shows the same interpretation of the structure without the seismic image. Restoration of slip on the main fault and the associated folding in panel D yields a simple interference structure. This example is intended to illustrate that interference structure may have complex geometries. Nevertheless, these structures can generally be interpreted using a combination of fault-related folding theories. This interpretation invokes the basic patterns of interference folding with the kink method (section 1A-1) and fault-bend folding (section 1B-1) to describe the hanging wall structure. The hanging wall portion of the offset monocline is refolded using the concept of folding vectors described in section 1B-5. Interference structures also generally exhibit very distinct patterns in map view and three dimensions. For a description of these patterns, see Novoa et al. (1998) and Camerlo et al. (section 224, this volume).



2

Case Studies 59 Part 2: Case Studies


2-1: Pitas Point Anticline, California, U.S.A. John H. Shaw, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. Stephen C. Hook, Texaco Inc., Houston, Texas, U.S.A. John Suppe, Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey, U.S.A. Location: Eastern Santa Barbara Channel, California, U.S.A. Topics: Fault-bend folding, growth structure, map patterns Reserves: Gas in Pliocene clastic reservoirs

Pitas Point anticline

The Pitas Point anticline is located in an active fold and thrust belt in the eastern Santa Barbara Channel, California (Figure 1) (Namson and Davis, 1988; Shaw and Suppe, 1994). The fold has a flat crest separating gently north- and south-dipping limbs that are bounded by the Rincon and offshore Oak Ridge trends, respectively (Figure 2). Both fold limbs terminate downwards at about 5 km depth, suggesting the presence of a detachment in the Miocene Monterey Formation. Upper Pliocene and Quaternary strata thin onto the crest of the anticline, suggesting that these are growth or syntectonic units. Moving upward in section, the crest of the fold narrows and migrates to the north. Thus, shallow strata penetrated by the Texaco 234 #7 well dip gently to the south, whereas, deeper units are horizontal or dip gently to the north. In the following discussion, we present an interpretation of this structure as a growth faultbend fold that is compatible with these basic observations.

Figure 1: Map of fold trends in the eastern Santa Barbara Channel, California, showing locations of the Pitas Point trend and seismic profile shown in Figure 2. RT = Rincon trend; ORT = offshore Oak Ridge trend; ORF = Oak Ridge fault; MCT = Mid-Channel (Blue Bottle) trend.

Figure 2: Post-stack, time-migrated, and depth converted 3-D seismic reflection profile across the Pitas Point trend, with formation tops and dipmeter from the Texaco 234 #7 well. Downward terminating kink bands (2) indicate a detachment at about 5 km depth (see section 1A-2, Recognizing thrust and reverse faults). Shallow gas sag is documented by Mastoris (1990).


Part 2: Case Studies

2-1: Seismic interpretation

Pitas Point anticline

We propose a simple growth fault-bend fold model (Figure 3) and interpretation (Figure 4) to describe the geometry and kinematic evolution of the Pitas Point anticline.

growth

1

pre-growth crestal uplift stage 2

crestal uplift stage dipping over horizontal strata 3

crestal broadening stage shallow detachment 4

slip on shallow detachment Figure 3: Sequential models of a growth fault-bend fold (Suppe et al., 1992). Model 1 contains two fold limbs developed above a ramp between decollements. The fold is in the crestal uplift stage of growth (Shaw et al., 1994), as fault slip is less than ramp width. In Model 2, additional slip widens kink bands, which narrow upward in the growth section (Suppe et al., 1992). In Model 3, fault slip is greater than ramp width. Thus, strata are refolded from the back limb (A-A) onto the crest of the structure, which now widens with fault slip (crestal broadening stage, Shaw et al., 1994). Growth strata are also folded above the crest, generating a pattern of dipping over horizontal beds and offsetting the shallow crest from the deep crest of the fold. These patterns, as well as the fault cutoffs, are observed in the 3-D seismic data from the center of the Pitas Point anticline (Figure 2). Model 4 includes minor displacement on a shallow detachment producing subtle folding that is similar to patterns observed above the Pitas Point anticline in Figure 4.

Figure 4: Post-stack, time-migrated and depth converted 3-D seismic reflection profile across the Pitas Point trend, with formation tops and dipmeter from the Texaco 234 #7 well. Downward terminating kink bands, that are highlighted in Figure 2, indicate a detachment at about 5 km depth (see section 1A-2, Recognizing thrust and reverse faults). Labeled axial surfaces correspond to those modeled in Figure 3.

Fault-bend folds in the crestal broadening stage exhibit dipping over horizontal strata in the growth section on the fold crest (Figure 3). This pattern is observed in the seismic image of the Pitas Point anticline and in the dipmeter of the Texaco 234 #7 well. In Figure 4, we interpret the anticline as a simple fault-bend fold developed above a north-dipping (13°N) thrust ramp that connects detachments in the Miocene Monterey Formation. Based on fault-bend fold theory, where θ = φ = 13°, the forelimb should dip 14°S (β = 14°) and be slightly narrower than the backlimb (R=.95). These values were used to guide the interpretation, which generally conforms to reflection geometries. The fold is slightly modified by slip on the shallow Montalvo thrust, which is described by Shaw et al. (1996). 61



2-1: Map-view analysis To describe the three-dimensional geometry of the Pitas Point anticline, we present a structure contour and axial surface map at the top of the Pliocene Repetto Formation. The axial surface map is generated using the vertical projection method of Shaw et al. (1994), as described in Figure 5. We interpret that the fold along section X-X (Figure 4) is in the crestal broadening stage of growth. Folds in the crestal broadening stage have a distinct axial surface map pattern (Figure 6). This pattern is observed in the axial surface map of the Pitas Point trend (Figure 7), and in a time-slice from the 3-D seismic survey (Figure 8).

Figure 7: Axial surface map at the top of the Pliocene Repetto Formation, superimposed on a structure contour map of the same horizon that was generated independently from well control. The plunge of the fold is reflected by pairs of axial surfaces (A-Aand B-B) that converge toward the fold terminations. In the center of the trend, the forelimb axial surfaces (B-B) are deflected southward. This pattern is consistent with the crestal uplift stage of growth (Figure 6) in the center of the trend and along section X-X. For a more detailed discussion of the map pattern, see Shaw et al. (1994).

Figure 5: Perspective view of a plunging fault-bend fold. (top): Between sections 3 and 2, fault slip is greater than the ramp width and the fold is in the crestal broadening stage. As slip decreases to the right of section 2, the fold enters the crestal uplift stage. Fold plunge is denoted by converging pairs of axial surfaces. (bottom): Axial surfaces are mapped by projecting their intersections with the mapped horizon vertically to a horizontal datum.

Figure 8: Enlarged (2X) portion of the axial surface map superimposed on a time slice (2.3s TWTT) from the 3-D seismic survey. Note that in the zone of crestal broadening the trend of axial surfaces B and B are parallel to the seismic reflections in the forelimb. The wide fold crest is imaged as a broad negative (white) amplitude surrounding platform Habitat.

Conclusions: Figure 6: The axial surface map pattern of a doubly plunging fault-bend fold is characterized by pairs of axial surfaces that converge at the fold terminations. The zone where the fold is in the crestal broadening stage is defined by the deflection of the forelimb kink band (B-B) away from the backlimb kink band (A-A), yielding a wider fold crest.

• The Pitas Point anticline is a south-vergent, fault-bend fold developed above a thrust ramp and detachment within the Miocene Monterey Formation. Maximum slip on the fault is about 3.5 km. • Upper Pliocene and Quaternary strata are syntectonic units folded by displacement on the thrust. • The fold is in the crestal broadening stage of growth in the center of the trend beneath platform Habitat.



2-2: Toldado Anticline, Upper Magdalena, Colombia Alexis Rosero, HOCOL S.A., Bogota, Colombia Juan Carlos Ramon, HOCOL S.A., Bogota, Colombia Location: Upper Magdalena basin, Tolima, Colombia Topics: Fault-bend folding, growth strata Reserves: Oil in the Albian Caballos sandstones

Avechucos Syncline / Toldado oilfield

The Toldado anticline is part of the buried NNE trending Ortega fold and thrust belt (Figure 1). This belt is located below and to the west of the Avechucos syncline. The Toldado anticline is a NNE-SSW-trending anticline with a wide, low-angle crest separating gently east- and west-dipping limbs (Figure 2). The structure is interpreted as a faultbend fold. Two models are geometrically possible based on the fold shape (in Cretaceous strata) and partially constrained fault geometries (Figures 3 and 4). The geometry of growth strata is used to constrain the degree of fold evolution and to distinguish the structural interpretation. The near-horizontal growth strata (Paleocene) across the fold crest indicates that the fold is on the Crestal Uplift Stage (see Shaw et al., section 2-1, this volume). Paleocene growth strata gets thinner along the crest of the fold. This is partly due to variable uplift over the fold and partly due to erosion on the Eocene unconformity. These data indicate that sedimentation rate during the Paleocene was close to, or slightly higher than, the uplifting rate.

Figure 1: Location map of the Toldado anticline, showing the main structural features of the study area. Note that the Toldado anticline is located close to the trace of the Avechucos syncline. Location of the seismic line on Figure 2 is shown.

Figure 2: Post-stack, time-migrated, 2-D seismic reflection profile across the Toldado anticline. The line is in TWT but is displayed in 1:1 scale using the velocity function of the Toldado-3 well. Toldado-3 well and formation tops are shown. Note thinning of Paleocene growth strata (1) across the fold crest. Minor erosion occurs along the crest of the fold associated with the Eocene unconformity. The forelimb downward termination (2) defines an intra-Villeta detachment.

63 Part 2: Case Studies


2-2: Toldado Anticline In this interpretation (#1) we consider that the upper detachment is parallel to strata in its footwall and the top of the ramp is located at the base of axial surface A. We observe a forelimb cutoff angle (β) of 32°. However, before this value is used to calculate the ramp dip, the forelimb must be “unfolded” across the 8° bend in the upper detachment (φ). Using fault-bend fold theory, we obtain a 35° dip of the forelimb before it was folded across the bend in the detachment. This value serves as the forelimb cut-off angle (β = 35°) that, along with the “unfolded” inter limb angle ( γ = 74°) is used to calculate the change in fault dip (φ = 23°) an initial cut-off angle of the ramp (θ = 26°). This yields a 30° dipping ramp. 0

The resulting interpretation implies that the fault-bend fold is in the crestal broadening stage. The models presented by Shaw et al. (section 2-1, this volume) show that at this stage there should be dipping growing-strata on top of horizontal crestal beds (see Figure 3, insert). The seismic does not support this geometry and thus this model is discarded.

0

dipping over horizontal strata


Figure 3: Interpreted seismic section assuming a fault-bend fold in the crestal broadening stage (Interpretation #1).



2-2: Toldado Anticline In this interpretation (#2) we use the observed forelimb hanging wall cutoff angle, (β = 32°), the dip of the upper detachment (13° SE) and the forelimb interlimb angle (γ = 79.5°) to predict the change in fault dip (φ), the initial cutoff angle (θ), and the slip ratio (R). Using the anticlinal fault bend fold graphs (see section 1B-1) we derive a φ value of about 16°. With this angle, we obtain an initial cutoff angle of about 27°. The slip ratio R is calculated as 0.85. This agrees with the slip ratio (S1/S0) measured on the seismic section.

Note that in this interpretation the shortening and slip is smaller than in interpretation #1, and that the axial surface (A) is not fixed to the top of the ramp (where the ramp meets the upper detachment). This implies that this fold is on the crestal uplift stage (Suppe, 1983; Shaw et al., 1999). In this case, the horizontal growth strata seen on the seismic above the fold crest agrees with the crestal uplift model (see insert), making this second interpretation more plausible than the previous one.

In conclusion, based on fault and pre-growth fold geometries, two structural models are possible for the Toldado anticline. Growth strata are used to distinguish between these alternative models, and support our interpretation of the Toldado anticline as a fault-bend fold in the crestal uplift stage of growth.


Figure 4: Interpreted seismic section assuming a fault-bend fold in the crestal uplift stage (Interpretation # 2).



2-3: Sequatchie Anticline, Tennessee, U.S.A.: A small displacement fault-bend fold Shankar Mitra, University of Oklahoma, Norman, Oklahoma U.S.A. Location: Appalachian Plateau, Tennessee, U.S.A. Topics: Fault-bend fold, multiple-bend ramp

Figure 1: Generalized geological map of the Sequatchie anticline in Cumberland and Rhea Counties, Tennessee (modified from Hardeman, 1966), showing the location of the seismic profile shown in Figures 2 and 3. Omu-S = Middle to Upper Ordovician and Silurian. D-M = Devonian to Mississippian. Pg-Pco = Pennsylvanian Gizzard Group and Crab Orchard Mountains Group. Pl = Lower Pennsylvanian units above the Crab Orchard Mountains Group.

The Sequatchie anticline (Figures 1 and 2) is the frontal structure of the Southern Appalachian thrust belt in southern Tennessee, Georgia, and Alabama. An interpretation of the northern part of the structure in Cumberland and Rhea counties, Tennessee, is presented based on surface data, a seismic profile, and data from the ARCO-Ladd #1 well. The structure has a low relief and exposes Mississippian to Pennsylvanian units on the crest of the structure (Figure 1). Farther south, the relief increases, and Middle Ordovician to Devonian units are exposed at the surface (Hardeman, 1966; Harris and Milici, 1977).

Figure 2: Part of time-migrated seismic profile through the Sequatchie anticline, Tennessee.



2-3: Sequatchie Anticline Seismic interpretation is based on both a time-section (Figure 2), and a post-stack depth migration (Figure 3), based on a velocity model constructed from a preliminary depth model. The structure is related to a thrust fault that originates at the base of the Cambrian Rome Formation, and climbs to the base of the Pennsylvanian Gizzard Group. The seismic data, and the ARCO Ladd #1 well indicate that the fault has a low dip (approximately 5°) in the Cambrian Rome and Conasauga Formations, but has a much steeper dip (15°) in the Cambro-Ordovician Knox Group and the remaining Ordovician to Devonian units. The steeper fault dip in the Knox Formation is probably related to the higher competence of this unit. Surface data suggest that the front limb of the structure has a very steep dip, ranging from 30 to 85°.

The Sequatchie anticline is interpreted to be a low-displacement fault-bend fold, related to a multi-bend ramp (Figure 4a). There are four bends in the fault, each of which defines an active axial plane. Movement of the hanging wall over the fault bends results in the development of a series of passive axial surfaces, which originate at the active axial surfaces and migrate away from them. The active and and passive axial surfaces separate panels of relatively constant dip, which can be identified from surface and seismic data, and from the dipmeter data in the ARCO-Ladd #1 well.

The seismic data show a low westward dip of the basement between shot points 390 and 485, and a steeper eastward dip between shot points 390 and 325. This geometry is partly due to a seismic pull-up under high velocity carbonates in the hanging wall of the Sequatchie thrust, which was apparently uncorrected in the depth model used for the post-stack depth migration. However, there appears to be a very low-dipping eastward ramp under the Sequatchie thrust, which drops all units down to the east. The presence of the ramp is also indicated by dipmeter data in the footwall. This ramp may have formed along a zone of weakness in the basement during loading associated with the emplacement of Valley and Ridge thrusts. The formation of the Sequatchie thrust fault may have been influenced by the location of this ramp. DATUM = 1100 feet

DATUM = 1100 feet

Figure 3: Uninterpreted (a) and interpreted (b) time-migrated sections through the Sequatchie anticline. Active axial surfaces are shown in green, and passive axial surfaces in orange. The seismic section and the interpretation do not correlate 1:1 with the structural cross section because of crooked line effects.



2-3: Sequatchie Anticline The cross section presented below (Figure 4a) is based on the seismic interpretation and was restored using line-length balancing (Figure 4b). The restoration shows that the shortening for the base of the Rome Formation is approximately 5100 ft. The fault displacement decreases from 5100 ft at the base of the

Rome Formation to 4500 ft at the top of the Knox Group and 2200 ft at the top of the Mississippian units. The forward shear of the loose line in the restored section suggests a small amount of differential penetrative strain at the mesoscopic and microscopic scales within the Silurian to Mississippian units.

This inclined shear profile and proposed penetrative deformation is consistent with the steep front limb of the fold, and the small fault displacement in the Mississippian units.

ARCO-LADD #1 JEWETT HEIRS

Figure 4: Structural cross section through the Sequatchie anticline, Tennessee, based on seismic data (Figure 2), surface data, and data from the ARCO-Ladd #1 Jewett Heirs well. Active axial surfaces are shown in green and passive axial surfaces in orange. b. Line-length restoration of the structural cross section in a.

Conclusions • The Sequatchie anticline is a fault-bend fold related to a multibend fault ramp connecting major detachments in the Cambrian Rome Formation and the Pennsylvanian Gizzard Group.

• Macroscopic shortening associated with the formation of the structure is approximately 5100 ft. • The front limb of the structure is fairly steep, suggesting penetrative deformation within the Silurian to Mississippian units, which have been transported onto the upper detachment.



2-4: El Furrial Field cross section, Eastern Basin, Venezuela Enrique Novoa, Departamento de Analisis Exploratorio Integrado, Gerencia de Exploracion y Produccion, PDVSA-INTEVEP, Venezuela László Benkovics, Departamento de Analisis Exploratorio Integrado, Gerencia de Exploracion y Produccion, PDVSA-INTEVEP, Venezuela Claudia Fintina, Departamento de Analisis Exploratorio Integrado, Gerencia de Exploracion y Produccion, PDVSA-INTEVEP, Venezuela Javier De Mena, Departamento de Delineacion y Caracterizacion de Yacimientos, Gerencia de Exploracion y Produccion, PDVSA-INTEVEP, Venezuela Location: Eastern Venezuela Basin, Venezuela Topics: Fault-bend fold, growth strata Reserves: 2.0 billion barrels

El Furrial Oil Field

El Furrial Trend is located in the deformation front of the Serranía del Interior fold belt to the south of the Pirital Fault, Eastern Venezuela Basin (Figure 1). It is divided into three giant oil fields: El Furrial, Carito, and Santa Barbara fields. The boundaries between these fields are tear faults and/or lateral ramps. This structural trend contains actual recoverable reserves of about 2.0 billion barrels of medium gravity oil. A balanced cross section through El Furrial field is presented. The structure is asymmetrical, with the backlimb much wider than the forelimb. The backlimb includes two inclined dip domains while the front limb is composed of one domain (Figures 2 and 3). We interpret the fold as developing above a two-bend thrust fault that accommodates about 14 km of shortening (Figure 4). Growth strata suggest that the fold started growing during the early-to-middle Miocene. We present a kinematic model that shows how this structure may have developed.

Figure 2: A depth-migrated 3-D seismic reflection profile that images El Furrial structure. Notice that the image deteriorates between X and X. The blue ticks show the top of Oligocene picks in the wells. Profile provided by PDVSA E&P.

Figure 1: El Furrial trend (B) is located in the Eastern Basin of Venezuela (A).



2-4: El Furrial Field cross section

El Furrial Oil Field

A balanced cross section across El Furrial field is shown in Figure 3. The structure is characterized by a very narrow forelimb and wide backlimb. The backlimb is made of two inclined dip domains (A-C and C-C). The kink band A-C dips 27° NW and C-C dips 12° NW. The forelimb is composed of a dip domain (B-B) that dips 20° SE. The kink bands A-C and C-C are parallel to the fault plane. El Furrial fold started to grow in the early-to-middle Miocene as shown by a growth triangle (Suppe et al., 1992) in the sequence of this age interpreted on top of the kink band B-B. This observation is supported by geochemical data which show biodegradation of oil (Talukdar et al., 1987). Presumably, this early oil was accumulated when the reservoir was at shallow depth during an early stage of fold development. Normal faults have been interpreted in both the hanging wall and footwall. These normal faults may be related to the development of the foreland basin in front of the fold belt. A kinematic model (Figure 4) shows the evolution of the Furrial Fold through time.

Figure 3: A balanced, retrodeformable cross section (X-X)across El Furrial Trend that integrates seismic reflection (Figure 2) and well data. El Furrial trend develops above a two-bend thrust fault, which causes a very wide backlimb and a narrow forelimb. The steeper portion of the thrust fault is short and the gentle part is very long. The backlimb is interpreted to be composed of two inclined dip domains (A-C and C-C) which are parallel to the El Furrial fault. On the other hand, the forelimb is composed of a single inclined dip domain (B-B). The seismic data illuminate the kink bands B-B and C-C very well, however the dip panels A-C and C-A are not well defined by the data. Notice the growth axial surface (G) on top of the kink band B-B which shows a growth triangle in the early-middle Miocene sequence. This structural trend accommodates around 14 km of total slip.

Figure 4: A balanced, kinematic model of development of the El Furrial trend. a: Incipient fault and active axial surfaces (A and B) in undeformed strata. b: Slip on the two-bend thrust fault generates inactive axial surfaces A and Bthat are rigidly translated away from active axial surfaces A and B. Once axial surface A arrives at the convex bend of the fault, an incipient active surface (C) is generated. Moreover, axial surfaces A, B, and B become inactive and will be rigidly translated along the upper portion of the thrust fault. c: Additional slip on the fault causes the development of an inactive axial surface Cand the kink band C-Cstarts to grow until the present geometry is reached. Similar kinematic models for multibend faults are shown by Medwedeff and Suppe (1997).

Conclusions: • El Furrial field is located within a very asymmetric fault-bend fold where the backlimb is much wider than the forelimb. • Growth strata and geochemical data suggest that it started to grow during early-middle Miocene times. • The fault plane has two bends and is divided into two sections: a narrow, steep ramp and a long, more gentle ramp. • The fault accommodates about 14 km of shortening.



2-5: Rosario Field, Maracaibo Basin, Venezuela Ted Apotria, ExxonMobil Development Company, Houston, Texas, U.S.A. M. Scott Wilkerson, Department of Geology and Geography, DePauw University, Greencastle, Indiana, U.S.A. Location: Maracaibo Basin, Venezuela Topics: Fault-related fold termination geometry and kinematics Reserves: ~50 MMB (Molina, 1992) Overview: Fault-related fold terminations typically form due to loss of displacement on the genetically-related thrust fault, an along-strike change in fault attitude, or both. Constraints on footwall cutoffs and along-strike displacement are needed to determine the termination mechanism, which can often be determined from reflection seismic data. Fold geometry from a single profile does not uniquely establish kinematics. The Rosario structure is a contractional fault-related fold located in the western Maracaibo Basin, Venezuela (Figure 1). The plunging southern termination is constrained by industry reflection seismic and well data, and is interpreted to be due to an along-strike decrease in displacement. The fault geometry changes from a flat-ramp-flat at the crest of the structure where displacement is greatest, to simply a ramp near the lateral fault tip. These observations suggest a kinematic model in which the structure initiated as a modified fault-propagation fold with an isolated fault ramp within the “stiff” layer. With increased shortening, the fault grew to link with upper and lower detachments in the weaker shale units resulting in a hybridized fault-bend fold. The geometric elements of a single profile at the crest are consistent with the Suppe (1983) fault-bend fold model. However, interpretation of the structure in 3-D suggest different kinematics. Note: A full presentation of the seismic data and interpretation is in Apotria and Wilkerson (2002). A .mov-format animation of the 3-D structural model of Rosario Field can be downloaded at the AAPG Datashare web page (http://www.aapg.org/datashare/) and is on the CD-ROM accompanying this book. Generalized stratigraphy of the western Maracaibo Basin is summarized in Figure 2. Interpretation of the Rosario structure is constrained by 2-D time-migrated, 1985- and 1990-vintage seismic lines (Figure 3). Interpretations from these lines were converted to depth (e.g., Figure 4) using interval velocities calculated from seismic well ties from the CR-12 well (Figure 3C). Our discussion will focus on seismic lines CCT-90c-14 and CAT85-1 (Figure 3A–C), which cross the crest of Rosario, where the

Figure 1: Principal structural features of the western Maracaibo Basin, Venezuela. The Rosario oil field (white box) was discovered in 1954, with production from fractured Cretaceous carbonates and Eocene fluvial clastic reservoirs (Molina, 1992). The La Luna Quarry is highlighted in red.

primary geometric elements are best imaged. Two highimpedance and continuous reflections mark the top and bottom of the carbonate section (Figure 3A, B). The first reflection occurs between the Colon Shale and the top of the carbonates, and marks the mechanical transition between the “stiff” unit below and the “weak” clastic unit above. A second strong impedance contrast occurs at the base of the carbonate section and the top of the underlying Rio Negro clastic section. These

Figure 2: Generalized stratigraphy of the western Maracaibo Basin. Formation tops in depth (meters below a 33m KB) and interval velocities used for depth conversion posted from the CR-12 well. Qualitative mechanical stratigraphy and the location of inferred detachments is also depicted.

reflections bound a total carbonate section that is about 547 m thick in the CR-12 well (Figure 3C). The Tertiary section consists of alternating sands, silts, and shales and exhibits parallel folding. The Cretaceous Colon Shale normally has a thickness of about 550 m, except when structurally thickened where faults emerge from the underlying Cretaceous carbonate package (Figure 3).



2-5: Rosario Field

The Rosario Fault exhibits a flat-ramp-flat geometry at the crest (Figure 3A–C). La Luna and portions of Rio Negro strata are truncated and displaced onto a footwall flat near the base of the Colon Shale. Near the crest, the forelimb dips are slightly greater than those on the backlimb, defining a weak asymmetry toward the foreland (Figures 3, 4). The exact positions of the La Luna and Rio Negro footwall cutoffs are not well-imaged and are obscured by velocity pull-up beneath the hanging-wall carbonates (dashed line in footwall in Figure 3). However, we estimate that apparent displacement onto the upper flat is a maximum of 2.4 km in the plane of this section (Figure 3B), or approximately 2.0 km if projected into the transport plane (see transport direction in Figure 4E). Deformation interpreted at Rosario occurred during a middleMiocene and younger Andean Orogeny (Roure et al., 1997). Based on present-day topographic relief, the structure remained active into the Pleistocene and recent.

Figure 3: 2-D time-migrated seismic lines over the Rosario structure (see Figure 4 for location). Seismic lines are about 1:1 in the vicinity of the Cretaceous section. (A) uninterpreted line CCT-90c-14. Tie-line locations are labeled in gray. (B) interpreted line CCT-90c-14. Formation tops are labeled on the right; the approximate middle-Miocene surface is dashed. RF = Rosario Fault.; REF = Rosario East Fault. (C) line CAT-85-1. (continued on the next page).



2-5: Rosario Field Other regional seismic data in the vicinity of Rosario suggest that shortening of the Tertiary section is not transferred to the foreland. Instead, we interpret a wedge structure that transfers displacement back towards the hinterland (Figure 3A–D), similar to structures seen in nearby outcrops at La Luna quarry (Figure 5). South of line CCT-90c-14 (Figure 3A, B), significant differences in the geometry of the Rosario structure exist relative to the crest. Over a distance of 4 km, the fold loses a welldeveloped backlimb and a hanging-wall ramp on footwall flat (compare Figure 3D, E). The Rosario Fault also changes from a flat-rampflat geometry to simply a ramp (Figure 4E). A lower flat may accommodate the shortening observed near the ramp, but there is no direct evidence for it based on fold shape. Apparent offset of the La Luna Formation decreases from about 2 km at the crest (Figure 3A, B), to 1.5 km (Figure 3C), to 1 km (Figure 3D), to about 100 m near the termination (Figure 3E). This loss of displacement is primarily accommodated by transfer to the Rosario East Fault, which gains displacement to the south. This is evident from the top La Luna structure map as two discrete en echelon anticlines separated by the Rosario Fault (two dashed lines in Figure 4A, B). Where the Colon Shale dampens the displacement transfer between the two faults, the relay between the two folds becomes less evident in the shallower Tertiary section, and is only reflected as a subtle change in fold axis trend above the transfer zone (single dashed line in Figure 4C, D). In addition, dip magnitude near the crest is less on the Colon Shale reflector compared to the top La Luna (Figure 4). Figure 3 (continued): (D) CAT-85-2. (E) CAT-85-3. (F) CAT-85-4. (G) CAT-85-5. The top of La Luna (blue) is mapped in Figure 4A. The top of Colon Shale (green) is mapped in Figure 4C. See Figure 4 for location.



2-5: Rosario Field

Figure 4: (A) Top La Luna Formation sub-sea depth structure (blue reflector in Figure 3). Solid red lines represent the position of seismic lines published in this section of this volume. Other lines are published in Apotria and Wilkerson (2002). Dashed lines mark fold hinges for Rosario and Rosario East. Contour depths range from -4925 m (blue) to -4025 m (red) with a contour interval of 50 m. (B) Top La Luna dip magnitude map with superposed structure contours. Dip is a maximum of 22° (red) and a minimum of 0° (blue). (C) Top Colon Shale sub-sea depth structure map (green reflector in Figure 3). Dashed line represents a single fold axis for both Rosario and Rosario East. Contour interval is 50 m. (D) Top Colon Shale dip magnitude map with a maximum of 18° (red) and a minimum of 0° (blue) with superposed structure contours. (E) Sub-sea depth structure-contour map of the Rosario Fault. Contour interval is 100 m. Red arrow indicates the assumed regional transport direction perpendicular to the Perija Mountain Front (Figure 1). Dashed lines represent boundaries between the ramp and the two flats. The flats die out to the south, with only a ramp near the termination. The Rosario Fault also changes attitude toward the north, defining an oblique ramp. The oblique ramp is associated with fold closure to the north, but does not appear to directly influence the fold termination to the south.

The present-day geometry at the crest of the Rosario structure has the essential characteristics of a fault-bend fold (Figure 3A–C). However, the lateral variation in fold and fault geometry suggests that a flat-ramp-flat is not present near the termination, and may have been absent during the structure’s early development. Eisenstadt and DePaor (1987) proposed a 2-D model for fault growth in which a fault ramp initially nucleates in the “stiff” layer with associated tip strains

accommodated by folding. As shortening accrues, the ramp grows up and down section, eventually linking with upper and lower stratigraphically-controlled flats. In the kinematic model that follows, we extend Eisenstadt and DePaor’s (1987) 2-D model to 3-D, and assert that spatial variation in geometry is also a proxy for temporal variation.



2-5: Rosario Field Kinematic Model of the Rosario Structure

Figure 5: Outcrop analog from the La Luna quarry (see Figure 1 for location). The lower massive unit is the Maraca Member of the Cogollo Group carbonates, which is overlain by the thin-bedded, La Luna Formation. The deformation style that occurs at outcrop scale where the fault emerges from the Maraca is similar to that seen at seismic scale where faults emerge from the “stiff” carbonate section into the “weak” Colon Shale (Figure 3). This style of deformation at the tip of an emerging thrust fault could account for some of the apparent thickening in the Colon Shale seen on seismic sections near the ramp upper-flat transition.

Assuming that the spatial variation in the fault-fold geometry also represents the temporal variation of the fold’s development, we suggest the following kinematic model for the Rosario structure. Each interpreted fold growth stage is consistent with observed geometry from tip to crest. Stage 1 (pre-middle-Miocene, Figure 6A). Cretaceous and younger strata are essentially undeformed. Regional uplift and erosion occurred within the western Maracaibo Basin in the Eocene, but no local fold developed at Rosario during this time.

Figure 6: Model for the 3-D development of the Rosario structure. Each profile represents stages in both the temporal and spatial evolution of the structure from (A) earliest/least displacement to (D) latest/most displacement. See explanation in text below.

Stage 2 (middle-Miocene, Figure 6B). Shortening of the section initiates with small reverse offset near the top of the Cretaceous carbonates. Folding of the Colon Shale occurs with fault propagation through the “stiff” carbonate interval. Sub-seismic scale deformation of the Tertiary section is manifested by layer-parallel shortening. No evidence for a backlimb is observed. This line resembles Figure 3F and 3G observed near the present-day southern termination. Stage 3 (Figure 6C). The fault ramp links with an upper detachment in the Colon Shale with potential structural thickening near the upper flat (analogous to Figure 5). Increased displace-

ment places the hanging-wall ramp onto the upper flat, and the forelimb begins to steepen. This is supported by the decrease in forelimb dip toward the present-day termination (Figure 4B, D). Stage 4 (Figure 6D). As displacement accrues, the Rosario Fault continues to propagate downward and eventually connects to the basal flat within the Rio Negro or La Quinta Formations. This produces a discrete backlimb-lower flat transition that is observed at the present-day fold crest (e.g., Figure 3A–D). When both upper and lower flats are operative, additional fault displacement is accommodated by fault-bend folding. 75



2-5: Rosario Field The 3-D interpretation of the Rosario structure highlights the importance of distinguishing fault-bend fold geometry from fault-bend fold kinematics. At the crest, the Rosario structure exhibits characteristics of a fault-bend fold (e.g., a lower and upper detachment, intervening ramp, and a hanging-wall ramp on footwall flat geometry). However, the diagnostic geometric elements of a single profile do not uniquely establish the kinematic development. The Suppe (1983) fault-bend fold model assumes flexural slip as the deformation mechanism, and results in passive folding above a pre-existing fault. The prescribed kinematic model results in fold geometry that is a function of the underlying fault geometry, and a forelimb dip that remains constant with slip. Given the assumptions (Suppe, 1983), one can predict the orientation of one element (e.g., the ramp dip) given two other elements (e.g., forelimb dip and the axial angles). We measured the same geometric elements near the crest of the Rosario structure on seismic line CAT-85-1 (Figure 7) and compared them to theory (Suppe 1983). The forelimb dip (β) is 22°, consistent with the dip map on the top of the La Luna (Figure 4B). The axial angle (γ) is more difficult to determine due to smooth, parallel folding, but our estimate is 80°. Using these measurements, the Suppe (1983) model predicts a ramp step-up angle (θ) of 17°, which is our observation on CAT-85-1, if backlimb dip is used as a proxy for the ramp dip. The natural example matches the Suppe (1983) model prediction of the geometry of a single profile. However, our observations of the structure in 3-D suggest the kinematics of the Rosario structure are different than in the Suppe (1983) model. Our observations are consistent with the model in Figure 6, in which the Rosario structure evolved from a “faultpropagation fold” into a “fault-bend fold” (geometric rather than kinematic description). We also note that the forelimb dip decreases along strike (Figure 4B), further supporting a hybridized model. Although the Rosario crest has present-day geometry consistent with a simple fault-bend fold, the kinematics are more complicated than a single 2D profile would suggest. Preservation of growth strata, poorly defined in this study area, would be of further use to constrain the kinematic development of the fold.

Conclusions

Figure 7: (A) Seismic line CAT-85-1 (Figure 3C) with the addition of interpreted dip domain boundaries (red). (B) Angular measures of fold geometry where γ = axial angle, β = forelimb dip, θ = ramp dip. The geometric elements of this single profile are consistent with the Suppe (1983) fault-bend fold model. However, based on interpretation of the structure in 3-D, the inferred kinematic development is different. Instead, the structure develops from a fault-propagation fold (active fold above a buried fault tip) into a fault-bend fold (passive fold above an existing fault) as slip increases (Figure 6).

• The southern termination of the Rosario structure likely formed due to an along-strike decrease in displacement. Key elements of the interpretation include: a) the fault geometry changes from flat-ramp-flat at the crest to a fault ramp near the southern tip of the structure, b) forelimb dip decreases toward the southern termination, and c) the backlimb is indistinct toward the southern termination. • These observations suggest kinematics in which the structure initially developed as a simple fault ramp in the “stiff” layer (fault-propagation fold stage) and later propagated to connect with upper and lower detachments (fault-bend fold stage). Our model is a 3-D extension of a 2-D model proposed by Eisenstadt and DePaor (1987) in which fault ramps nucleate in “stiff” units. • Rosario provides a natural example of a structure where spatial differences may reflect temporal stages in the evolution of a fault-related fold. The model departs from previous models of rigid self-similarity and permits variations in fold style and deformation mechanisms influenced by mechanical stratigraphy.

Acknowledgments Permission to reproduce the seismic data was provided by Elsevier Press and The Journal of Structural Geology. We also thank ExxonMobil Exploration Company, ExxonMobil Upstream Research Company, and PDVSA (Venezuela) for permission to publish. 76 Shaw, Connors, and Suppe


2-6: Medina Anticline, Eastern Cordillera, Colombia Mark G. Rowan, Rowan Consulting, Inc., Boulder, Colorado, U.S.A. Roberto Linares, Ecopetrol, Instituto Colombiano del Petroleo, Piedecuesta, Santander, Colombia Location: Llanos foothills, Eastern Cordillera, Colombia Topics: Fault-bend fold, axial-trace map, fold-evolution matrices Reserves: Giant fields along trend (e.g., Cusiana, Cupiagua)

The Medina Anticline is located in the Llanos Foothills province along the border of the Eastern Cordillera, Colombia, approximately 100 km southwest of the giant fields of Cusiana and Cupiagua (Figure 1). It is interpreted as a simple fault-bend fold because of its symmetrical shape, kink-band geometry, and horizontal crestal domain (Figure 2). Shallow structural levels are well imaged, but the deep geometry and the detachment level and trajectory of the underlying fault are unknown. In order to address these issues, we use a grid of time-migrated, 2-D seismic data to generate an axial-trace map (Shaw et al., 1994) of the anticline. We then generate fold-evolution matrices and models, which illustrate the effects of two independent variables on fold geometry (Rowan and Linares, 2000), to determine the factors controlling the three-dimensional geometry of the fold. This allows us to identify active and inactive axial planes, construct the three-dimensional fault geometry, and complete the structural interpretation to depth. Axial-surface analysis shows that the three-dimensional geometry of the Medina Anticline is compatible with a faultbend fold interpretation in which displacement increases to the northeast, the ramp dip decreases to the southwest, and the length of an intermediate flat increases to the northeast.

Figure 2: Uninterpreted 2-D time-migrated seismic profile across the Medina Anticline, the adjacent Rio Amarillo Syncline, and the frontal Aguaclara Fault (location shown on Figure 4). The fold geometry, with symmetrical limbs, a horizontal crestal domain, and sharp hinges separating planar dip domains, suggests a fault-bend fold origin. Seismic data courtesy of Ecopetrol.

Figure 1: Map showing the location of the Medina anticline along the eastern border of the Eastern Cordillera of Colombia, between the Quetame basement massif and the frontal Aguaclara fault. Insert shows the location of the larger map in the northwestern corner of South America.



2-6: Medina Anticline Axial-trace maps display structure contours and the plan-view traces of fold hinges (Wilkerson et al., 1991; Shaw et al., 1994; Shaw and Suppe, 1994). Models of fault-bend folds in which displacement increases along strike produce a pattern of active and inactive axial traces and a three-dimensional geometry in which the crestal domain narrows and then widens (Figure 3). Axial traces interpreted on individual seismic profiles across the Medina Anticline (Figure 4) were projected vertically and connected to create the axial-trace map for the fold (Figure 5). Although this map is broadly similar to the model pattern (Figure 3), there are also significant complications. Furthermore, it is possible to generate comparable patterns by varying parameters other than displacement (Rowan and Linares, 2000). Thus, further analysis is needed to identify active and inactive axial traces, understand the critical variables, and complete the interpretation.

Figure 3: Perspective view and axial-trace map of fault-bend fold in which displacement increases along strike (after Shaw et al., 1994). Active and inactive axial traces are green and red, respectively.

Figure 5: Time-structure map of the top of the Mirador Formation (contour interval is 400 msec; depth is relative to arbitrary datum near surface). Contours are in black (tick marks point downdip), seismic lines are in grey, faults are in red, wells are in orange, and erosional truncation is shown by the thick dashed line. The blue lines are the axial traces at this structural level, and the arrows point in the dip direction. The Medina Anticline is bounded by the broad Nazareth Syncline to the northwest and the tight Rio Amarillo Syncline to the southeast. The crestal domain is most narrow at the fold culmination and plunges to the southwest and then south to where it intersects the Aguaclara Fault where it curves west. Similarly, the backlimb curves and becomes less steep toward the southwest and south.

Figure 4: Partial interpretation of the line shown in Figure 2. The red dashed line is the top of the upper Eocene Mirador sandstone (the main reservoir in the area), which is constrained by nearby well control in both the hanging wall and footwall (see Figure 5). The steep grey lines are axial traces along the fold hinges separating planar dip domains; the offset of axial traces in the Medina Anticline is caused by a minor detachment at the base of the Oligocene to lower Miocene Carbonera shales.



2-6: Medina Anticline Fold-evolution matrices show the profile geometry of folds with two varying parameters and are used to construct model axial-trace maps and corresponding perspective views (Rowan and Linares, 2000). Figures 6, 7, and 8 show the fold-evolution matrix, axial-trace maps, and perspective views, respectively, for a model in which both displacement and ramp dip vary linearly. Changes in ramp dip result in curved structure contours (Figures 7c, d and 8c, d) rather than the straight contours produced by varying displacement (Figures 7a, b and 8a, b). However, this difference is distinctive only for the linear gradients used; for example, a nonlinear displacement gradient would result in curved axial traces. A more reliable criterion for distinguishing between changing displacement and changing ramp dip is limb dip. As the ramp dip decreases, both forelimb and backlimb dips decrease (Figures 7c, d and 8c, d). Thus, parallel structure contours (Figures 7a, b and 8a, b) indicate only changing displacement, whereas divergent structure contours (Figures 7c, d, e, f and 8c, d, e, f) show that the ramp dip is varying along strike.

Figure 7: Axial-trace maps of the six panel combinations indicated in Figure 6, in which displacement and/or ramp dip vary linearly along strike. Black lines are structure contours, and dip symbols show the orientation of dip domains.

Figure 6: Fold evolution matrix for linear increase in displacement and linear decrease in ramp dip (28.5, 22.5, 16.5, and 10.5 degrees). (a) through (e) indicate six different combinations of four profile geometries used to construct the corresponding axialtrace maps (Figure 7) and perspective views (Figure 8).

Figure 8: Perspective views corresponding to the axial-trace maps of Figure 7, in which displacement and/or ramp dip vary linearly along strike.



2-6: Medina Anticline The presence of the Rio Amarillo Syncline between the Medina Anticline and the immediate hanging wall of the Aguaclara Fault suggests that there is a flat between the Medina ramp and the Aguaclara ramp. Such a flat could change length along strike, so we illustrate a fold-evolution matrix, axial-trace maps, and perspective views (Figures 9, 10, and 11, respectively) in which both displacement and flat length vary linearly. The resulting axial-trace patterns and three-dimensional geometries are complicated by the presence and interference of new axial surfaces (C, C, D, D) associated with the syncline and frontal ramp. The effects are best seen in the third column of Figure 9, where displacement increases over a flat of fixed length. B migrates toward the foot of the upper ramp, intersecting with C to form D. When B reaches the upper ramp, it becomes fixed and C now migrates up the ramp. In the meantime, B is also migrating forward; when it reaches the foot of the upper ramp, B and B are eliminated and replaced by a new set of A and A axial traces associated with the frontal ramp. Farther up the ramp are C, D, and an offset B (e.g., top of second column).

Figure 10: Axial-trace maps of the six panel combinations indicated in Figure 9, in which displacement and/or flat length vary linearly along strike. Thin black lines are structure contours, thick black lines are faults, and dip symbols show the orientation of dip domains.

Figure 9: Fold evolution matrix for linear increase in displacement and linear increase in flat length. (a) through (e) indicate six different combinations of four profile geometries used to construct the corresponding axial-trace maps (Figure 10) and perspective views (Figure 11). Red numbers (1–6) indicate geometries used to construct the model axial-trace map in Figure 12; 3 is intermediate between the middle two profiles in the top row, and 4 is intermediate between the top two profiles in the second column.

Figure 11: Perspective views corresponding to the axial-trace maps of Figure 10, in which displacement and/or flat length vary linearly along strike.



2-6: Medina Anticline

Figure 12: (a) Synthetic axial-plane map constructed using geometries 1–6 in Figure 9; and (b) corresponding interpretation of the Medina Anticline. In (a), the six profile geometries from Figure 9 were spaced equally and rotated to match the orientation and approximate the scale of the Medina Anticline. Thus, from southwest to northeast: flat length first increases as displacement is held constant (1–3), flat length then decreases as displacement increases (3, 4), flat length again increases as displacement is held constant (4, 5), and then both flat length and displacement increase (5, 6). The dashed line in (a) is the approximate location of the Aguaclara Fault, and the number 1 in (b) indicates the narrowest point of the crestal domain where axial traces A and B switch between active and inactive.

To model the Medina Anticline, selected profiles from Figure 9 are joined in map view to create the synthetic axial-trace map in Figure 12a. This model accurately depicts most of the features of the Medina Anticline, so that axial traces can now be identified (Figure 12b). However, the model has a narrow backlimb and horizontal crest to the southwest (Figure 12a), whereas the observed geometry shows a widening backlimb and dipping, curving crestal domain (Figure 12b). We infer that this is caused by a southwestern decrease in ramp dip, as modeled in Figures 7c and 8c. The model-constrained map interpretation is then used to complete the interpretation. Where the crestal domain is narrowest (location 1, Figure 12b), axial traces A and B should intersect at the top of the lower ramp (Shaw et al., 1994). The fault geometry on each profile is then determined as shown and explained in Figure 13. Figure 13: Finished interpretation of the line shown in Figure 2. The axial-trace analysis and comparison of the observed map to the model map (Figure 12) allows the axial traces to be identified. The level of the flat is determined by the intersection of A and B where the crestal domain is narrowest (location 1 on Figure 12b) and then correlated along strike. The length of the flat is determined by the intersection of active axial traces B and C at the top of the lower ramp and base of the upper ramp, respectively. Note that the length of the hanging-wall flat (B-C) approximately balances that of the footwall flat (B-C).



2-6: Medina Anticline The fault geometry constructed on each profile is shown in map view in Figure 14. Also shown are deeper normal faults visible in the southwest; their continuation to the northeast is not imaged but is likely, and we speculate that the Medina fault-bend fold formed where the Aguaclara Fault ramped up over an underlying basement normal fault (Figure 15). Both the thick-skinned uplift of the Quetame basement massif and the thin-skinned development of the Medina Anticline are interpreted to have formed during Tertiary inversion of a Jurassic rift basin (Figure 15) (Rowan and Linares, 2000; see also Cooper et al., 1995).

Figure 15: (a) Regional 1:1 cross section through the culmination of the Median Anticline showing its relationship to the inverted Quetame basement massif; and (b) Schematic reconstruction (not to scale) showing the infilled rift geometry at the end of the Cretaceous. Aguaclara fault is shown as an out-of-the-syncline thrust, but it could also be rooted in basement. Tan = prerift basement; blue = Jurassic synrift; green = Lower Cretaceous; orange = Upper Cretaceous; yellow = Tertiary/Quaternary.

Conclusions: Figure 14: Map of the fault geometry underlying the Medina Anticline as constructed using the axial-surface analysis. Most of the fault consists of a lower ramp, an intermediate flat that widens to the northeast, and an upper ramp. To the southwest, the fault curves westward, forming an oblique ramp and thus a lower ramp angle. This is apparently in response to curving traces of deeper, rift-related normal faults that offset prerift basement (blue). Contour interval is 400 msec; depth is relative to an arbitrary datum near surface.

• The Medina Anticline is a fault-bend fold, probably formed as the Aguaclara Fault ramped up over a basement normal fault during Tertiary inversion of a Jurassic rift basin. • The three-dimensional geometry is controlled by: (1) An increase in displacement to the northeast; (2) An increase in flat length to the northeast; and (3) A decrease in ramp dip to the southwest. • Axial-surface analysis is a useful tool for constraining subsurface geometry where axial traces are easily defined, but must be used in conjunction with other data/techniques to avoid model-driven interpretations.



2-7: Three-dimensional interpretation of the El Furrial Trend, Eastern Venezuela Basin, Venezuela Miguel Morales, PDVSA, EPM, Venezuela. Enrique Hung, PDVSA, EPM, Venezuela. Richard Bischke, Subsurface Consultants & Associates, LLC., Houston, Texas, U.S.A. Location: Eastern Venezuela Basin, Venezuela Topic: Fault-bend fold folding Reserves: 11,100 MMBO (in place) The super giant El Furrial field is a fault-related anticline located in the Eastern Venezuela Basin. The field was discovered in 1986, based on good quality 2-D seismic data. The El Furrial trend continues to the west as a series of fault-related structures that make up the Serrania del Interior southeast-verging fold and thrust belt (Aymard et al., 1990). The El Furrial trend defines the frontal edge of this fold and thrust belt. The stratigraphy of the fold belt consists of a 3- to 5-km thick Cretaceous to Paleogene passive margin section and a 0- to 8-km thick sequence of Neogene to recent syntectonic and post compressional foredeep fill deposits. The large Pirital fault overthrusts the westernmost structures, repeating 5000 m (16,000 ft) of Cretaceous section. Fault surface mapping and structural interpretations indicate that the major faults and their associated anticlines form a linked en echelon system related to dextral transpression south of the El Pilar right lateral strike-slip fault system.

Figure 2: Depth-corrected profile.

El Furrial is the easternmost of three major structures (El Furrial, Carito, and Tejero, from east to west) that form the North Monagas fields in the Serrania del Interior fold and thrust belt (Figure 1). The trend has 11 billion bbl of oil in place and presently produces 400,000 bbl/day. The structures trend northeast-southwest across northeastern Venezuela, and are offset in a dextral en echelon relationship to each other (Figure 3). These offsets are caused by northwest trending lateral ramps in the underlying major thrust faults (Bischke et al., 1997). These structures are the result of mid to late Miocene dextral transpressional displacements south of the El Pilar strike-slip fault (Figure 1). In the northern part of the South American Plate, the transpressional displacements produced a series of northnorthwest-trending dextral tear faults and lateral ramps that turn to the east-northeast to become ramp and flat thrust faults (Figure 2). Maps constructed of the fault surfaces indicate that many of the faults interconnect to form a linked fault system (Boyer and Elliott, 1982). Figures 4 and 5 describe these general relationships. In Figure 4, the Tejero ramp branches off the Urica Fault trend, and the offset Carito ramp creates another lateral ramp that trends subparallel to the Urica Trend. In turn, the Carito ramp is offset from the Furrial ramp by another lateral ramp. Figure 1: Simplified regional map showing the main structural elements of the Serrania del Interior fold and thrust belt. See Figure 2 for a depth-corrected profile.



2-7: 3-D El Furrial Trend The three ramp systems link to each other and to the Urica Fault trend along a lower fault flat (Figure 4). Displacement of the hanging wall over the three ramps creates the three offset folds, which form the structural traps for the fields. Based on interpretation of high quality 3-D seismic data (samples of which are shown on the following pages), we interpret that these structures developed mainly as fault-bend folds (Suppe, 1983, 1985).

Figure 3: Simplified depth map of the El Furrial trend showing offset fields.

The El Furrial trend is overlain by the Pirital thrust, one of the largest faults in the regional system. The Pirital thrust dips to the north over a horizontal distance of 20 km (Figure 6, sp 100 to 600 between 3 to 10 s). In the west, the Pirital branches off the northwest-southeast trending Urica lateral ramp system, forming the western flank of the Serrania del Interior fold and thrust belt (Figure 1) (Bischke et al., 1997). The Pirital fault overthrusts the Oligocene Naricual reservoir unit, repeating about 500 m (16,000 ft) of the Cretaceous San Juan Formation (Figure 6). The main reservoir unit in the area is the Oligocene Naricual Formation, which contains fluvial deltaic to shallow marine sands (Prieto et al., 1990). The Naricual sands are approximately 500 m (1700 ft) thick (Figure 6), and can contain 250 m (800 ft) of net pay. This northeastward prograding sequence of sands is contemporaneous with the trailing shelf margin of the South American Plate. Later overthrusting loaded and down warped the plate forming a foredeep basin and most likely an outer rise, similar to the outer rise and gravity high observed seaward of oceanic trenches (Watts and Talwani, 1974). Seaward of the trenches normal faults tend to occur on the upwarped highs, which extend due to flexure. The Naricual Formation contains many normal faults that may have originated in a similar fashion when the overthrust sheets of the Serrania del Interior advanced toward the south, loading and flexing the South American Plate.

Figure 4: Block diagram illustrating a linked ramp-flat and lateral ramp system.

Figure 6: Regional cross section and stratigraphic column showing the main tectonostratigraphic elements from the Caribbean plate to the Orinoco tar belt (modified from PDVSA Report).

Figure 5: Block diagram showing the hanging wall above the El Furrial, Carito, and Tejero ramps forming three offset fault-bend folds.



2-7: 3-D El Furrial Trend Seismic Overview Here we present seismic profiles from a 3-D survey that define the geometry of the El Furrial trend. The profiles are migrated and displayed in time (Figures 7, 8) and depth (Figures 9, 10). Figure 7 is an uninterpreted dip section (A) that trends northwest-southeast across the structure. On dip line A, near the trace of strike section B, there is a panel of reflections that dips to the southeast (between 3 and 4.3 s) that defines the forelimb of the El Furrial structure. This south dipping panel overlies a prominent horizontal reflector at about 4.3 s. We interpret that this horizontal reflector is a fault-plane reflection originating from the upper flat (detachment) of a ramp-flat system. The fault is located at the downward termination or discontinuities in the dip panel (Dahlstrom, 1969; Tearpock and Bischke, 2002; see section 1A-2, this volume). The horizontal reflector can be followed to the north where it joins a group of north-dipping reflections. We interpret these north-dipping reflections to represent the backlimb of a fold, which is thrust to the southeast above the frontal ramp (Suppe, 1983, 1985).

Figure 7: Section A — Seismic time profile images dip panels forming a south-verging anticlinal fault-bend fold. Intersection with section B is shown in black line. Arrows highlight fault position.

Figure 8 is a time section along the strike of the El Furrial structure. Note that at about 4.3 s a near-horizontal reflector extends across the strike profile. This feature corresponds with the fault-plane reflection described in dip section A. Above the fault, a panel of reflections that dips to the east represents the folded hanging wall of the El Furrial structure. The nearly horizontal reflections below the thrust correspond to the relatively undeformed footwall. Figure 8 images a strike or lateral ramp (Tearpock and Bischke, 2002), or more precisely the inverted portion of the lateral ramp that is thrust up a frontal ramp and onto an upper flat.

Figure 8: Section B — Strike seismic profile in time showing dip panels formed above the main detachment surface.



2-7: 3-D El Furrial Trend Structural Interpretation In this section, we present interpreted depth profiles (Figures 9 and 10). The high impedance package above 5 km represents the Oligocene Naricual sands (orange horizon), and the horizontal reflection just above 6 km is the upper flat of the thrust fault. In section A (Figure 10), the fold contains a flat crest separating a narrow, southeast-dipping forelimb and a wide, northwest-dipping back limb. Based on the fold and fault geometry, the structure appears to be a fault-bend fold (Suppe, 1983, 1985; Novoa et al., section 24, this volume). The El Furrial thrust fault repeats a minimum of 2.0 km of section and suggests at least 50% shortening (≈ 4 km) (Figure 10). We can define only a minimum estimate of slip on the El Furrial fault because the backlimb of the structure extends beyond the three-dimensional data set. Naricual equivalent sands produce to the south of the trend. Our interpretation suggests that the Naricual sands project beneath the El Furrial structure at about the 7.0- to 8.0-km level.

Figure 9: Interpreted strike line B.

In summary, the El Furrial anticline is a well-defined example of fault-bend fold, similar to structures initially described in the Appalachian Mountains, U.S.A. (Rich, 1934) and the Canadian Rockies (Bally et al., 1966). This fold style is common in other parts of South America (e.g. Dengo and Covey, 1993) and across Venezuela. Thus, faultrelated folding techniques serve as powerful tools for describing many of the hydrocarbon-producing structures in these regions.

Conclusions • The super giant El Furrial trend is formed by three offset fault-bend folds. • The folds are related to a linked dextral en echelon ramp-flat and lateral ramp system. • Shortening is estimated at 50%. Figure 10: Area-balanced interpreted of dip line A.



2-8: Shear fault-bend fold, Deep-Water Niger Delta Freddy Corredor, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. John H. Shaw, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. John Suppe, Department of Geosciences, Princeton University, Princeton, New Jersey, U.S.A. Location: Niger Delta, West Africa, Nigeria Topics: Shear fault-bend folding, growth sedimentation

Using fold shapes, fault plane reflections, and patterns of growth sedimentation, we model a fault-related fold in the deep-water Niger Delta using shear fault-related folding theory. The Niger Delta offers a unique opportunity to study fault-related folds, as the structures are well imaged at deep levels in seismic reflection profiles and because they preserve growth strata that record fold kinematics. Individual fault-related folds are characterized by long

planar backlimbs with increasingly shallower dips to growth strata, suggesting a component of progressive limb rotation. Forelimbs are short compared to backlimbs, but growth strata show more consistent dips that suggests a component of folding by kink-band migration. Combined mechanisms of kink-band migration and limb rotation are thus invoked to model the kinematics of this fault-realted fold.

Figure 1: Uninterpreted, migrated, and depth-converted 2-D seismic profile through a fault-related fold in the deep-water Niger Delta. We observed three basic structural patterns that are consistent with pure shear fault-bend folding kinematics: First, a long planar backlimb that dips less than the fault ramp with increasing shallower dips of growth strata, second, a short forelimb compared to the backlimb, and third, a synclinal axial surface that does not bysect the syncline. Seismic data courtesy of MABON LTD.



2-8: Shear fault-bend fold, Niger Delta The Niger Delta is situated in the Gulf of Guinea (Figure 2) on the margin of West Africa. Sourced by the Niger River, it is one of the largest regressive deltas in the world with an area of roughly 300,000 km2, a sediment volume of 500,000 km3, and a sediment thickness of more than 10 km in the basin depocenter. The northern delta boundary is the Benin flank, an east-trending hinge line south of the West Africa basement massif. Cretaceous outcrops on the Abakaliki Fold Belt define the northeastern delta boundary. The offshore boundary of the delta is defined by the Cameroon volcanic line to the east, the border of the Dahomey basin to the west, and the 4000 m bathymetric contour. From the Eocene to the present, the delta has prograded southwestward into the Gulf of Guinea. The Niger Delta basin consists of Cretaceous through recent marine clastic strata that overlie oceanic and fragments of continental crust. The compressional fault-related fold structures in the deep-water Niger Delta are the product of contraction due to gravity-driven extension on the shelf.

Figure 3: 2-D seismic section through the fault-related fold interpreted in this contribution showing some important characteristics including (1) sea floor reflection, (2) top of oceanic crust reflector, and thrust fault plane seismic reflection indicated by red arrows. Notice how the backlimb dips much less than the fault ramp. See text for detail of (3) and (4).

The stratigraphic sequences imaged in the seismic profile shown above (Figure 3) correspond to Tertiary deepmarine and deltaic sediments. At the bottom of this sequence, the Akata Formation, which can be observed above the Top of oceanic crust reflection (2), is up to 3000 m thick in this portion of the delta, and is composed of thick deep marine shale sequences (potential source rocks), and may contain some interbedded turbidite sands (potential reservoirs in deep water environments). On seismic sections, the Akata Formation is generally devoid of internal reflections (3), and exhibits low P-wave velocities that produce a pull-down velocity effect in time sections, and may indicate regional fluid overpressures. This Formation corresponds to the weak decollement layer that undergoes an externally imposed shear deformation in this fault-related fold. We use shear fault-bend fold kinematics (section 1B-4, this volume) to interpret this structure. Shear fault-bend folds are characterized by long planar backlimbs that dip less, or much less than the fault ramp, as observed in Figure 3 (4), and shows increasingly shallower dips to growth strata suggesting a component of folding by limb rotation. A fault plane reflection is clearly observed (red arrows) that constrains the fault geometry and its planar nature. The fault ramp dips at an angle of 26°. The long planar backlimb dips at an angle of 7.5°, which is much less than the dip of the fault. Also notice that, unlike conventional fault-bend folds, the length of the backlimb does not represent the amount of slip along the fault, and that is represented by the distance between the green dots. This difference between the fault displacement and backlimb length is due to the combined limb rotation and kink-band migration folding mechanisms that occur in shear faultbend folding kinematics. Two end-member interpretations are possible: Simple shear and pure shear fault-bend folding. We will discuss the main structural and stratigraphic features to distinguish between these two end members. Figure 2: High-resolution shaded relief and seafloor bathymetry image of the Niger Delta showing the approximate location of the seismic line used in this study (1).



2-8: Shear fault-bend fold, Niger Delta This fault-related fold can be modelled using the pure shear or simple shear fault-bend folding kinematics (section 1B-4, this volume), (Figures 4 and 5). In simple shear fault-bend folding a weak decollement layer of finite thickness (The Akata Formation) at the base of fault ramps undergoes an externally imposed bedding-parallel simple shear with no basal fault. In pure shear fault-bend folding the deformation of a weak decollement layer of finite thickness is locally confined to the rock volume in the inmediate vicinity of the fault ramp where stresses are high. The basal decollement layer slides above a basal fault and shortens and thickens above the ramp with no externally applied bed parallel simple shear. The slip along the basal detachment decreases to zero at the bottom of the fault ramp. The total slip, then, is accomodated by slip along the fault ramp, and by thickening of the weak decollement layer. In simple shear fault-bend fold kinematics the synclinal axial surface at the bottom of the fault ramp bysects the syncline, while in the pure shear fault-bend fold kinematics this synclinal axial surface is not the angle bisector of the syncline.

axial surfaces

Growth axial surface

axial surfaces

Growth axial surface

Figure 4: Two kinematic models of simple and pure shear fault-bend folds constructed using the end member theory graphs of Figure 5. A) Simple kinematic model of a pure shear fault-bend fold showing downward propagation of shear with the resulting patterns of growth strata, where the slip rates along the fault ramp are equal to the rates of growth sedimentation. The distance between the bottom of the growth axial surface and the synclinal axial surface at the top of the pre-growth sequence is equal to the maximun slip along the basal fault. B) Simple kinematic model of a simple shear fault-bend fold with patterns of growth strata, where the slip rates along the fault ramp are equal to the rates of growth sedimentation. The final geometry of the fault-related fold is the same in both models. Pure shear fault-bend folding kinematics require a shallower detachment level compared to the calculated detachment using simple shear fault-bend folding.

Figure 5: A) Pure shear fault-bend folding end member theory graph (section 1B-4, this volume) showing the relationship between ramp dip, back dip, and dip of the syncline axial surface within the weak decollement layer. The yellow square in the graph corresponds to the fault-related fold interpreted in this contribution. B) Simple shear fault-bend folding end member theory graph (section 1B-4, this volume). The yellow squares in the graphs correspond to the fault-related fold interpreted using the backlimb and cut-off angles interpreted in this section (2-8).



2-8: Shear fault-bend fold, Niger Delta

Figure 6: Simple shear fault-bend fold interpretation of the migrated 2-D seismic profile in the deep-water Niger Delta. A shear profile is included that shows the deformation of a line originally perpendicular to bedding before deformation. This profile shows how the shear decreases upwards. The shear is concentrated between the bottom of the fault ramp and the yellow horizon. An overall simple shear (αe) of 40° is interpreted in the lower 1000 m that terminates in the top of the kink-band (a-b), which agrees with the value predicted via theory from the back-limb dip (δb) of 7.5° for kink-band (a-b) and a fault dip (θ) of 26°. A simple shear (αe) of 15° is interpreted in the next 500 m that terminates at the fault in kink-band b-c, which agrees well with a shear predicted via theory from the back-limb dip (δb) of 6° for kink-band (b-c) and a fault dip (θ) of 26°. Notice that fault slip decreases from a maximum at the top of the ramp to zero at the base of the ramp. Shallow growth strata over the backlimb suggests limb rotation. The synclinal axial surface in this case was interpreted at the point of maximum curvature between the synclinal dip domains. It bisects the syncline across the weak decollement layer. A lower detachment is interpreted at 6500 m depth where the synclinal axial surface intercepts the bottom of the fault ramp. Notice how the length of the backlimb does not reflect the amount of slip along the fault as predicted by conventional fault-bend fold theory. The forelimb is interpreted using multibend fault-bend folding theory. The growth strata onlap the forelimb according to the theory when the rate of growth sedimentation is lower than the rate of structural growth. The gentle dips of the growth strata could be the result of differential compaction and drape.

Simple Shear Fault-bend Fold




Figure 7: Pure shear fault-bend fold interpretation of the migrated 2-D seismic profile in the deep-water Niger Delta. A shear profile is included that shows the deformation of a line originally perpendicular to bedding before deformation. This profile also shows how the shear decreases upwards. The shear is concentrated between the bottom of the fault ramp and the yellow and green horizons. An overall pure shear (αe) of 60° is observed in the lower 700 m that terminates in the top of the kink-band (a-b), which agrees well with the value predicted via theory from the back-limb dip (δb) of 7.5° for kinkband (a-b) and a fault dip (θ) of 26°. An additional pure shear is observed in the next 500 m that terminates at the fault in kink-band (b-c), which produces a back-limb dip (δb) of 6.0° for kink-band (b-c). Notice that fault slip goes to zero at the base of the ramp. A much higher detachment is interpreted in this case at 5700 m depth where the synclinal axial surface also intercepts the bottom of the fault ramp. Notice how the length of the backlimb does not reflect the amount of slip along the fault as predicted by conventional fault-bend fold theory, and requires less slip than the simple shear case. The synclinal axial surface in this case was interpreted at the location of maximum change in dip domain. It does not bisect the syncline across the weak decollement layer. The synclinal back angle (ψ) is 23.5°, which agrees well with the value predicted via theory for the observed back-limb dip and ramp angles, and the calculated shear angle.

Pure Shear Fault-bend Fold




Figure 8: Close-up view of the forelimb. The forelimb is interpreted using multibend fault-bend folding theory. The growth strata onlap the forelimb as predicted by the model when slip rates are greater than growth sedimentation rates. The gentle dips are probably produced by differential compaction and drape.

Conclusions: • A pure shear fault-bend fold is described in the outer fold belt of the deep-water Niger Delta where the weak decollement layer corresponds to the deep marine Tertiary Akata Formation. • The pure shear fault-bend fold described in this section (28) is characterized by a long planar backlimb with increasingly shallower dips to growth strata, a short forelimb compared to the backlimb with onlapping growth strata, and a synclinal axial surface that does not bisect the syncline due

Figure 9: Close-up view of the syncline showing the interpreted picks of the synclinal axial surface across different stratigaphic levels (green dots). The synclinal axial surface, in this interpretation, does not bisect the syncline. Thickening along the decollement layer (Akata Formation) can be observed above the fault ramp, on the left flank of the syncline. These two observations suggest a pure shear fault-bend fold.

to the thickening of the weak decollement layer across the axial surface. • The forelimb is interpreted using the multibend fault-bend folding theory. • The length of the backlimb does not reflect the amount of slip along the fault ramp. • The patterns of growth sedimentation suggest increasing limb rotation by progressive increase of shear along the backlimb. • Rates of syntectonic growth sedimentation are lower than

rates of uplift along the fault ramp during initial stages of fold growth producing onlap over the forelimb, and are increased later such that no bathymetric relief develops. • The main feature that allows differentiation between single and pure shear fault-bend folds in seismic sections is the synclinal axial surface. This axial surface is an angle bisector in simple shear fault-bend folds, but not in pure shear fault-bend folds due to the thickening of the weak decollement layer across the axial surface as illustrated in the structure interpreted in this section (2-8).



2-9: Basil anticline, Northern Apennines, Italy Fabrizio Storti, Dipartimento di Scienze Geologiche, Università degli Studi “Roma Tre,” Roma, Italy Stefano Tavani, Dipartimento di Scienze Geologiche, Università degli Studi “Roma Tre,” Roma, Italy Saverio Merlini, ENI/Agip Division, San Donato Milanese, Milano, Italy Alessandro Mosconi, ENI/Agip Division, San Donato Milanese, Milano, Italy Francesco Salvini, Dipartimento di Scienze Geologiche, Università degli Studi “Roma Tre,” Roma, Italy Location: Northern Adriatic Sea, Italy Topics: Fault-propagation folding, growth structure, foreland flexure Reserves: Gas in Pliocene clastic reservoirs Figure 1: Location of the seismic profile.

Figure 2: Post-stack, time-migrated 3-D seismic reflection profile across the Basil anticline and the Apenninic foredeep. The presence of gas is indicated by the pull-down effect in Pliocene sediments.

Figure 3: Interpretation of the seismic profile. Basic sedimentary and tectonic features are highlighted. The lateral transitions among middle Eocene-Miocene sediments are well imaged, as well as the outstanding Pleistocene unconformity and the overlying progradational sedimentary structures.

The Basil anticline is located at the toe of the Apennines fold and thrust belt, in the northern Adriatic Sea (Figure 1). In the regional seismic line (Figures 2, 3) the pre-, syn-, and postorogenic sedimentary architectures are well imaged, as well as two major thrust-related structures and their overlying growth section. An outstanding feature in the preorogenic succession is the transition from a middle Eocene-Miocene carbonate platform (easternmost sector) to a basinal sequence, through a slope domain. An upper Messinian unconformity marks the onset of foreland flexure and the sedimentation of Pliocene synorogenic deposits in the sinking foredeep. A Pleistocene unconformity marks the end of the major contractional event, followed by the progressive filling of the depocenter. 93 Part 2: Case Studies


2-9: Basil anticline The Basil anticline provides a spectacular example of a thrust-related anticline. Almost the entire fold shape and its interactions with surface processes (syntectonic sedimentation and erosion) are very well recorded. Deformation terminated before the occurrence of any fault breakthrough within forelimb and this prevented any distortion induced by further forelandward translation. Upper Pliocene strata thin onto the crest of the anticline, suggesting that they are growth strata (e.g. Suppe et al., 1992). Erosion of part of the crest and the forelimb indicates that the late Pliocene evolution of the anticline progressed in subaerial conditions. The probable presence of wedge geometries in the Pleistocene sediments may support a late reactivation of the fault-fold pair. We interpret this anticline as a growth fault-propagation fold (Figure 5). Details of the basic observations discussed above are provided in Figure 6. Basil anticline

Outward propagation of the sole thrust along the bottom of the synorogenic sediments. Displacement on the upward migrating frontal ramp is accommodated by the development of the Basil fault-propagation anticline. Folding occurs in a high sedimentation environment and well developed growth wedges form on both limbs.

Flexural sinking of the foreland and deposition of synorogenic clastic sediments in a foredeep environment.

Preorogenic succession Figure 5: Numerically modeled (HCA; Salvini et al., 2001) cartoon showing the reconstructed evolutionary steps for this sector of the Apenninic foreland system and the interpretation of the Basil anticline as a growth fault-propagation fold. Figure 4: Seismic image of the Basil anticline.



2-9: Basil anticline The Basil anticline has a gently dipping backlimb and a steep forelimb truncated by the thrust ramp. The shape of the anticline is well rounded in the upper part and becomes more angular in the fold core. The two axial surfaces bounding the flay-lying crest can be traced in this region of the fold and their downward prosecution indicates that they merge at a point located near the upper Messinian unconformity. This suggests that the basal decollement is located at the bottom of the Pliocene sediments. Upper Pliocene growth wedges are well imaged in both limbs, supporting their rotation during fold evolution (e.g. Hardy and Poblet, 1994). In particular, the large syntectonic fan in the forelimb indicates that most of its total rotation occurred in the late Pliocene. The occurrence of limb rotation in fault-propagation anticlines is predicted by the trishear kinematis (Erslev, 1991; Hardy and Ford, 1997; Allmendinger, 1998). Wedge geometries can be imaged in the lower Pleistocene sediments overlying the forelimb. They provide a reliable evidence of a limited fold activity post-dating the early Pleistocene erosional event. Two other sedimentary wedges are tentatively imaged in younger strata deposited on both limbs.

Conclusions: • The Basil anticline is a notheast-verging fault propagation fold developed at the tip of a thrust ramp that soles down into the upper Messinian unconformity. • Upper Pliocene and, possibly, lower Pleistocene strata are syntectonic units folded during fault motion.

Figure 6: Interpreted seismic image of the Basil anticline showing basic features that have been used for the reconstruction of fold kinematics.



2-10: Salt weld detached fault-propagation folds Frank Bilotti, Timothy Brickner, Thomas Elliott, Chip Morgan, Richard Redhead, Yusri, Unocal, Sugar Land, Texas, U.S.A. Figure 2: Pre-stack time migrated seismic profile converted to depth. This line images two contractional structures that detach from the welded autochthonous salt level.

Location: Deepwater Espirito Santo Basin, Brazil Topics: Fault-propagation folding, salt welds, detachments An early Tertiary, north-south–oriented compressional event in the Espirito Santo Basin formed a mixture of salt-weld detached fault-propagation folds and compressed salt walls. The larger structures preserve a combination of salt-deflation stratigraphic geometry and contractional fold geometry. In this example we model one of these asymmetric contractional structures as a constant thickness fault-propagation fold. We find that two solutions fit the data depending on where one defines kink bands and measures the limb dips. Of the two solutions, a low-angle breakthrough solution that honors the deep fold geometry fits the data better.

autochthonous salt water

Figure 3: Summary of sub-regional structural history of the deepwater Espirito Santo Basin. Horizontal welds are less extensive in more distal parts of the basin.

96

Figure 1: Regional map of the Espirito Santo Basin.

Shaw, Connors, and Suppe


2-10: Salt weld detached fault-propagation folds

forelimb inactive axial surface

Figure 4: Main kink bands defined by axial surfaces.

backlimb active axial surface

single crestal axial surface Figure 5: Model for a simple fault-propagation fold from a flat detachment. Figure 6: Axial surface maps at 2 seismic depth slices. The pattern of a broadening flat crest with decreasing slip predicted by fault-propagation folding is supported by the data. The deeper slice at 6400 ms shows the intersection of the crestal axial surfaces, which is also consistent with the fault-propagation folding model.

Discussion: We employ a fault propagation fold model to interpret this structure based on its first-order structural geometry as an asymmetric fold with a steep or faulted forelimb. The well-defined basal detachment and the lack of structural relief across the structure indicate that the structure soles to a horizontal detachment.



2-10: Salt weld detached fault-propagation folds Solution 1: Using shallow fold geometry We utilize fault-propagation folding theory to provide balancing constraints for the poorly imaged core of this structure. Seismic loop ties and bisecting axial surfaces are the basis for the geometric interpretation. Disagreement between the model structure and the seismic image are due to either geologic complexity not being accounted for in the model or shortcomings of the seismic image.

a. Predicted forelimb dip based on bisecting axial surfaces matches seismic tie, probably a steep fold limb.

Figure 8. Fault-propagation fold solution for the structure using the shallow geometry as the main constraint.

b. Seismic tie is extended across structure and bisecting axial surfaces projected downward. There is no net structural relief across the structure so we postulate a flat basal detachment.

c. FPF theory predicts a basal step-up angle φ = 15° for γ = 30°. Since θ2 = φ, backlimb dips should be equal to the basal step-up angle. In this section the backlimb dips at 17°. Using these parameters we predict the location of the fault.

Figure 7. Interpretation of the structure using fault-propagation fold (FPF) theory.

This balanced section works well for the shallow geometry that we used to constrain the fault-propagation fold model; however, it does not agree well with the bed dips in the deeper part of the structure. In fact, the steeper bed dips at depth suggest that the model fault geometry would actually cut down section with respect to the hanging-wall rocks. This leads us to explore another interpretation of the structure.



2-10: Salt weld detached fault-propagation folds Solution 2: Using deep fold geometry We again utilize fault-propagation folding theory to provide balancing constraints for the poorly imaged core of this structure. In this case, however, we choose to respect the geometry of the deeper fold. The deep fold limbs are steeper and have more inflections in dip, yielding a more complex solution. Here we invoke a low-angle breakthrough of a FPF. kink bands

balanced model section

low-angle breakthrough

a. Defining kink bands from the deep fold geometry yields 7 dip panels. The overall geometry still fits the FPF model; however, another detail must be added to explain the extra kink band. Using δf=60° and θ2=φ, constant thickness FPF theory predicts backlimb dip δb=34°. This predicted dip matches the dip of the deep reflectivity of the backlimb.

Figure 10. The final fault geometry results from the addition of the breakthrough fault as well as folding of the existing fault in the core of the fault-propagation fold. The shallow fold geometry generally reflects the fold shape, but not the exact dip angles. The disagreement can be due to the changing thickness of strata in the section or mechanical thickening or thinning of beds. Because the history of salt deflation causes variation in stratigraphic thickness we propose that this fold forms in rocks with pre-existing thickness variations.

c. We postulate a low-angle fault breakthrough to explain the additional kink band. Using the model developed in Suppe and Medwedeff, 1991, we balance the model using an additional kink band whose width is the same as the amount of the breakthrough slip.

predicted fault geometry

b. Using the FPF fold geometry we can predict the firstorder fault geometry.

Conclusions:

Figure 9. Interpretation of the structure using fault-propagation fold theory and geometric constraints from the deeper part of the structure.

• After nearly complete deflation and welding, the autochthonous salt level still provides a sub-horizontal detachment surface for thin-skinned contractional structures • This structure fits the basic geometry and kinematics described by fault-propagation folding theory. • Two models were tested; of these, a more complex model utilizing deep geometry as the primary constraints provides a better fit to the data. 99



2-11: Structural inversion along the Sakala Fault, East Java Sea, Indonesia Shankar Mitra, University of Oklahoma, Norman, Oklahoma, U.S.A. Location: East Java Sea, Indonesia Topics: Inversion, fault-related folding

The Sakala structure (Figure 1) is a fault-related inversion structure in the East Java Sea (Figure 2), located in a back arc setting behind the Java trench. Along this trench, the Australian plate is subducted under the Eurasian plate, along a north-dipping subduction zone (Hamilton, 1979). Inversion structures in this area resulted from north-south extension in the Eocene and Oligocene, followed by compression in the same general direction, in the early Miocene.

Figure 2: Generalized map of the East Java Sea, showing the location of the Sakala inversion structure, and a seismic reflection profile through the structure.

Figure 1: Time section through the Sakala structure.



2-11: Sakala inversion structure In order to interpret the detailed geometry and evolution of the structure, a pre-stack depth migrated section (Figure 3) was used. The structure is interpreted as an inversion structure formed along the south-dipping Sakala fault. The interval

between the Ngimbang and Prupuh Formations shows a significant increase in thickness from the footwall to the uplifted hanging wall across the fault. The thickness also increases gradually away from the fault zone. The structural geometry of the units

closely resembles that formed by the compressive reactivation of an extensional fault-propagation fold with fault breakthrough. The fold geometry was used to model the fault geometry, which is poorly imaged on the seismic sections (Figure 4a).

Figure 3: Pre-stack depth-migrated seismic profile through the Sakala structure. a. Uninterpreted profile. b. Shaded area represents the interval between the tops of the Ngimbang and Prupuh Formations. Note that the thickened section is in the uplifted block, suggesting structural inversion.

Figure 4: a. Interpreted depth profile through the Sakala structure. b. Restoration of the interpreted seismic profile to the pre-compressional stage, using antithetic inclined shear. Part of the compressional fault-propagation fold is not restored, so that the true restored geometry of the top of the Prupuh Formation is given by the solid gray line. If A1=A2, the structure is area balanced.



2-11: Sakala inversion structure Forward modeling and restoration was used to decipher the detailed evolution of the structure (Mitra, 1993). An experimental clay model (Figure 5; Mitra, 1993; Mitra and Islam, 1994) demonstrates the development of an extensional fault-propagation (drape) fold, and the drape dip resulting from breakthrough of the fault. The experiment simulates the deformation

in pre-extensional units, modeled by a clay layer, above a basement fault dipping 45° degrees. Extension initially results in the formation of a broad fault-propagation fold (Figure 5a). The deformation occurs by the sequential development of a large number of small normal faults, which are progressively rotated to steeper dips with increasing extension. The extension even-

Top Ngimhang Formation

tually results in a major fault breaking through the clay unit at a steeper angle than the basement fault. The fault-propagation or drape dip is preserved in the hanging wall, and is rotated as it passes through the synclinal hinge (Figure 5b).

In late Eocene and Oligocene time, extension resulted in the development of a fault-propagation fold above a deep-seated planar fault dipping approximately 40° (Figure 6a and b). The fault propagated at a steeper angle (55°) through the fold in the Ngimbang and older units and subsequently through the synextensional Prupuh Formation. The propagation of the fault through the extensional fault propagation fold resulted in a basinward drape dip A-B (Figure 6b). This drape panel was rotated to a shallower dip (B-C) as it passed through the synclinal hinge. Basinward of C, units show a small dip into the fault. The synextensional growth units deposited in the hanging wall showed a progressive increase in thickness into the basin through the three major dip panels.

Top Prupuh Formation

Compressive deformation in the early Miocene resulted in folding of units as they passed through fault bends (Figure 6c). The panel B-C was folded to its original steeper dip, and horizontal beds basinward of C were folded to the dipping panel C-D. In addition, a tight fold developed within the synextensional units at the tip of the fault. The compressive folding in the possibly unconsolidated synextensional sediments may have resulted in some area loss, although the structure can be area balanced by assuming some penetrative deformation. Figure 4b shows the restoration of the interpreted seismic profile to a postextensional stage, using the model described above. Variable inclined shear was used to restore parts of the hanging wall deformed by fault-bend folding. The restoration did not remove the effects of compressive fault-propagation folding at the leading edge of the structure. The post-extensional top of the Prupuh Formation possibly had the geometry shown by the dark gray line in Figure 4b. The leading edge of the structure was folded to the geometry shown by the dashed line during compressive deformation, with the area A1 = A2.

Figure 5 (above): Clay model showing the development of an extensional faultpropagation fold, and the subsequent breakthrough of a major fault. Note the development of a drape dip panel in the hanging wall. Figure 6 (right): Evolution of the Sakala structure. a. Pre-extensional geometry. b. Post-extensional geometry. Note the development of a drape dip due to extensional fault propagation folding in the hanging wall. c. Final structure, resulting from compressive reactivation of the Sakala fault.

Conclusions: • The Sakala structure in the East Java Sea is interpreted to be an inversion structure formed by Miocene compressive reactivation of an Eocene-Oligocene extensional structure. • Extension along the Sakala fault resulted in an extensional fault-propagation (drape) fold with subsequent fault breakthrough, resulting in the preservation of a drape dip in the hanging wall. • Compressive reactivation along the fault occurred by fault-bend folding, accompanied by fault-propagation folding at the leading edge of the structure. 102 Shaw, Connors, and Suppe


2-12: Detachment fold, Niger Delta Frank Bilotti, Texaco, Bellaire, Texas, U.S.A. John H. Shaw, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. Ronald M. Cupich, Texaco, Bellaire, Texas, U.S.A. Roisin M. Lakings, Texaco, Bellaire, Texas, U.S.A. Location: Deepwater Niger Delta Topics: Detachment folding, growth strata, restoration We describe a large detachment fold located between the inner and outer fold and thrust belts of the deepwater Niger Delta (Figure 1). Seismic lines define the structure as a broad, symmetric anticline involving Miocene and lower Pliocene deltaic strata (Figure 2). The structure is overlain by syntectonic growth strata that show an upward fanning of limb dips in the upper Pliocene and Pleistocene section, indicating that the fold grew by limb rotation. A continuous, relatively flat basement underlies the fold indicating the presence of a sub-horizontal detachment surface in the pro-delta Akata Formation. Detachment folding requires ductile thickening of the Akata Formation above the basal detachment in the core of the fold.

Figure 2: Migrated 2-D seismic profile through the detachment fold. We observe two basic structural patterns in the seismic profile (top) that are consistent with detachment folds (left): 1) symmetric, dipping fold limbs situated over flat reflectors in the Akata Formation and basement; and 2) syntectonic growth strata with bed dips that shallow upward toward the seafloor. These observations, and the lack of an obvious thrust ramp beneath the fold, indicate that the structure is a detachment fold formed primarily by limb rotation. The structure grew during the Pliocene and Quaternary. Figure 1: Bathymetry of the offshore Niger Delta showing the major structural belts and the location of the study area. Modified from Connors et al. (1998).



2-12: Niger Delta detachment fold We present a kinematic model (Figure 3) and interpreted seismic reflection profile (Figure 4) across the detachment fold. The kinematics of fold growth are recorded by the geometry of syntectonic strata. In Figure 3, we compare the growth of a model detachment fold with a restoration of the seismic interpretation using heterogeneous inclined-shear. The restoration demonstrates that the structure grew primarily by limb-rotation, with a minor component of limb widening between restoration steps B and C. The structure is cored by pro-delta marine sediments of the Akata Formation (Figure 4). Detachment folds require that material in their cores deform and thicken to accommodate fold amplification. The Akata Formation, a marine shale and the probable hydrocarbon source rock, exhibits this increased thickness.

Figure 4: Interpreted seismic section and geologic cross-section through the detachment structure.

Conclusions: Figure 3: Sequential model (0-3) of a detachment fold (left) with fixed limb widths that grows by limb rotation. The model is compared with a balanced restoration of the structure (right) derived using variable inclined-shear (Novoa et al., 1999).

• This structure is a detachment anticline that grew primarily by limb-rotation since the early Pliocene. • The basal detachment is located in the Akata Formation marine shales, which thickened in the core of the fold during growth.



2-13: Mississippi Fan Fold Belt, Gulf of Mexico Mark G. Rowan, Rowan Consulting, Inc., Boulder, Colorado, U.S.A. Frank J. Peel, BHP Billiton Petroleum, Houston, Texas, U.S.A. Location: Offshore Louisiana, northern Gulf of Mexico Topics: Salt-cored detachment folds, reverse faults, growth strata Reserves: Giant fields along trend to west (e.g., Mad Dog)

Figure 1: Map of the northern Gulf of Mexico showing the distribution of allochthonous salt (black), basinward- and landward-dipping faults (blue and red), and deepwater folds (green). Modified from Diegel et al. (1995) and reprinted by permission of the AAPG.

Figure 2: Uninterpreted and interpreted views of Profile A showing symmetric, rounded, unfaulted detachment fold cored by salt. Blue — top and base of allochthonous salt and equivalent weld (indicated by pair of dots); purple — undated horizon, possibly Upper Jurassic; green — MCU (midCretaceous unconformity), alternatively identified as top Cretaceous using new well data (T. Dohmen, 2001, personal communication); red — top Oligocene; orange — intraMiocene; yellow — time-transgressive growth unconformity/onlap surface. Horizon correlation around the plunge termination of the fold shows that strata truncated by the unconformity at (1) are age-equivalent to those onlapping the surface at (2); deeper growth strata are thinned and rotated on both flanks (3). 3-D data courtesy of WesternGeco; location shown on Figures 7 and 10.

The Mississippi Fan fold belt is one of several deepwater contractional provinces that formed in response to gravitational failure of the northern Gulf of Mexico passive margin (e.g., Diegel et al., 1995; Peel et al., 1995; Rowan et al., 2004). It comprises salt-cored detachment folds and associated reverse faults that developed principally during the late Miocene (e.g., Weimer and Buffler, 1992; Rowan, 1997). Although all folds were originally thought to be cored by the autochthonous Louann salt, modern data show that the frontal folds are detached above an Upper Jurassic to Lower Cretaceous allochthonous nappe (Peel, 2001; Rowan et al., 2001, 2004). In this section (2-13), we examine the three-dimensional geometry of a composite frontal fold using a series of 3-D time-migrated seismic profiles and structure maps. The profile geometry varies considerably along strike from a relatively simple, symmetric, unfaulted detachment fold (Figures 2, 3) to an asymmetric, faulted fold that is vergent either basinward (Figures 4, 6) or landward (Figure 5). Also, an earlier (Mesozoic) deformation phase complicates the deep geometry. Thus, no simple 2-D or 3-D structural model adequately explains the relationship between the fold and associated faults, and geometric and/or quantitative models are of minimal use in aiding seismic interpretation in this case. 105 Part 2: Case Studies


2-13: Mississippi Fan fold belt

Figure 3: Uninterpreted and interpreted views of Profile B showing a rounded detachment fold with a very slight asymmetry and a minor, high-angle reverse fault on the forelimb. Again, note the differences in growth strata between the two limbs. Horizons as in Figure 2; location shown on Figures 7 and 10. 3-D data courtesy of WesternGeco.

Figure 4: Uninterpreted and interpreted views of Profile C showing an asymmetric detachment fold with a long, planar backlimb and a steep forelimb cut by a basinwardvergent, high-angle reverse fault zone. The early deformation stage is clearly shown by the structural thinning and thickening (4) between the top salt (blue) and the top Oligocene (red). Horizons as in Figure 2 (dashed where approximate, dotted where uncertain); location shown on Figures 7 and 10. 3-D data courtesy of WesternGeco.



2-13: Mississippi Fan fold belt

Figure 5: Uninterpreted and interpreted views of Profile D showing a broadly symmetric fold with a larger reverse fault on the basinward limb but a deep-level crest on the landward side. Again, note the differences in growth strata between the two limbs. Horizons as in Figure 2 (dashed where approximate, dotted where uncertain); location shown on Figures 7 and 10. 3-D data courtesy of WesternGeco.

Figure 6: Uninterpreted and interpreted views of Profile E showing an asymmetric fold with a long, gentle backlimb and a steeper, faulted forelimb. Again, note the differences in growth strata between the two limbs and the early deformation visible at depth on the backlimb. Horizons as in Figure 2 (dashed where approximate, dotted where uncertain); location shown on Figures 7 and 10. 3-D data courtesy of WesternGeco. The well was dry.



2-13: Mississippi Fan fold belt The three-dimensional geometry of this salt-cored fold is complex, consisting of three en-echelon segments (I, II, and III) and a smaller segment (IV), with individual culminations separated by saddles (Figure 7). Thus, any given profile through the fold is likely to display significantly different geometries (Figures 2-6). Furthermore, the relationship between the fold and its associated faults is highly variable: Segments I and II each have a frontal reverse fault (linked at prominent cusp), segment IV has a small landward-vergent fault, and segment III is unfaulted (Figures 7 and 8). Initial growth strata are thinned and rotated on both limbs (3 on Figure 2), consistent with detachment folding with progressive limb rotation (e.g., Hardy and Poblet, 1994). However, shallow growth geometries (backlimb truncation at 1 and forelimb onlap at 2 in Figure 2) are similar to those modeled for fault-bend folds with synkinematic erosion (Figure 9) (Suppe et al., 1992; Hardy and Poblet, 1995). Although the detachment (base of salt nappe) does indeed ramp up, the studied fold is a detachment fold with no higher-level flat or wedge thrust. Thus, a growth pattern of backlimb truncation and forelimb onlap does not necessarily define a fault-bend fold. In this case, it was generated by a salt-cored detachment fold in which the forelimb locked up as the backlimb continued to rotate.

Figure 8: Variation of shortening along the strike of the fold, divided into faulting and folding components. A through E are the five profiles illustrated in Figures 2 through 6, respectively. Modified from an earlier interpretation (Rowan, 1997), with shortening values determined from line-length restoration of fourteen equally spaced profiles. There is no direct correlation between fault and fold geometries because faults are secondary structures that may or may not develop and modify preexisting detachment folds.

Figure 7: Time-structure contour map of the studied fold (segments I, II, III, and IV) and more landward structures. Yellows and reds are highs, blues and purples are lows; thin black lines are reverse faults and black blobs are salt diapirs. The three-dimensional geometry is very complex: individual fold segments may have different orientations, plunge angles, fold-fault relationships, and diapiric influence. Grey lines show seismic profiles of Figures 2-6.

Figure 9: Modeled fault-bend fold with synkinematic erosion and sedimentation (modified from Hardy and Poblet, 1995). The red horizon is a time-transgressive growth unconformity (analogous to the yellow horizon in Figures 2-6), with time-equivalent strata truncated on the backlimb (1) and onlapping the forelimb (2). The resulting growth geometry is very similar to that observed in salt-cored detachment folds of the Mississippi Fan fold belt, which do not contain an upper detachment and a connecting ramp (compare this figure with Figure 2b). Thus, backlimb truncation and forelimb onlap do not uniquely define a fault-bend fold, but simply show that the two limbs behaved differently.



2-13: Mississippi Fan fold belt Another complicating factor is an earlier stage of deformation. Thickness variations in the section between the top salt and the top Oligocene (e.g., 4 in Figure 4) reflect a dominantly Cretaceous tectonic event. An isochron map of an interval immediately above salt shows a complex pattern of paleohighs and paleo-lows (Figure 10). A related feature is that the suprasalt Cretaceous and Paleogene section is generally thinner than in the area basinward of salt (e.g., between the purple and red horizons in Figure 2) because of distal salt inflation during the early history of the margin (Hall, 2000). The variable profile geometry of this fold is thus partly due to the multi-phase deformation history illustrated in Figure 11. Deformation began almost immediately after salt deposition due to differential thermal subsidence and the consequent basinward tilt. This resulted in a combination of distal inflation, nappe extrusion, and folding beneath a thin overburden. The complex geometry of these structures (Figure 10) is interpreted as an interference pattern during convergent gliding off both the Florida and Louisiana margins. The early structures then served as buckling instabilities for the later (Neogene) deformation, but only some were reactivated because of the thicker overburden, and thus longer wavelength, of the detachment folding.

Figure 11: Schematic evolution based on quantitative restorations and regional considerations (modified from Rowan et al., 2000): (a) Upper Jurassic salt deposition; (b) gravity gliding caused by Cretaceous thermal subsidence and basinward tilting results in distal inflation, nappe extrusion, and small-wavelength folds; (c) relative quiescence during the Paleogene as thermal subsidence and tilting wane; (d) gravity spreading of the Neogene progradational margin, resulting in larger-wavelength folding; and (e) cessation of deformation as the Pleistocene deepwater Mississippi fan is deposited. Sections are not drawn to scale, and the effects of salt withdrawal and diapirism are not shown.

Conclusions:

Figure 10: Isochron map of an interval immediately above salt showing the geometry of the early (dominantly Cretaceous) deformation. Thins corresponding to paleo-highs are in yellow and red; thicks corresponding to paleo-lows are in blue and purple. The complex pattern influenced the development of the later (Miocene) structures, shown by the black lines with arrows, resulting in the larger wavelength, variable fold geometries observed today.

• Salt-cored detachment folds in the Mississippi Fan fold belt have complex threedimensional geometries with significant variations along strike caused by variable fold-fault relationships and the effects of an earlier deformation phase. • Patterns of growth strata are ambiguous and cannot always be used to determine the fold style and nature of underlying faults. • In the case of salt-detached fold belts on passive margins, therefore, applying simple geometric and quantitative models to shallow horizons in order to constrain the deeper interpretation is often inappropriate.



2-14: Yakeng detachment fold, South Tianshan, China Aurélia Hubert-Ferrari, Institut de Géologie, Université de Neuchâtel, Neuchâtel, Switzerland John Suppe, Department of Geosciences, Princeton University, Princeton, New Jersey, U.S.A. Xin Wang, Department of Geosciences, Zhejiang University, Hangzhou, China Chengzao Jia, PetroChina, Beijing, China Location: Kuche, Tarim Basin, Xinjiang, China Topic: Analysis of a detachment fold in the thickness domain Reserves: Exploration region Key Point: Yakeng anticline illustrates the importance of working in the thickness domain when interpreting detachment folds. Measurements in the thickness domain show that Yakeng has 1.2 km shortening above a basal two levels of major detachment, basal diapirism, basement folding, and a 2.4-km-thick growth sequence. Structural Setting: The active Yakeng anticline is topographically expressed by deformation of the alluvium at the front of the southern Tianshan thrust belt (Figure 1). Seismic imaging and drilling (Figure 2) show it to be a classic detachment fold lying above a decollement in the evaporite-rich Tertiary Jidikuh Formation, which roots northward (below horizon 4) into the massive 200-km-long Quilitak anticline (Figure 1). Just to the south of Yakeng anticline is the Yanan anticline, which is a basement-involved inversion structure whose north flank interferes with the south flank of the Yakeng anticline.

Figure 2a: Uninterpreted seismic (horizontal scale equals vertical scale, topography exaggerated x4).

Figure 2b: Interpreted seismic (horizontal scale equals vertical scale, including topography) showing the 27 horizons used in analysis of Yakeng. Two major detachments bound the thickening yellow and orange interval.

110 Figure 1: Yakeng fold at the front of the southern Tianshan. (Landsat TM)



2-14: Surface expression of fold growth and sediment trapping Geomorphic expression: The morphology of the 50–150-m-high topographic anticline illuminates the most recent increment of fold growth and sedimentation. It is a deformed and incised alluvial surface for which prior through-going drainage systems are still visible (Figures 3 and 4), showing that deposition previously exceeded uplift, similar to the present situation at Kuche where Yakeng is largely buried (Figure 1). Limb dips (3–4°) in the valley east of the seismic line (Figures 1, 2b) are a significant fraction of the seismic dips (4–6°), indicating the extreme youth of Yakeng anticline.

Figure 5: Low rounded morphology of the Yakeng anticline.

Drainage and sedimentation: The topographic anticline is a barrier to the river networks (Figures 1, 3); only regionally important rivers can now cross Yakeng. Smaller streams previously crossed Yakeng anticline as demonstrated by the numerous well preserved wind gaps (Figure 3) and by southward merging channel networks that are continuous across the wind gaps (Figures 3, 4). This implied reorganization of drainage networks is an effect of decreasing stream power caused by decreasing stream gradients associated with fold growth. As a result, sediment is preferentially trapped north and south of Yakeng (Figures 5, 6), producing a topographic expression that is narrower than the anticline at depth, especially on the north flank (Figures 2, 3, and 12).

Figure 3: The active Yakeng anticline forms a 6-km-wide rounded topographic ridge that few rivers can incise, as shown by the many wind gaps (w). Northward tilting of the north flank of Yakeng and alluvial deposition both decrease stream gradients, which favors the development of channels on the sides of the alluvial fans and along the northern limb of Yakeng (1). Others channels have a converging pattern (2) which increases their stream power sufficiently to keep incising Yakeng anticline. The increase of meander amplitude and wavelength across Yakeng also reflect these changes in gradient.

Figure 4: Southward-converging drainage networks are interrupted by wind gaps at the crest of Yakeng anticline. Flow is now to the north on the north flank of Yakeng. These southward converging networks formed before Yakeng anticline developed its present topographic expression. Seismic line in black.

Figure 6: A facies change on the northern limb of Yakeng anticline is visible in the field (top) and in the seismic reflection profile (bottom). The northern edge of the anticline is mainly formed by thick dark conglomerate (Xiyu F.) whereas its top is composed mainly of yellow-grey sandstone. Most coarse dark conglomerates began to be deposited during the glacial period (1.8 Ma to present). They progressively filled the basin between Quilitak and Yakeng anticlines.



2-14: Seismic characteristics and folding mechanism Initial assessment: Yakeng anticline dies out downward (Figure 7), suggesting it is a classic detachment fold that can be analyzed quantitatively for shortening and timing (Figure 8). However Yakeng is too complex because of regional variation in stratigraphic thickness below horizon 15 (Figure 9) and interference with Yanan anticline (Figure 7). This forces us to move our analysis of Yakeng from the depth domain to the thickness domain (Figures 10–14).

Figure 8: Classical detachment folds are characterized by a linear upward increase of area of structural relief A = hs within pregrowth strata (Epard and Groshong, 1993). By measuring the area of structural relief of many horizons the magnitude — and the timing — of shortening can be determined s = A/h. Shortening can also be determined for each layer from bed-length measurements s = δL = L2 – L2, but only if bed length is conserved. Yakeng anticline is significantly more complex than this model.

Figure 7: Yakeng anticline dies out downward in height and width, indicating a basal detachment (1, horizon 4), which extends to the north under Quilitak anticline. Yanan anticline is a basement-involved inversion structure that is young, as shown by changes in structural relief on its south flank (2). Yanan interferes with Yakeng anticline (3), making analysis of Yakeng more challenging.

Figure 9: Measurement of area of structural relief (A11) following the model of Figure 8 is ambiguous since the undeformed regional gradient (4) is hard to determine because the basement is folded and thickness varies regionally. Therefore we move our analysis to the thickness domain (Figures 10–14).



2-14: Analysis of Yakeng in the thickness domain By flattening the structure to appropriate horizons we can view the structure in the thickness domain and more easily determine the regional stratigraphic gradients (Figures 10–12), which are needed to measure areas of structural relief (Figures 10, 12). The analysis shows us that interval 4-15 has undergone significant shortening (1200 m) and interval 4-5 has undergone additional diapiric flow (0.8 km2). The overlying strata (15-27) show modest thinning over Yakeng and nearly constant thickness relief, which can be modeled as the beginning of growth. Strata above horizon 27 are more strongly thinned, showing a recent acceleration of growth of Yakeng, preceeding its emergence as a topographic feature.

Figure 12: Yakeng anticline flattened to horizon 4 (h=v). The analysis given below shows that horizons 5–14 have undergone 1200 m of shortening and thickening above an evaporitic detachment. There is and additional 0.8 km 2 of diapirism in the basal layer (4-5). Horizons 15–27 show a nearly linear upward dearease in shortening. After horizon 27 time shortening and uplift has accelerated leading to topographic emergence.

Figure 10: Section produced by flattening on horizon 4. It is easier in this thickness display than in the depth display (Figure 9) to identify the regional stratigraphic gradients (1, also see Figure 11) for measuring area of structural relief (A11) and undeformed height (h11). Note that Yanan anticline largely disappears in this thickness display because it is a flexural-slip fold conserving layer thickness, whereas Yakeng is visible because it has grown by layer thickening. Yanan anticline appears only as a depression (2) because of stratigraphic thinning due to growth (see interval 15–22 Figure 11), but at deeper levels (3) the slight depression is produced by flow of the basal evaporitic layer (4-5).

Figure 11: The keys to interpreting Yakeng anticline are revealed by its thickness variations. The basal evaporitic interval (5–4) shows thickness variation largely caused by diapiric flow (Figure 13). Interval 5-15 shows large regional northward stratigraphic thickening reflecting syndepositional flexure of the basement, plus local structural thickening at Yakeng. The overlying interval 15-27 shows stratigraphic thinning over both Yanan and Yakeng anticlines, indicating growth. The uppermost interval (topo-27) shows large thinning over Yakeng, indicating accelerated growth including diapirism.

Figure 13: Area of thickness relief increases linearly from layer 5 to 15 indicating a nearly constant shortening of 1200 m (compare Figure 8). The nonzero intercept indicates an additional 0.8 km2 of diapiric flow in the basal evaporitic interval (4-5). The interval of nearly constant relief (15–27) can be modeled as a growth internal (δS/δH = 0.2, assuming diapirism is after horizon 27.)

Conclusions:

Figure 14: Shortening is calculated from area of relief minus the diapiric area (see Figures 8 and 13). The nearly linear shortening within the growth interval (15-27) suggests that diapirism is late, leading to the topographic emergence of Yakeng. The larger apparent shortening of layers 5-6 may suggest a small additional diapiric component.

Yakeng anticline the value of analysis in the thickness domain:

• • • •

Thickness analysis clearly identifies the growth, pregrowth, and diapiric intervals. Beds in the pregrowth sequence have shortened by 1200 m. There a significant diapiric component in the basal evaporitic layer (0.8 km2). The growth of Yakeng between horizons 15–27 shows a nearly linear rate of shortening, followed by an acceleration of growth and topographic emergence. • The topography shows folding of previously through-flowing stream valleys This study was supported by NSF EAR-0073759, NSFC 49832040, TPEDB-PetroChina, and Princeton 3-D Structure Project.



2-15: Odd geologic structures of southern Oklahoma revisited, Oklahoma, U.S.A. Paul Genovese, Grizzly Energy Resources, LLC., Columbia Falls, Montana, U.S.A. Location: Ardmore Basin, Oklahoma, U.S.A. Topics: Fault-bend folding, fault-propagation folding, structural wedging, kinematic forward models, growth strata

Introduction C.W. Tomlinson (“Odd Geologic Structures of Southern Oklahoma,” 1952) observed that late Paleozoic “Structures of types somewhat unusual for the Mid-Continent region occur in the Ardmore district of Oklahoma.” Despite structural peculiarity, application of fault-related fold theory to a modern 2-D seismic profile can explain the geometry of Fox-Graham Field (Figure 2), one of Tomlinson’s “odd structures.” This section (2-15) illustrates 1) application of fault-bend fold theory to produce a model of fault shape and footwall structure, 2) how concepts of structural wedging and fault propagation folding combine to produce a model explaining the geometry and kinematics of the “rabbit-ear” fold (Figure 2) and, 3) how these models are synthesized to produce a retrodeformable, kinematically-viable, forward model that evolves to approximate the present geometry of the structure.

approximate 2-D seismic line location

Figure 2: Uninterpreted, depth-converted 2-D seismic profile across Fox-Graham Field and the Harrisburg Trough. Besides the well-imaged fold that dominates the profile, of particular importance in constraining a fault-related fold interpretation are the geometry of the pre-Pennsylvanian unconformity (1), recognition of fault plane reflections (2) and, recognition of footwall structure (3). Tomlinson (1952) described the rabbit-ear fold along trend of the one imaged here. No vertical exaggeration.

Figure 1: Location map.



2-15: Odd structures Fault-Bend Fold Analysis: The true utility of fault-bend fold theory (Suppe, 1983), is its capability to provide a complete fault/fold solution using limited data. Figures 3 and 4 illustrate data constraints and fault-bend fold solution for the large fold. The solution (Figure 4) is a multiple bend fault-bend fold. By itself, the solution is largely geometric with few explicit kinematic implications. Incorporating other observations into the interpretation makes it more robust. Recognizing the unconformity as an originally horizontal isochron, for instance, constrains possible kinematic solutions by defining the timing and location of folding. As an example, the small forelimb (+I in Figure 3) is not folded sympathetically with the unconformity, and is therefore interpreted as an older structure, as opposed to a limb formed by slip through Bend 3 (Figure 4).

Figure 3: Subdivision of the fold into dip domains (regions of equal dip, as in Suppe, 1983) in preparation for fault-bend fold analysis . Dips and the fault-plane segment are regarded as “hard” constraints for the purposes of interpretation. Question marks denote uncertainty in the downward continuation of the fault plane and fold axial surfaces (dashed black lines).

Figure 4: Constrained fault-bend fold solution fully predicts fault-plane geometry.



2-15: Odd structures

imbricate

Models of Footwall Deformation: The shape of the unconformity provides critical information about the kinematic evolution of the fold. Using the folded unconformity as a “strain gauge” and restoring fault slip, it is demonstrated that the hanging wall fold cannot be the product of slip on a single fault (Figure 5) but must result in part from folding in its footwall. Further, the folded unconformity helps constrain models of footwall folding (Figures 6 and 7). unconformity

Figures 6a and 6b are models of footwall imbricate structures proposed to explain the present geometry of the folded unconformity in Figure 5a. In each case, fault Bend 1 (and consequently Bends 2 and 3) are deactivated by, and passively transported by, the footwall imbricate. Folding is generated only at Bend 4, a synclinal bend in the imbricate fault. These models do not explain the shape of the folded unconformity in Figure 5a, and are discarded as possible solutions.

KEY

Bend 3

Bend 2

active axial surface (anchored to fault bend/tip, beds fold as they move through) inactive axial surface (anchored to the rock, former site of active folding) folding generated by footwall wedge folding generated by slip on upper fault Figures 7a through 7e: Sequence of steps in a kinematic forward model that, unlike those shown in Figure 6, reproduces the shape of the folded unconformity in Figure 5 and the seismic profile. In this model, a structural wedge folds the footwall and refolds the overlying hanging wall. Slip on the upper fault in Figure 7e completes the deformation, giving the unconformity its present shape. Note the “shoulder” produced, in part, by “rolling” a flat part of the unconformity through Bend 3 and tilting it forward onto the crest of the fold. Compare this feature with that shown on the seismic line in Figure 2. Folding generated in the footwall is shaded differently from that generated in the hanging wall for clarity in Figure 7d, 7e (refer to key). While there is no direct evidence for a thrusted footwall wedge on the seismic data, southwest-verging thrust and reverse faults are not uncommon in the Ardmore Basin (e.g. Overbrook Thrust, Caddo Fault).

Bend 1

Figure 5a: Kinematic model based on the fault-bed fold solution of Figure 4. Gray panels represent rock folded through fault bends after the unconformity was formed. Their width is consistent with fault slip applied at lower left. The red panel cannot be explained by the same slip.

“shoulder”

Figure 5b: Model restored by removing fault slip does not restore the unconformity to horizontal above the red panel. A footwall fold of some kind (indicated by the question mark) must exist below the red panel to account for the discrepancy. The width of kink band A-A represents the total slip of the hanging wall before the unconformity was formed. B-B, which locally refolds A-A, represents slip on an unspecified footwall fault after the time the unconformity formed.



2-15: Odd structures “Rabbit-Ear Fold:” An “ odd” type of structure described by Tomlinson (1952) is the rabbit-ears anticlinoria. Figure 8 compares Tomlinson’s rabbit-ear fold with one imaged on the seismic profile a few miles away. Figure 9 shows a generalized model of how this type of rabbit-ear fold could form as a type of structural wedge.

Forward Kinematic Model: Figure 10a through 10i shows stages in a balanced, kinematic forward model. Figure 10j shows that the model result is a good fit with the seismic data giving validity to the solution. It must be noted that the solution is not unique, especially for the footwall structure, but is kinematically viable and therefore more robust than a balanced cross section. Models like this are useful for considering any time-space dependent features in the petroleum system such as fracture distribution and intensity, migration pathways and traps, and source-rock/reservoir juxtaposition during generation. 10a

8a

10b

8b Figure 8 compares a kinematic model of a rabbit-ear fold superimposed on seismic profile (Figure 8a, enlarged from Figure 2), with the rabbit-ear modified from Tomlinson (1952) constructed from well control (Figure 8b). Although the interpreted structures are a few miles apart, they bear some similarity, most notably the folded pre-Pennsylvanian (1) and pre-Atokan (2) unconformities in the core of the structure.

9a

9b

9c

Figure 9 shows the kinematic development of a fold similar to the rabbit-ear folds in Figure 8, without the complications of unconformities and pre-existing structure.

Figure 10a: Initial conditions for kinematic model. (1) is a fold limb related to deep thrust, perhaps the Arbuckle Thrust, inferred by different authors to sole between ~ -30,000’ (Crawford et al., 1990) and -60,000' under the Ardmore Basin. (2) is an incipient thrust fault. (3) is a fault bend that is the locus for development of a fault-propagation fold.

10c

Figure 10b: (1) is a fault-propagation fold, with slight backward shear (2) applied to achieve balance. (3) is an incipient fault that will decapitate the faultpropagation fold consistent with the observation that the internal angle (4) of the fault-propagation fold (g* of Suppe and Medwedeff, 1990) is not found on the present-day hanging wall.

10d

Figure 9a shows the incipient thrust and backthrust (dashed red lines) that define the wedge tip. Fault-bend fold parameters q and f are used with standard fault-bend fold theory (Suppe, 1983) to calculate the dip of kink band (a) in Figure 9b. This theoretical treatment is identical to that of a “nondetachment wedge thrust” (Medwedeff, 1988). Figure 9b and 9c show how fault slip on the backthrust is consumed by the faultpropagation fold. Theoretically, the dip of forelimb (b) is equivalent to that predicted by fixed-axis fault-propagation fold theory (Suppe and Medwedeff, 1990) where q = f = dip of (a).

Figure 10c: Slip on fault (1) translates the hanging wall. Pre-Atokan unconformity (2) (erosion exceeds uplift) is folded forward (3) as it passes over fault bend (4). Atokan sediments onlap unconformity near (3) and fault (1) reaches the seafloor at (5) as a contractional growth fault.

Figure 10d: Emplacement of structural wedge (1) produces kink band (2) in the footwall and folds the overlying hanging wall. Subsequently, the pre-Pennsylvanian unconformity (3) truncates the hanging wall, preserving a remnant (4) of the pre-Atokan unconformity. Deposition of the Pennsylvanian Deese group follows erosion (5).



2-15: Odd structures 10e

Figure 10e: Initial wedge block fails and second wedge block (1) is emplaced producing kink band (2) and slight bend in fault (3).

10f

10f

Figure 10f: Renewed slip on deep (Arbuckle?) thrust widens initial kink band (1) by an amount (2) folding the entire section.

Figure 10g: Renewed slip fault (1) produces hanging wall folding at fault bends (2)–(5). Rabbit-ear fold (6) begins to form above fault tip (7).

10j 10h

Figure 10h: Continued slip on fault (1) amplifies growth of rabbit-ear fold.

10i

Figure 10i: Final increment of slip on fault (1) results in present structural geometry.

Figure 10j: Final stage of kinematic forward model from Figure 10i, enlarged and superimposed on seismic profile. Note in particular the good fit between the model and the unconformity (1), “shoulder” (2), rabbit-ear fold (3), and footwall reflections (4).



2-15: Odd structures Timing of structural events

Table 1: Timing of structural events interpreted in kinematic model and on 2-D seismic profile. Letters correspond to labels in Figure 11. Orogenies are adapted from Lang (1957) and Tomlinson and McBee (1959), based mainly on the presence of conglomerates in the stratigraphic section. Events A-G support punctuated orogenesis, whereas H demonstrates continuous deformation. The discrepancy in the thickness of Mississippian strata in well 3 vs. well 4 indicates that uplift related to slip on fault D probably began prior to deposition of basal Atoka, as opposed to after as illustrated in the kinematic model. Observation of growth folding (as in Suppe et al., 1992) constrain displacements in the kinematic model to show that the “shoulder” and rabbit-ear folds (Figure 11) formed coevally with G. Both of these folds trap and produce significant quantities of oil, demonstrating that migration occurred after the Pennsylvanian.

Conclusions: This case study synthesizes basic observations from seismic and well data (Figure 2), fault-bend fold analysis (Figures 3 and 4), kinematic constraints on models of footwall deformation (Figures 5–7), principles of rabbit-ear folding (Figures 8 and 9), and a fully-retrodeformable kinematic model (Figure 10) to reasonably match the shape and explain the origin and timing of structures observed on a seismic profile (Figure 11, Table 1). It is demonstrated that application of fault-related fold theory can even yield tractable geometric and kinematic solutions for odd structures, like those found in southern Oklahoma.

Acknowledgements: The author thanks Texaco Exploration and Production Inc. (in particular Frank Gaines) for providing the 2-D seismic line used in this study. Much of this study was completed as part of the author’s Ph.D. thesis research at Princeton University, special thanks to advisor John Suppe and colleagues John Shaw, Frank Bilotti, and Chris Connors. This section (2-15) benefitted from thorough and thoughtful reviews by Stephen Hook and Peter Brennan.

Figure 11: Cross-section interpretation of the seismic profile from Figure 2, incorporating the final stage of the kinematic model from Figure 10i (boundary shown in gray) plus additional well control (2, 3, and 4). Stratigraphy in the kinematic model is modified to better fit well control. Refer to Table 1 for labels A–H.



2-16: Fault-bend folds in the southern Caribbean Ranges, San Carlos, Venezuela P. E. Kraemer, Pecom Energia, S.A., Neuquen, Argentina J. Silvestro, Pecom Energia, S.A., Neuquen, Argentina Location: Caribbean Ranges, San Carlos,Venezuela Topics: Fault-bend folds structures, thrusting sequence, restoration

The San Carlos fold belt is located at the southern tip of the allocthonous thrust front of the Caribbean Ranges, Venezuela (Figure 1). The seismic example (Figure 2) shows a buried fold and thrust belt overlying a normal faulted authoctonous platform. The main structures are three folds (α, β, γ) with typical kink geometries (Suppe, 1983) overlain by a Quaternary unconformity (U). The anticlines β and γ are linked to a common decollement folded by the δ anticline and deep kink panels probably related to footwall shale flow. The shortening on the main thrust ramp is transferred to a structural wedge duplex at the front of the fold belt (d). The γ anticline is interpreted as a multi-bend-fault-bend fold (Medwedeff and Suppe, 1997) with two foot-wall (FWR1-2) and hanging-wall (HWR1-2) ramps. The β anticline is a folded single ramp fault-bend fold. The syncline (α) is interpreted as an early thrust sheet, folded by a late thrust sheet (ε).

Figure 1: Location map a) Geologic map of the Southern Caribbean thrust front, Venezuela. b) Main structural features at the base of Quaternary unconformity.

Figure 2: Uninterpreted and interpreted seismic section. Numbers 1, 2, and 3 indicate the suggested sequence of deformation of thrust sheets. Horizontal scale equals vertical scale. Section trace shown in Figure 1. Time migrated seismic section displayed in depth.



2-16: Structural restoration To test the validity of our structural interpretation and document the sequence of thrusting, we present a four-stage area balanced restoration of section A-A. The sequence of deformation is summarized as: Stage 0: Full restoration. Stage 1: Footwall shale flow was active in the early stages of emplacement of thrust sheet α until the late emplacement of thrust sheet ε. Stage 2: Emplacement of thrust sheet α. Stage 3: Emplacement of thrust sheet β, γ folded by a late anticline δ. Stage 4: Emplacement of thrust sheet ε that folds thrust sheet α. Total shortening is 3.2 km, distributed as follows: thrust sheet ε = 0.76 km, thrust sheets δ, β, γ = 1.7 km, thrust sheet α = 0.8 km.

Conclusions: • The seismic example shows typical kink-fold geometry. • Folds are interpreted as single- and multi-bend fault-bend folds. • Based on geometric constraints, the sequence of deformation is interpreted as shallow foreland propagating thrust sheets (α, β, γ) that are refolded by late deeper hinterland anticlines (ε, δ). Figure 3: Sequential restoration showing the proposed sequence of deformation. Active faults are indicated by solid red lines. Red dashed lines show fault trajectory prior to displacement.

Acknowledgements: The authors would like to thank Pecom Energia S.A. for the authorization to publish this section (2-16).



2-17: Quirk Creek anticline, Alberta, Canada Steven Lingrey, ExxonMobil Upstream Research Co., Houston, Texas, U.S.A. Location: Southern Alberta Foothills, Canada Topics: Fault-bend folding Reserves: Gas nearby in Lower Carboniferous carbonate reservoirs The Foothills of the Canadian Rocky Mountains have provided many examples of fold and thrust fault structural features considered to be characteristic of the detached contraction of sedimentary layering (Bally et al., 1966; Dahlstrom, 1970; Price, 1981; Boyer and Elliot, 1982). Surface exposures are augmented by extensive seismic reflection profiling and by oil and gas drilling. Fault-related fold mechanisms (Suppe, 1983; Jamison, 1987; Suppe and Medwedeff, 1990) offer a means of more systematic analysis and prediction of subsurface geometry within the context of seismic and well control. The southern Alberta Foothills are particularly well suited for this type of analysis because: 1) the area yields very good land seismic data, 2) well control is abundant (mature gas field drilling province), 3) well-documented and uncomplicated pre-tectonic stratigraphic geometry (Cordilleran miogeoclinal platform strata situated east of the hingeline), and 4) an empirically constrained system of bedding detachment horizons. In seismic data acquired over the northern Quirk Creek area (10 km south of Moose Mountain culmination; Figure 1) a clear example of a hanging-wall ramp (cutoff) is expressed by its time-migrated reflection image (Figures 2, 3). The full uninterpreted seismic line is displayed in Figure 4; its interpretation is displayed in Figure 5.

Figure 1: Location map for the seismic line showing position in inner Foothills between Moose Mountain culmination and Quirk Creek gas field. Paleozoic outcrop is shaded blue.

Figure 2: Detail view of time-migrated seismic image of hanging-wall ramp (cutoff) showing bedding reflection terminations against fault reflection. Lower Paleozoic terminations are clear between A-A and A-A; Upper Paleozoic terminations between B-B are off the end of the seismic line, but are visible in adjacent seismic data. The geometry of fold axes can be inferred in a fault-bend fold sense.

Figure 3: Fault-bend fold structural interpretation of detailed seismic image. Thrust fault trajectory is shown by dashed red line and its dotted projection off the end of the seismic data. Fold axes are shown by dashed green lines; long dashes relate to changes between hanging-wall flats and ramps, short dashes relate to secondary bends in the fault trajectory. Blue lines with arrowheads mark bedding orientations with cut-offs against the fault, blue lines with dashed ends mark bedding roughly parallel to the fault. The isochrons between reflections mark an increase towards the anticlinal crest interpreted to be caused by subseismic small thrust faults.



2-17: Quirk Creek

Figure 4: Post-stack time-migrated 2-D seismic reflection profile across the northern Quirk Creek area. Display is scaled to be 1:1 at an average interval velocity of 4000 m/s. Three wells indicate key stratigraphic tops and positions of fault repetitions. Three stratigraphically calibrated zones of distinctive reflections guide interpretation away from the well control (yellow boxes): 1) a very high-amplitude continuous reflection event (doublet) characterizes the Jurassic-Lowermost Cretaceous Fernie-Kootenay zone immediately overlying the Lower Carboniferous (Mississippian) Rundle Group, 2) a high-amplitude sporadically continuous reflection commonly occurs just above the top of the Devonian Palliser Formation, and 3) a system of higher amplitude reflections, three to four cycles long, indicates the Cambrian strata.



2-17: Quirk Creek

Figure 5: Post-stack time-migrated 2-D seismic reflection profile across the northern Quirk Creek area showing structural interpretation. Blue line marks top of the Rundle Group; dark blue line marks top of the Palliser Formation; pink line marks middle Devonian marker near the top of the Cambrian. Lines are dashed where seismic reflection imaging becomes uncertain. Thrust fault trajectories are marked by heavy, dark red lines. Bedding and faults allow subdivision of the Paleozoic into three layers: 1) Lower Carboniferous (shaded blue), 2) Devonian (shaded dark blue, and 3) Cambrian (shaded pink). A semi-continuous set of high-amplitude reflections interpreted to be the base of the Cambrian reflection set below the basal decollement are shaded light orange. The heavy, dashed orange line is the projected regional position for the base of the Cambrian assuming a flat, gentle (2–3 degrees) surface. The discrepancy between the two orange lines indicates a velocity anomaly that can be correlated to the number (net thickness) of repetitions of Paleozoic carbonates. Beneath the exploration well 2-23-21-6W5, the velocity “pull-up” effect reaches 600 m/s.



2-17: Quirk Creek To a first approximation, the Fernie-Kootenay reflection event separates the Mesozoic rocks above, which show interval velocities of 4000 m/s, from the Paleozoic rocks below, which show interval velocities of 6000 m/s. Imbrication of fast rocks above Paleozoic detachment horizons deflects (pulls up) the unfaulted sub-basal decollement reflection (lowest parts of the Cambrian reflections). The magnitude of pull-up can be matched to the excess thickness of Paleozoic imbricates contained in the overall structure. The profile expression of the anticline conforms to fault-bend fold theory in that its western limb dip is caused by footwall ramps and its eastern limb dips are caused by hangingwall ramps. Deeper elements show duplex-style deformation that slightly modifies the overlying hanging-wall ramp. Conversion of the time image to depth allows the degree of conformance to ideal fault bend fold theory to be measured (Figure 6). The hanging-wall ramp steps up from a lower detachment in the Cambrian to an upper detachment in the Fernie-Kootenay. The upper detachment begins just past the eastern end of this seismic line; its position in the hanging-wall can be traced on other seismic lines to the east where it is observed to place the JurassicCretaceous Fernie-Kootenay strata atop middle and Upper Cretaceous strata. As the hanging-wall ramp crosses the Paleozoic strata, it flattens briefly at an intermediate detachment horizon located at the base of the Lower Carboniferous layer (Banff detachment). Displacement is larger than the horizontal extent of the hanging-wall ramp (> 5 km) so that the cutoffs fully overlie the upper detachment. The corresponding footwall ramp must exist west of the seismic data. Figure 7 shows a restoration of the depth profile and an idealized geometric model using Suppe’s (1983) mathematical constraints of fault-bend folding.

Figure 6: Depth-converted profile of time-migrated seismic image. Time image velocity distortions due to lateral increase in fast Paleozoic rocks relative to slow Mesozoic rocks are removed. Basal fault trajectory consistently overlies Fernie-Kootenay strata and therefore is a footwall flat; the step in the central part of the fault is a bend caused by footwall deformation (northern plunge end of Quirk Creek gas trap). A-A and A-A mark position of lower hanging-wall ramp and B-B marks position of upper hanging-wall ramp.

Conclusions: • Fault-bend fold theory makes a good match with the Quirk Creek anticline observed in seismic data. • Westerly dips arise from thrust sheet strata overlying footwall ramps (cutoffs). • Easterly dips arise from rotation of the leading-edge cutoffs (hanging-wall ramp) onto an upper detachment. • Footwall duplexing complicates, but does not obscure the fault-bend fold.

Figure 7: Restoration and geometric modeling of fault-bend fold geometry. Top section (A) is a restoration of the depth profile using a flexural-slip mechanism. Restoration recovers the primary flat-ramp-flat fault geometry. Lower three sections (B, C, D) show the initial, middle, and final states of a forward geometric model using Suppe’s (1983) fault-bend fold theory. The final deformation state nearly matches depth profile in Figure 6. Second order internal shortening of Devonian and Cambrian layers are ignored by the geometric model.



2-18: Imbricate fault-related folding, South Caribbean Basin, Colombia Freddy Corredor, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. Tomas Villamil1, Exploration Vice-President, Ecopetrol, Bogotá, Colombia 1

Present address: Lukoil Overseas Colombia, Ltd., Bogotá, D.C., Colombia

John H. Shaw, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. Location: South Caribbean Basin, offshore northern Colombia Topics: Conventional and shear imbricate fault-bend folds The South Caribbean basin represents an accretionary prism that resulted from the transpressional collision between the Caribbean and South American plates during the Tertiary. An imbricate thrust system in the southern portion of the basin is clearly imaged with 2-D seismic reflection data, with which we interpret fold and fault geometries and patterns of growth sedimentation. We model this imbricate system using a combination of conventional and shear imbricate fault-related folding theories (Suppe, 1983; Corredor et al., 2002; Suppe et al., 2003), and trishear kinematics (Erslev, 1991; Allmendinger, 1998). The patterns of growth sedimentation that can be observed in this imbricate system are used to further constrain the models.

Figure 1: Regional topography, bathymetry, and tectonic elements of Colombia and location of the Seismic line (1) used for this study.

Seismic data courtesy of ECOPETROL Figure 2: Uninterpreted, migrated, and depth converted 2-D seismic profile across the Fuerte Imbricate System in the South Caribbean Basin, offshore northern Colombia. Two stacked thrust sheets are imaged (see detailed description in Figure 4). We observe two structural patterns that are consistent with break-forward imbricate systems: A) The upper thrust fault appears folded by the underlying thrust sheet, and B) younger growth strata are folded above the frontal thrust sheet.



2-18: Imbricate fault-related folding — Colombia The Fuerte Imbricate System is located in the Southwest Caribbean Basin, beneath a large deltaic system. This deltaic system was fed primarily by the paleo-Atrato, Sinú, and Magdalena rivers. The northern boundary of the imbricate system is the east-west trending Canoas fault. The western limit corresponds to the South Caribbean Deformation front. The deformation in this imbricate system began late during the Miocene(?) and continues through the present day, resulting in its pronounced bathymetry expression (Figure 3). The thrust sheets are composed of Miocene marine shales and turbidite sands (Potential Reservoirs). This imbricate system is detached at the bottom of an Oligocene(?) fine-grained section, which is a potential source rock. This system preserves growth strata that records fault and fold kinematics. These growth sediments are deposited in piggy-back basins formed over the backlimbs of individual imbricates and as onlapping sequences against the forelimbs (Figures 2 and 3). The piggy-back stratigraphy consist of distal marine, fine-grained sediments and condense sections. In the uppermost portion of the seismic profile, a spectacular Pleistocene prograding deltaic sequence can be observed.

Figure 4: 2-D post-stack migrated and depth-converted seismic section through the imbricate system interpreted in this contribution showing some important characteristics including: (1) Sea floor reflection, (2) Growth sedimentation, and thrust faults defined by fault-plane reflections and cutoff (red arrows). Notice how the upper thrust fault is folded across the syncline by the lower thrust sheet.

Figure 3: High resolution sea floor bathymetric image of a region north of the Fuerte Imbricate system interpreted in this contribution, and regional map showing the location of (1) the bathymetric image, and (2) the seismic line across the Fuerte imbricate system. The ridges on the southern portion of the image (3) represent northeast-trending thrust-related folds that are actively deforming the sea floor and controlling the course of meandering turbidity channels (4). The low regions between ridges (5) correspond to the piggy-back basins formed above the backlimbs of individual fault imbricates. On the upper right corner (6), the southern limit of the Magdalena delta system is burying these active folds and faults. The limit between these two systems corresponds to the Canoas Fault (7).

The stratigraphic sequences imaged in the seismic profile (Figure 4) correspond to Tertiary marine and deltaic sediments. At the bottom of the section an Oligocene(?) sequence is interpreted (3), and is composed of thick deep marine shale sequences (potential source rocks), and may contain some interbedded turbidite sands (potential reservoirs in deep water environments). On seismic sections, this sequence is generally devoid of internal reflections. This formation is interpreted to correspond to a weak decollement layer that undergoes an externally imposed shear deformation in this imbricate system. Seismic reflections beneath this sequence are generally continuous laterally, suggesting that the decollement for this system is located at the bottom of the Oligocene(?) shale. The Oligocene(?) sequence is overlaid by Miocene-Pliocene interbedded shallow marine shales and sandstones (4) that produce seismic reflections with higher amplitudes and lower frequencies. In the uppermost portion of the seismic profile, a spectacular Pleistocene prograding deltaic sequence can be observed (5). This deltaic sequence is not folded by the underlying imbricate system constraining the age for the end of deformation in these particular thrust sheets. Fault plane reflections and cutoffs are clearly observed (red arrows) that constrain the geometry of both thrust faults. The upper thrust fault is folded by the lower fault suggesting a break-forward sequence of imbrication. Break-forward imbrication results from a new thrust being developed in the footwall of what was previously the active thrust. 127



2-18: Imbricate fault-related folding — Colombia Folding vectors describe folding shear strains, which are angular measures of the change in orientations of beds and faults across a fold limb or kink band (see section 1B-5). Thus, folding vectors can describe the refolding of overlying thrust sheets due to imbrication (Shaw et al., 1999; see section 1B5). As mentioned earlier, we observe two structural patterns in the seismic image that are consistent with this being a break-forward imbricate system: A) The upper thrust fault appears folded by the underlying thrust sheet, and B) younger growth strata are folded above the frontal thrust sheet. Using folding vectors, we test the idea that the shallow thrust sheet (red arrows in Figure 5A) is folded by a deeper thrust. Slip and shear on the deep thrust has produced multiple kink bands that should have refolded the overlying thrust sheet if this is a break-forward system. Hence, the orientation of the shallow thrust, and beds in its hanging wall, should change as the thrust sheet passes over the underlying kink bands. The folding vector method is used to predict the amount of deflection of the shallow thrust as it is refolded by the two underlying kink bands, which are bounded by axial surfaces (A-A; B-B). If the predicted shape of the fault is consistent with the observed fault shape, it will confirm that this is a break-forward imbricate system. The deflections of bedding across the deep kink bands are used to determine the folding vectors (U and V). Folding vectors are measured parallel to axial surface orientations. In a break-forward system, folding vectors U and V should be equal to the deflections of the shallow thrust described by vectors X and Y, respectively. (Note: This method is based on conservation of shear, and hence line length, parallel to the axial surface orientation). The thrust fault on the right side of the section, before entering the kink band AA, has a dip of approximately 10°. The folding vector U is measured as the deflection of the light blue layer (Figure 5B) across axial surface A in the footwall of the thrust fault. The folding vector U is then used to predict the deflection (X) of the shallow thrust fault (U = X). The predicted dip of the folded fault above kink band A-A is 19°, consistent with the dip of the fault plane observed in Figure 5A. Moving to the left, the thrust fault next enters the kink band B-B at its dip of 19°. The folding vector V is measured as the deflection of the light blue layer across the axial surface B, and is used to predict the deflection (Y) of the shallow thrust fault across the kink band BB (V = Y). The predicted dip (31.5°) of the refolded fault above kink band BB is also consistent with the dip of the fault plane seismic reflection. This confirms that these thrust sheets form a break-forward imbricate system.

Figure 5. Uninterpreted (A) and interpreted (B) close-up view of the Fuerte Imbricate System, offshore northern Colombia, to illustrate how folding vectors (see section 1B-5) are used to interpret this break-forward imbricate system.



2-18: Imbricate fault-related folding — Colombia

Part 2X: Imbricate Systems — offshore Colombia

This imbricate system can be modelled using combined conventional and shear imbricate fault-bend folding theories. Using folding vectors we have shown on the previous page (Figure 5) that this system has a break-forward sequence of imbrication. Additionally, we observe three structural patterns that suggest shear fault-bend folding (see section 1B-4) is an important mechanism in this imbricate system (Figure 6). The thrust sheets show gentle back-limbs that dip less than the fault ramps, growth sediments show evidences of limb rotation, and a broad anticline in the shallow thrust sheet overlies a synclinal bend on the thrust fault. In simple shear fault-bend folding, the weak decollement layer (Oligocene?) at the base of fault ramps undergoes an externally imposed bedding-parallel simple shear. The total slip produced by the shear is accommodated by increasing slip along the fault ramps, and by rotation along the back-limbs. Further frontal imbrication and the transfer of shear produce a decrease in the ramp and bedding angles in younger and shallower thrust faults (Figure 7), occasionally producing folds not directly related to a fault-bend.

Figure 6: 2-D depth-converted seismic section through the imbricate system interpreted in this contribution showing the characteristics that suggest this system involves shear imbricate fault-bend folding: (1) Backlimbs dip less than fault ramps, (2) Small forelimbs compared to backlimbs, (3) Growth sedimentation show evidences of limb rotation, and (4) Anticline is underlaid by a synclinal bend (5) in the associated thrust fault. Notice also that the upper thrust fault is folded across the syncline by the lower thrust sheet, as described on the previous page.

Incipient thrust Fault

Figure 7: Forward model of a break-forward sequence of imbrication by forward distributed transfer of shear showing the resulting patterns of growth sedimentation. Imbricate fault-bend fold theory describes refolding of shallow thrust sheets by younger and deeper faults. A) an incipient thrust. B) and C) a simple shear fault-bend fold grows by increasing simple shear across the weak decollement layer. The growth strata show evidences of limb rotation and kink band migration. D-E) a frontal thrust sheet is formed by simple shear fault-bend folding. This additional shear produces forward (counterclockwise) rotation of the shallower and younger thrust sheet, effectively decreasing the dip values of the fault ramp and folding bedding. A portion of the flat crest of the fold rotates forward, forming a forelimb with no associated fault-bend comparable to that observed in Figure 6. The growth sediments deposited in the earlier stages are also refolded by the younger thrust.



2-18: Imbricate fault-related folding — Colombia

Figure 8: Interpreted depth converted seismic profile across the Fuerte Imbricate System, combining the results from the structural analyses presented on the preceding pages. The stratigraphic correlation is based on regional interpretation of seismic facies across a large seismic survey. The complex geometry of the lower thrust ramp is interpreted to result from an externally imposed simple shear in its footwall (Shear Profile 2) caused by thrust sheets that lie to the northwest of this image. The shallower thrust sheet shows an anticline in the right portion of the section, which is underlaid by a synclinal fault bend. This relationship is interpreted as the result of forward rotation of the bedding and the thrust fault due to the simple shear fault-bend folding in the underlying thrust sheet (Shear Profile 1). Shear faultbend folding is consistent with growth sediments showing evidence of limb rotation, and with back limbs dipping less than the fault ramps. Shearing in the lower thrust sheet has also refolded the shallower thrust fault and the beds in its hanging wall, indicating that this is a breakforward imbricate system.

Conclusions: • The Fuerte structure is a break-forward, shear imbricate fault-bend fold system in the southern Caribbean basin, offshore northern Colombia. • Folding vectors are used to interpret the thrusting sequence. • Several characteristics allow the interpretation of shear imbrication in this system, including: A) beds on the backlimbs dip less than fault ramps, B) growth sediments show evidences of both limb rotation and kink-band migration, and C) an anticline in the shallower thrust sheet is underlied by a synclinal bend in the associated thrust fault. • The Fuerte structure was active early during the Miocene-Pliocene with thrust faults emerging in the sea floor. Further foreland thrusting has sheared this system and passively transported it forward along an Oligocene(?) basal detachment during the Pliocene to the present time. 130 Shaw, Connors, and Suppe


2-19: Oligocene fold belt, western Gulf of Mexico, U.S.A. Thomas W. Bjerstedt1, ChevronTexaco Exploration, Bellaire, Texas, U.S.A. 1Present

address: U.S. Department of the Interior, Minerals Management Service, New Orleans, Louisiana, U.S.A. water bottom

Location: Western Gulf of Mexico; Port Isabel and Alaminos Canyon OCS areas Topics: Detachment anticlines among salt-withdrawl mini-basins, syndepositional tectonism Reserves: Rank exploration area

The Oligocene fold belt is a series of NE-SW–trending detachment anticlines in the western Gulf of Mexico. Detachment folds are down-dip elements of a coupled extensional to compressional transition. Deposition updip of upper Oligocene Frio fluvio-deltaic systems on the Texas coast loaded prodelta and slope environments to the E–SE (Figure 1). Detachment anticlines are common at terminations of low-angle thrusts that cut early Oligocene section. Nickoli is a well-imaged example of such a fold in this structural trend (Figure 2). Two exploration wells were drilled in southern Port Isabel OCS area in 1996 (Figure 3). One well lacked a reservoir and the other lacked charge. 3-D basin modeling suggests that the northern part of the fold-belt trend has more favorable timing relationships for hydrocarbon generation, reservoir deposition, and trap and seal formation.

erosional truncation

Figure 2: (Above) SW-NE cross section through 3-D post-stack seismic volume. The location of the arbitrary line is shown in Figure 5. Shown are over-thickened anticline core (1), backlimb erosional unconformity, and main growth phase of Nickoli structure (2). No vertical exaggeration.

Figure 1: (Left) W-E schematic crosssection along Oligocene depositional dip from Texas outcrop belt to deepwater. Updip fluvio-deltaic and deltaic systems transitioned down dip to slope and basin floor fans in Port Isabel and Alaminos Canyon OCS areas. Exploration targets are detachment anticlines cored by upper Oligocene sandprone fan systems in the Frio. Salt diapirism was contemporaneous with late Oligocene deposition and also influenced facies patterns.

Figure 3: (Left) Map showing the general location of the Oligocene fold belt and the Nickoli structure in Alaminos Canyon OCS blocks 51 and 52. Dots show location of 1996 exploration wells in Port Isabel OCS area.



2-19: Oligocene fold belt, western Gulf of Mexico, U.S.A. A detachment anticline forms Nickoli, a typical Oligocene fold belt structure. A deep seated detachment surface, probably late Eocene, is mappable on 2-D regional seismic throughout the fold belt. The oldest thrusts cut the early Oligocene section. The history of the Nickoli fold included low-angle thrusting that produced an overthickened core, rotation of the thrust backlimb and erosion, the main growth phase, onlap by younger deposits, draping, and eventual burial. The stratigraphic section indicated by (1) in Figure 4 is the overthickened core of the detachment anticline. The section indicated by (2) onlaps the eroded surface and indicates that the main growth phase of Nickoli fold began at the horizon interpreted to be near top of the late Oligocene Frio interval. Frio deposystems are expected to be sand-prone, low-stand basin floor fans. Transgressive, shale-prone, high-stand systems overlying the Frio are recognized in the western Gulf of Mexico as the latest Oligocene Anahuac interval. The youngest thrust was detached along the top Frio erosional surface, possibly within condensed zone(s) at the sequence boundary. Early Miocene deposystems that drape Nickoli and other fold belt detachment structures are expected to be shale prone as sand was diverted into lows until folds were buried completely. Figures 5 and 6 show mapped structure and seismic time slice. water bottom

Figure 5 mapped surface

regional detachment surface

Figure 5: Structure contour map on the top Anahuac (Figure 4 labeled surface). The map contour interval = 500 ft and the grid lines represent OCS blocks. The buttressing effect of the salt diapir to the west rotated the Nickoli anticlinal axis to an east-west orientation. Salt withdrawl from diapirs that surround the Nickoli mini-basin overprinted the basin-center structures with normal faults.

Figure 6: Showing about the same area as Figure 5. Seismic time slice near 3,050 m (10,000 ft) showing structural configuration and position of the youngest thrust fault.

Figure 4: Annotated seismic showing regional detachment surface and detachment anticline formed by thrust faults, (1) overthickend core of regressive Frio sand-prone deposystems (middle to late Oligocene), and erosional unconformity at the top of the Frio, and (2) onlap of Frio by Anahuac transgressive shale-prone deposystems (latest Oligocene). Early Miocene deposits drape the fold until completely buried.



2-20: Edge-Sigsbee Folds, Gulf of Mexico, U.S.A. Robert J. Alexander1, Thomas W. Bjerstedt2, and Sharon L. Moate3, ChevronTexaco, Bellaire, Texas, U.S.A. 1Present

address: BHP Billiton, Houston, Texas, U.S.A.; 2Present address: Texaco Minerals Management Service, New Orleans, Louisiana, U.S.A; 3Present address: Consultant, Bellaire, Texas, U.S.A.

Location: Western Gulf of Mexico, Keathley Canyon OCS area Topics: Detachment anticlines, Sigsbee Escarpment Reserves: Rank exploration area Low-relief folds at the edge of the Sigsbee Escarpment (toe of the bathymetric slope) have prompted questions about their origin and seismic imaging. There are two likely hypotheses for how these structures formed: 1) they are not “real,” and their seismic expression results from velocity anomalies caused by abrupt changes in thickness of the salt canopy at the edge of the Sigsbee and rapid water deepening onto the abyssal plain; and 2) they are formed by a combination of lateral compression, detachment folding, and isostatic response to salt sheet and sediment loading, (i.e., foreland bulge analogy). A velocity anomaly origin (hypothesis #1) is tenuous where these folds are continuous inboard and outboard of the salt edge. Furthermore, these folds do not occur everywhere along the salt edge and Sigsbee Escarpment. An origin due to gravity-gliding and lateral compression with down-dip lateral shear on a Paleogene detachment surface (hypothesis #2) is more likely. Good quality 3-D prestack depth-migrated seismic data in the salt canopy reentrant of Keathley Canyon show that a low-relief edgeSigsbee anticlinal structure is cored by a subtle, incipient imbricate duplex which thickens the section, arching the overlying sediments into a probable detachment anticline. There is seismic evidence for coupling of the laterally prograding salt canopy with subjacent sediments. Compression at the Sigsbee Escarpment (toe of the slope) is caused by salt and sediment movement toward the abyssal plain, which forms a duplex of thrusts above an over-pressured basal detachment surface. The overlying sediments arch in response to this structural thickening as is seen in foreland “triangle zones.” Although we don’t offer this explanation for all edge-Sigsbee structures, the interpretation should be considered when wellimaged pre-stack depth-migrated seismic data is available to test hypotheses.

Figure 1: (Above) 3-D pre-stack depth-migrated seismic line down the axis of Keathley Canyon and showing a fold and the leading edge of the Sigsbee salt canopy (light blue). Figure 2: (Right) Bathymetry of the deepwater Gulf of Mexico showing the location of Keathley Canyon, the Sigsbee Escarpment and the seismic line. Red color is ~600' (183 m) and blue color is ~9000' (2744 m) water depth.



2-20: Edge-Sigsbee Folds, Gulf of Mexico, U.S.A. Velocity modeling of 3-D pre-stack depth-migrated seismic line through Keathley Canyon shows a velocity inversion and inferred over-pressured zone. A detachment surface occurs at the base of a late Paleogene shale unit. Edge-Sigsbee detachment anticlines are formed by lateral compression of the sedimentary section as the salt canopy expands onto the abyssal plain generating an over-pressured zone of low strength.

Figure 3: Velocity model along annotated seismic line showing velocity inversion and inferred over-pressured section; A) Normal model, B) High velocities are clipped to better show a subtle velocity inversion in the Tertiary age section. In Figure 3A, red color is ~14700 ft/s (4480 m/s) and blue color is ~4980 ft/s (1518 m/s). Datum is near base Paleogene. Figure 5: Interpreted and annotated seismic line showing a low-relief, detachment anticline in subsalt sediments at the edge of the Sigsbee Escarpment. Also shown are interpreted faults, and a late Paleogene shale unit that is interpreted as a detachment surface at the base of an inferred over-pressured zone. Velocity analysis modeling for pre-stack depth migration and pore pressure prediction analysis identified the velocity inversion, which probably continues further northward under the salt than is shown in Figure 3.

Conclusion:

Figure 4: Enlarged inset from Figure 5. Note duplex structure in the sediments below the base of salt (blue). Compression occurs where the base of salt is “stepped,” which confirms some degree of coupling with subjacent sedimentary units.

Detachment anticlines with incipient duplex structures can form the cores of low-relief, four-way closures at the edge of the Sigsbee Escarpment, western Gulf of Mexico. Our model invokes a detachment surface in an overpressured section coupled with south-directed compression due to gravity gliding of the overlying salt+sediment load. The suprajacent section is arched by the developing duplex structure in the core of the fold (i.e., between purple and orange lines). There is no evidence for compressional deformation south of this seismic line (left side) indicating this is analogous to a triangle zone in the foreland of fold and thrust belts.



2-21: Fault–related folding in reactivated offshore basins, California Carlos Rivero and John H. Shaw, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. Location: Inner California Borderland, California, U.S.A. Topics: Basin inversion fault reactivation, fault-bend folding, structural wedges, blind-thrust We present seismic examples that illustrate basin inversion processes in the Inner California Borderland (ICB), offshore southern California. The ICB developed by Miocene crustal-scale extension dominated by a pair of regional low-angle detachments (Crouch and Suppe, 1993; Nicholson et al., 1993). During the Pliocene, the onset of the modern transpressional regime generated several large contractional fault-related fold trends spatially controlled by reactivation of these detachments (Rivero et al., 2000). We use a dense grid of industry seismic reflection profiles (Figure 1), and fault-related fold theories to analyze an anticline structure within these trends. The San Mateo anticline developed by the upward propagation of reverse slip during the inversion of Miocene half-grabens. Based on the analyses of kink-band panels, and growth and pre-growth sequences, we propose a structural interpretation for this fold consistent with seismic and well data. In addition, the structural interpretation provides insight into the kinematics of the basin inversion processes.

Figure 2: Time-migrated seismic reflection profile across the San Mateo Anticline. Note the regional oceanside detachment (1) extending beneath the San Mateo Anticline (2). This detachment is not folded by the contractional structures; thus we interpret that the San Mateo Anticline is formed by thrusting ramping up from this detachment surface. A preserved extensional rollover is also visible on the left-hand side of the section (3).

Figure 1: Location of the San Mateo trend in the Inner California Borderlands. The study area is defined by the grid of seismic reflection data. Yellow circles are well locations. L.A. = Los Angeles Basin. Figure 3: Time-migrated seismic reflection profile across the San Mateo Anticline. Stratigraphic tops are correlated from the San Clemente CH–1 well. Well-illuminated cutoffs and fault plane reflections (1) constrain the location of a reactivated normal fault and overlying thrust ramps that form the San Mateo structure. Note the Miocene syn-rift section penetrated by the well that expands toward the normal fault in a rollover structure. The rollover structure shows evidences of bivergent tectonic inversion or “bipolar extrusion” (Copper and Williams, 1989; Hayward and Graham, 1989), with both fore (1) and backthrust (2) anticlines developed by the inversion. Gently dipping continuous reflections and three-dimensional mapping define the location of the Oceanside Thrust, one of the regional Miocene detachments reactivated in the Pliocene (3). Syn-extensional deposits and unconformities define the presence of other normal faults (4) that are not inverted.



2-21: Fault-related folding in reactivated offshore basins, California Seismic interpretation

Figure 4: Balanced structural interpretation of the San Mateo Anticline. The interpretation highlights the relationship of the contractional structure to pre-existing normal faults reactivated during the phase of basin inversion. Kink-band domains in the back-limb of the anticline, and direct fault plane reflections constrain the geometry of the San Mateo thrust from shallow to deeper levels, where it is linked to an older normal fault. The seismic image also indicates that the San Mateo ramp is refolded by a younger, deeper fault. We interpret that both thrust faults terminate in structural wedges, as no foreland structures that could account for the transfer of slip exist beyond the San Mateo anticline. Formation tops from the well San Clemente #1. Labeled axial surfaces correspond to those modeled in Figure 6.

We interpret the San Mateo Anticline as an imbricated fault-bend fold produced by the upward propagation of contractional slip from an inverted normal fault into multiple detachment levels (Figures 4, 5, and 6). The backlimb geometry of the anticline exhibiting multiple dip-domains indicates the presence of a deeper structure. This sub-thrust structure refolds the shallow thrust sheet of the San Mateo Anticline in a way consistent with a break-forward system (Figure 6). The thrust front terminations of the San Mateo thrust and the underlying thrust are defined by two structural wedges that propagate slip back to the hinterland. At this location, the interaction between the synclinal axial surfaces of the upper detachments produces a complex geometry of the thrust front.

Stage 1: The San Mateo Thrust forms and slip produces the kink-bands A-A and B-B that define the shallow San Mateo Anticline. A structural wedge in the thrust termination of the upper detachment transfers slip back to the hinterland. Stage 2: Development of the sub-thrust structure with minor displacement that generates incipient kink-band C-C. A lower structural wedge is also formed in the thrust front position, analogous to San Mateo thrust. Stage 3: Final configuration of the imbricated system, consistent with a break-forward model (Shaw et al., 1999). Displacement on the deeper thrust refolds the San Mateo thrust sheet, and forms a structural wedge (triangle zone) at the western border of the fold belt.

Figure 5: Restoration of the proposed structural interpretation for the San Mateo anticline to the top of the Pliocene Repetto Formation. The restoration highlights the role of the extensional system controlling the geometry of the Miocene depocenters, and locating the Pliocene compressional structures. Estimated total shortening is 2.5 km.

136

Figure 6: Balanced sequential model of the development of the San Mateo structure.



2-21: Fault-related folding in reactivated offshore basins, California 3-D modeling Integration of seismic data, fold-related folding theories, and 3-D visualization techniques are used to illustrate the complex of basin reactivation along the San Mateo trend. Modeling of the shallow and deep structural styles, represented by the San Mateo thrust, the backthrust, and the basal Oceanside thrust highlights the role of the regional structural wedge in generating the contractional foreland and hinterland-directed structures.

San Mateo Anticline

Anticline trends associated with the Inversion

San Mateo Thrust

Bac k-T hru st

Figure 7: (a) Oblique view of a three-dimensional model incorporating a representation of the San Mateo thrust, and the structural wedge defined by the Oceanside thrust and the back thrust fault. Seismic image corresponds with profile Y-Y shown in Figure 3. The blue surface is the top of the syn-rift sequence (Monterrey Formation). (b) Seismic dataset used in the definition of the surfaces shown in the 3-D model. Contours represent bathymetry of the seafloor. (c) Same view as in (a) with the seismic image removed to highlight the lateral continuity of the structural wedge, as well as the contractional folding of the Monterrey Formation. Oceanside Thrust

Conclusions: • The San Mateo anticline is an imbricated fault-bend fold originated by basin inversion processes. The San Mateo thrust reactivated a segment of a northeast-dipping Miocene normal fault.

• The phase of basin inversion also reactivated a Miocene lowangle detachment as the oceanside thrust. The oceanside thrust transferred contractional slip to associated synthetic and antithetic normal structures, inverting a major grabenboundary fault, and generating a regional structural wedge defined by the oceanside thrust and a backthrust zone. This structural wedge controls the location of a prominent monocline with bathymetric expression.

• The structural style varies considerably across these inverted basins. In some areas, the pre-inversion geometry of the Miocene basins has not been modified, as it is expressed in well-developed rollovers preserved in the hanging wall of lowangle and high-angle normal faults. In contrast, uplifting and folding of the sedimentary fill, and reactivation of half-grabens, document the later phase of basin inversion. Footwall and hanging wall short-cuts associated with reverse and thrust faults are also documented by the seismic data. 137



2-22: Coalinga anticline, San Joaquin basin, California, U.S.A. Chris A. Guzofski, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. John H. Shaw, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. Location: Western San Joaquin basin, California, U.S.A. Topics: Structural wedges, backthrusting, imbrication Reserves: 906 Mbbl of hydrocarbons from lower to middle Miocene Temblor Formation The Coalinga anticline is located in an active fold and thrust belt in the western San Joaquin basin, California (Figure 1) (Namson and Davis, 1988; Stein and Ekström, 1992). At the surface the Coalinga structure is expressed as a southwest-vergent anticline that is defined by a narrow forelimb with a broad backlimb (Figure 2). We interpret that this structure developed above a northeast-dipping thrust ramp. At depth, the anticline is northeast-vergent and structural relief across the anticline provides evidence that a deeper, southwest-dipping ramp has uplifted the anticline. Herein, we use these observations to interpret this structure as a structural wedge, having grown through multiple stages of fault-bend folding.

Figure 1: Landsat TM image of the Coalinga anticline showing the locations of the seismic lines used in this study. Locations of wells 1. Pleasant-Valley #1; 2. LeavittHintze #1; and 3. PVF-11X are shown from Meltzer (1989) and Bartow (1990).

Figure 2: Migrated and depth-converted seismic profile with several wells showing formation tops across the Coalinga anticline. The structural relief between the synclines (1 and 2) bounding the anticline (3) provides evidence that one or more southwest-dipping thrust ramps underlie the structure. The asymmetry of the central anticline (3) demonstrates that an additional northeast-dipping thrust ramp underlies the structural crest. The absence of Tertiary deformation east of the Coalinga anticline (beyond this section) provides evidence that the southwest-dipping fault ramp or detachment does not extend basinward of the Coalinga anticline. This argues for the presence of a structural wedge, where slip is sent back to the hinterland on the inferred backthrust beneath the Coalinga structure.



2-22: Coalinga anticline We interpret the Coalinga anticline as being comprised primarily of a stack of imbricated structural wedges (Figure 3). Two structural wedges, with separate dipping forethrust ramps and a common upper detachment surface forming a backthrust, generate the gross morphology of the Coalinga anticline (Figures 3 and 4). The structural relief across the Coalinga anticline (between Pleasant Valley and the San Joaquin basin; Figure 3) is due to the accumulation of slip and uplift on the Coalinga thrust ramp, where the tip of the wedge is pinned by an active synclinal axial surface (B0). Similarly, structural relief across fold A4 to A5 in the San Joaquin basin is due to slip on the San Joaquin thrust ramp (Figure 4). The prominent forelimb of the Coalinga anticline (defined by A0 to A1 in Figure 3) records slip on a fault that has branched off of the upper detachment surface.

Figure 4: Migrated seismic line showing the location of the active synclinal axial surface (A4) in the San Joaquin basin, which is used to constrain the tip of the structural wedge associated with the San Joaquin thrust ramp (inset).

Figure 3: Balanced structural interpretation of the Coalinga anticline, in which several imbricated faults generate the main fold. Slip on the Coalinga and San Joaquin ramps generates two anticlinal fault-bend folds, where slip is sent back to the hinterland on folded backthrusts. The width of the forelimb of the Coalinga structural wedge is constrained by a pair of axial surfaces (B0 and B1), where the wedge tip is pinned by the active synclinal axial surface (B0). Imbrication of two older and shallower thrusts by the Coalinga wedge is demonstrated by the “capture” of fold limbs associated with these older faults by the Coalinga structural wedge. The forelimb of one of these older structures is constrained by axial surfaces C0 and C1. Growth strata within this kink band indicate that slip on its causative fault occurred at some point between the deposition of the Moreno shale (~ 65 Ma), and the deposition of the Kreyenhagen shale (~ 37 Ma), clearly before the development of the broad limb (B0-B1) that refolds it. A shallow thrust that branches off the main detachment generates the prominent anticlinal fault-bend fold defined by kink bands A0 and A1. The dip of this thrust ramp was determined based on the forelimb dip using fault-bend folding theory. However, the observation that the backlimb dips less than the fault ramp suggests that the backlimb is deforming by shear fault-bend folding mechanisms (see section 1B-4). Deformation of a shear band pinned by axial surface A2 (shaded yellow), leads to a minor rotation of the backlimb. The location of a regional angular unconformity is shown by yellow arrows. The axis of this unconformity (i.e. where rocks change from horizontal to dipping) is shown by a dashed yellow line. Formation depths are from Bartow (1990).



2-22: Coalinga anticline Here we present a balanced sequential model of the development of the Coalinga structure (Figure 5).

Figure 5A: Initial geometry of the Cretaceus (and older?) sedimentary sequence beneath the Coalinga structure. The development of a dipping panel beneath the shallow angular unconformity (shown by yellow arrows) is possibly related to slip on an unimaged fault (shown by queried red dashed line) that steps up to a local detachment.

Figure 5B: Initiation of slip (between 65 and 37 Ma) on the Coalinga ramp makes a structural wedge involving a detachment and a backthrust. Slip generates an anticline above the backthrust, with growth triangles associated with both the forelimb of the wedge (defined by axial surfaces C0-C1) and the forelimb above the upper detachment/backthrust.

Figure 5C: Initiation of the San Joaquin thrust ramp and development of the southwest-vergent anticline (defined by axial surfaces A0A1) by slip on the backthrust associated with the nascent San Joaquin structural wedge. The low angle of the backlimb of the southwest-verging anticline, relative to the underlying ramp, is due to simple shear fault-bend folding where e = 61° (see section 1B-4).

Figure 5D: Slip on the Coalinga thrust ramp generates a structural wedge (whose forelimb is defined by axial surfaces B0- B1) that captures and refolds the kink bands associated with the deformation modeled in panels B and C. The backthrusts of the Coalinga wedge and the San Joaquin wedge merge at the regional detachment, as the summed slip is sent back to the hinterland.

Conclusions: • The Coalinga structure is underlain by two independent southwest-dipping thrust ramps that generate two structural wedges that sole into a common backthrust/roof thrust. • The Coalinga structural wedge refolds at least two older structures, a Tertiary structural wedge with well-defined growth strata and a younger southwest-vergent anticline that has accumulated slip from the San Joaquin thrust ramp. While this southwest-vergent anticline is the most prominent surface feature of the Coalinga structure, slip on the two underlying thrust ramps produce the deeper fold architecture.



2-23: Wedge structure, Nias Basin, Sumatra, Indonesia Peter A. Brennan, Tellumetrics LLC, Sugar Land, Texas, U.S.A. John H. Shaw, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A. Location: Nias basin, Sumatra, Indonesia Topics: Structural wedge, growth structure, inversion We describe a complex structure located in the Nias basin, which lies between Nias Island and the southwest coast of Sumatra (Figure 1). Nias Island lies along the presentday plate margin between the Indonesian and Indian Ocean plates. The basin contains Miocene and younger sedimentary rocks deposited over a basement composed of an earlier Tertiary subduction complex. The basin underwent a period of extension during the early and middle Miocene, and a subsequent phase of contraction during the late Miocene, Pliocene, and Pleistocene. The structure we describe has been structurally inverted, such that it reflects both extensional and contractional components. We interpret the structural geometry and kinematics of this anticline using patterns of syntectonic growth strata, structural relief, and fault cutoffs (Figure 2).

Figure 2: Post-stack, time-migrated and depth converted seismic reflection profile of the Nias basin that images a contractional fault-related fold. The structure is composed of a monoclinal fold limb that is underlain by a fault, which appears to offset basement and uplift the southern portion of the fold. Two distinct stratigraphic sections (1 and 2) thicken to the north across the fold limb, suggesting that they are syntectonic (growth) strata.

Figure 1: Map of Sumatra showing the location of the Nias basin.



2-23: Structural interpretation and growth section 1

A: Seismic Example: Sumatra, Indonesia

We propose a structural wedge model (Figure 3) to explain the geometry of the structure and the pattern of growth section 1 (Figure 4). Based on interpreted fault cut-offs and structural relief, the main thrust ramp beneath the Nias structure dips to the south (Figure 4A) indicating that the monoclinal fold limb is a forelimb. The forelimb is bound by a roughly linear synclinal axial surface that extends upward through growth section 1, and by a curved anticlinal axial surface that has different orientations in growth and pre-growth sections (Figure 4A). Based on this axial surface pattern, we interpret the forelimb as a growth structure developed by kink-band migration, with an active synclinal axial surface and an inactive anticlinal axial surface (see section 1A-3). Given the fault dip direction, this growth patterns is inconsisitent with a simple forelimb fault-bend fold model (Figure 3A), but consistent with a decollement wedge model (Figure 3B). Thus, we interpret the structure as a decollement wedge (Figure 4B).

Kinematic models A: anticlinal fault-bend fold

B: decollement wedge

B: Interpreted section

Figure 3: Balanced kinematic models of an anticlinal fault-bend fold (A) and decollement wedge (B). In model A, the fault-bend fold is developed above a ramp that flattens to an upper decollement. The anticlinal axial surface is active, and thus linearly extends through pregrowth and growth sections. The synclinal axial surface is inactive, and thus changes orientation at the boundary between growth and pregrowth section (see section 1B-1). In model B, slip on the upper detachment is transferred to a backthrust forming a structural wedge (Medwedeff, 1989) (see section 1B-6). The synclinal axial surface is active and the anticlinal axial surface is inactive, in contrast to model A. Thus, simple anticlinal fault-bend folds and structural wedges can be readily distinguished based on patterns of growth strata.

Figure 4: Seismic reflection profile across the Nias basin structure with: A) basic interpretations of the fault position, axial surface traces, and growth sections; and B) structural interpretation. The geometries of the synclinal and anticlinal axial surface resemble those in Figure 3B, indicating that the structure is a decollement wedge. In this interpretation, the synclinal axial surface is pinned to the wedge tip, and the backthrust generally conforms to the bed dips in its hanging wall. This interpretations is consistent with the pattern of growth section 1, but does not yet explain the origins of growth section 2.



2-23: Structural interpretation and growth section 2 The structural wedge interpretation presented on the previous page (Figure 4B) explained the pattern of growth section 1, but did not address the origin and pattern of growth section 2. Two scenarios may explain this older growth structure. First, the older growth structure may reflect an early phase of contractional folding above the forethrust, developing as a structural wedge or perhaps a fault-propagation fold. Alternatively, the early growth structure could represent synrift fill of an extensional half graben developed by normal motion on the fault. Growth section 2 is middle Miocene in age, corresponding to a period of regional extension. Thus, we prefer the second scenario to explain the origin of the older growth structure. This implies that the structure is inverted, with a middle Miocene phase of extension followed by an upper Miocene phase of contraction. This structural inversion in modeled in Figure 5 and interpreted on the seismic section in Figure 6.

Interpreted section with inverted normal fault

Kinematic model

Figure 6: Interpreted seismic profile, showing an inverted half graben in the core of the Nias anticline. Growth section 2 is interpreted as synrift strata, similar to the model shown in Figure 5.

Conclusions: • Nias anticline formed by inversion of a Miocene normal fault and associated half graben. • Thrust motion on the inverted normal fault is transferred to a backthrust at the base of the post-rift sequence, forming a structural wedge. • Patterns of folded syntectonic growth strata were used to decipher the inversion history, and to support our kinematic interpretation of this structural wedge. Figure 5: Sequential kinematic model (stages 0 through 5) of the development of the Nias anticline. Model 0 shows an incipient normal fault and active axial surface. Slip on the normal fault (models 1–2) generates a roll-over panel and half graben, which is filled with synrift strata equivalent to growth section 2. In model 3, strata are deposited above the half graben after rifting has ceased. In models 4 and 5, the lower segment of the normal fault is reactivated as a thrust, which propagates up dip and shallows to a detachment at the base of the postrift sequence. Slip is transferred to a backthrust that is parallel to the overlying strata, forming a structural wedge.



2-24: Interference Structure, Gulf of Mexico, U.S.A. Rion H. Camerlo, ChevronTexaco, New Orleans, Louisiana, U.S.A. Thomas W. Bjerstedt, Minerals Management Service, New Orleans, Louisiana, U.S.A. Edward F. Benson, NuTec Energy Services, Stafford, Texas, U.S.A. Location: Perdido fold belt, western Gulf of Mexico, U.S.A. Topics: Kink-band interference and linkage, mechanical stratigraphy, kink-fold class detachment folds Reserves: Rank exploration area The Perdido fold belt (Figure 1) is a deep-water fold belt created by updip extension and sediment loading on the Texas Gulf Coast. Line A-A (Figure 2) shows two large anticlines within the Perdido fold belt that are bounded by tabular bands of angular folded units (kink bands) along both limbs (Figures 3 and 4). The inner kink bands intersect, producing an interference structure (Figure 5). This structure is strikingly similar to the model geometry developed by Medwedeff and Suppe (1997) of a counter-clockwise interference structure, and that modeled by Mount (1989) and Novoa et al., (1998). The natural structure deviates from this ideal parallel geometry due to the mechanical stratigraphy of the deforming units. Shortening across the kink bands (Figure 6), in general, shows two shortening maxima, one to the northeast on the western kink band and one to the southwest on the eastern kink band. The maxima are likely initiation points of the bands and demonstrate that kink band shortening is acting in relay, wherein the western kink band loses displacement to the south and the eastern kink band gains displacement to the south. The interference structure formed in the overlapping zone of the relay. In detail, the interference structure is a zone of diminished shortening that may be the result of inhibition of kink band growth in the zone of interference. The steep shortening gradient northeast of line B-B may result from the difficulty in forming an interference structure above the weak unit. A second shortening minima is seen in the western kink band at line D-D as well as a distinct swing in orientation. These are the result of linkage of two synthetic (with respect to dip) kink bands during kink band growth (Line C-C). This is similar to the interaction of faults, fractures, and folds in plan view. Both kink bands in the structure are more narrow above the interpreted green horizon in Figure 7 than below it, and consequently accommodate less shortening. The unit between the yellow and green horizon also has appreciable deformation induced thickness changes. This unit is interpreted as a secondary detachment horizon in the lower Eocene (a known detachment level across south Texas).

Figure 1: Perdido fold belt and area of interest for kink-band interference structure, western Gulf of Mexico. Uninterpreted profile plane vertical seismic section A-A stretched to depth, located in Figure 2. 3-D post-stack time migrated seismic courtesy of WesternGeco.



2-24: Interference structure, Gulf of Mexico, U.S.A.

Figure 2: Map of green horizon on seismic sections. Dashed blue lines are axial surfaces (kink plane intersections with the mapped horizon), the two kink bands discussed in this section (2-24) are in bold. Contour interval is 200 ft. Figure 3: Simplified fence diagram of lines A-A through D-D (Figures 4 and 7). The axial surfaces of the three kink bands are shown in different colors for clarification. Figure 4: Profile-plane vertical seismic sections B-B through D-D, uninterpreted and interpreted. Colored dashed lines are axial surfaces. All normal faults are omitted for simplification. Line C-C shows the linkage of kink band “X” and kink band “Z”. Line B-B’ shows distortion of the kink bands above the lower Eocene detachment at their intersection point prior to crossing in line A-A (Figure 7). No vertical exaggeration.



2-24: Interference structure, Gulf of Mexico, U.S.A. Vertical seismic profile shows interpreted structure resulting from the interference of two kink bands and it is similar to structures observed in nature. Displacement gradients along the kink bands in map view are modified by the interaction of the interfering kink bands and influenced by the mechanical stratigraphy.

Figure 5: Line drawing of a natural kink-band interference structure in black Carboniferous slates in the Beara Peninsula, Cork, Ireland, after Dewey, 1965. Dewey reported disharmonic folding at interference points.

Figure 7: Line A-A interpreted. Dashed blue lines are axial surfaces (kink planes). Kink band annotation follows the naming convention of Medwedeff and Suppe (1997). The two tan colored horizons were added to show details of the kink bands’ intersection. An additional kink band (labeled 33X11T), and requisite branch points P5 and P6, are deviations from the model geometry. No vertical exaggeration.

Conclusions:

Figure 6: Color-filled contours of amount of shortening across kink bands, in ft. Total displacement across both kink bands is shown in red text. Blue arrows indicate the location of measurement locations in addition to those of the cross sections.

• The reflection seismic data illuminates megascopic-scale kink bands. • The geometric fold model of kink-band interference agrees well with the observed structure and is a very useful tool in interpretation. • Appreciable deviations from the geometric model result from effects of the mechanical stratigraphy. Weak layers influence, and may control, the location of kink-band intersections. • Kink bands intersect and interact in relay similar to published examples of faults, folds, and fractures.



2-25: End members of gravitational fold and thrust belts (GFTBs) in the deep waters of Brazil Pedro Victor Zalán, PETROBRAS/E&P/E&P-CORP/TSP, Rio de Janeiro, Brazil Location: Deep waters of Brazil Topics: Gravitational fault-related folding, growth strata

Gravitational fold and thrust belts (GFTBs) associated to linked extensional-compressional systems occur in the deep waters offshore Brazil, and show two end members regarding structural and syndepositional styles. One end member, related to longer-lived linked extensional-compressional systems, is dominated by low rates of sedimentation. Thus, deformation rates are also low, giving rise to fold belts with growth folds topped by a series of younging- and steppingupward time-transgressive unconformities separating strongly deformed (below) and non-deformed (above) strata of the same age (growth strata). This fold belt type is well illustrated by a seismic section from the Pará-Maranhão Basin, as well as by another seismic line from the Barreirinhas Basin (Figure 1). The other end member, relat-

Figure 1: Location map of the gravitational fold and thrust belts (GFTBs) studied in this work. AM = Amazon Mouth Basin, PM = ParáMaranhão Basin, BA = Barreirinhas Basin, PE = Pelotas Basin.

ed to short-lived, linked extensional-compressional systems, is dominated by high rates of sedimentation. Thus, deformation rates are also higher, giving rise to very thick, harmonically folded and thrusted sedimentary strata, displaying simpler syngrowth relationships. In this case, thick syn-tectonic packages are deposited in the synclines and thinner (or absent) correlative packages on the anticlines. Time-transgressive unconformities are markedly absent. This type is illustrated by seismic sections from two major Miocene-Recent progradational sedimentary cones: the Amazon Mouth and the Rio Grande (Figure 1). The four cases presented in this section (2-25) are shale-detached/shale-cored fold belts.

Development of Passive Margins and GFTBs Continental margins build outward into deep and ultra-deep waters via denudation of the adjoining shields and deposition of the resulting debris, forming the continental shelves and slopes. The rifted/thinned edge of the continental plates cool exponentially as they move away from the heat source (mid-oceanic ridges) that initiated break-up of the continental plate. These continuous events create a very unstable situation since large volumes of sediments pile up at the margin of the continental shelves, in the upper slope, at the same time the whole area is gradually tilting oceanward due to thermal flexural bending. Large deltaic deposits of major rivers may create similar unstable conditions. Gravity failure occurs and allochtonous masses of sediments slide down the slope, over a ductile lithology that detaches the traveling rocks above from the autochtonous rocks below. When the frontal parts of the allochton diminish their velocity due either to a decrease in the gradient of such

Figure 2: Depth-migrated seismic section from the Pará-Maranhão Basin illustrating a complete linked extensional-compressional system. See interpreted section in Figure 3.



2-25: GFTBs in the deep waters of Brazil detachment zone or to a physical barrier (commonly a volcanic edifice) the incoming allochtons collide and strong contraction/compression occurs. The sliding of huge masses of recently deposited, slightly indurated sedimentary rocks takes place along well defined, seismically evident, closely spaced detachment zones, nucleated in ductile beds with regional distribution; salt or thick laminated shales. The detachment zones provide the linkage between the extensional and compressional provinces. When in significant volumes, the ductile beds may be involved in the folding, giving rise to huge diapir-nucleated folds. The nature of the detachment zone is the main factor determining the structural style of the associated GFTB. They may be of two strikingly different types: (a) salt-detached/salt-cored fold belts, e.g. the Perdido and Mississippi Fan fold belts (GOM) (Trudgill et al., 1999), and (b) shale-detached/shale-cored foldbelts, e.g. the Mexican Ridges (GOM) (Trudgill et al., 1999), Amazon Cone (Silva and Maciel, 1998) and Niger Delta (Hermann, 1998) fold belts. The failure occurs when vertical stresses due to overburden are weakened in relation to subhorizontal stresses due to several possibilities, including overpressure in shales (due to petroleum generation or any other classical overpressure mechanism) or ductile flow in salt. Shear stresses develop parallel to the slightly dipping bedding and overcome the vertical stresses.

The translational domain is a predominantly non-deformed region that passively traveled over the detachment zone. Weak arching may affect the rocks present in this area. Usually, increasing amounts of detachment folding occur oceanwards/basinwards, marking the passage of the translational domain into the compressional realm. The compressional domain may present spectacular deformation, with all kinds of reverse and thrust faults and fault-related folding (detachment, fault-propagation, and fault-bend folding). When detached on shales, the structural styles, the structural relief, and the overall dimensions may resemble those found in truly orogenic belts (Zalán, 1998). When salt is the lubricant, or is otherwise involved, deformation is more complex and salt tongues and canopies (Rowan et al., 2001) or nappes (Hudec et al., 2001) develop. The specific name gravitational fold and thrust belts (GFTBs) has been applied to such entities. Zalán (1999) studied some Brazilian GFTBs in detail and devised a tripartite structural model that predicts an orderly succession, from the internides to the foreland, of detachment folding, followed by closely spaced high-angle reverse faults and associated tight fault-propagation folds (also referred to as toe thrusts), ending in more widely spaced, low-angle rampflat thrusts with associated more open fault-bend folding. Important oil discoveries have been achieved in these compressional provinces in deep waters off GOM, Nigeria, Angola, and Brazil. The dimensions of these three domains may vary greatly. Usually the extensional and compressional domains are the widest but it is very difficult to exactly balance the amount of extension updip with the amount of contraction downdip, because of the details of the severe deformation that is usually non-resolvable by seismic data. Since they cover huge areas, on the order of several thousand square kilometers, it is difficult to have them all covered by 3-D seismic, and it is not unusual that extension and compression are divided into two or three belts of deformation.

These deformed masses of allochtonous rocks are referred to as linked extensional-compressional systems and have been found in the deep/ultra-deep water regions of most continental margins around the world. It is easily understood, and very well displayed in modern seismic sections, that these systems are composed of three major tectonic domains, each one presenting different and peculiar deformation (Figure 3). The extensional domain comprises highly strained subsided terrains at the upper continental slope, dominated by arcuated listric normal faults that sole out at the detachment level. Major listric faults present significant associated rollover anticlines and growth depositional wedges that thicken from the crest of the anticline towards the listric fault. Subsidiary listric normal faults, antithetic to the major downdip listric faults, are also abundant, as well as crestal fracturing/faulting in the rollover anticlines.

Figure 3: Depth-migrated seismic section from the Pará-Maranhão GFTB. The major components of a linked extensional-compressional system are clearly visible: The extensional, translational, and compressional (GFTB) domains and the linking detachment zone.



2-25: GFTBs in the deep waters of Brazil Petroleum Potential of the GFTBs The potential of these compressional structures for petroleum exploration seems to be very high. Structural closures are usually four-way and on the order of tens of square kilometers, and vertical reliefs are on the order of hundreds of meters. Common reservoir targets include vertically stacked, laterally confined, porous turbidite sandstones, deposited in meandering channels, that are encased within shales in the hearts of the anticlines. Detachment seems to preferentially take place in weakened, highly pressurized organic-rich shales located in oil generation windows. Reverse/thrust faults that splay upwards from the detachment zones into the folds serve as migrating routes for the ascending, released hydrocarbons. When salt is the detachment media, salt windows are required to allow migration from sub-salt source rocks. Traps are the fault-related folds and petroleum fields in deep water GOM, Nigeria, Angola and Brazil have been found in all three major types of contractional fault-related folds (detachment, fault-propagation and fault-bend anticlines).

Figure 4: Detailed view of the GFTB shown in Figure 3, displaying the internal architecture and structural styles of the compressional domain. Notice predominance of fault-bend folding in the more external part of the fold belt (yellow arrows), fault-propagation folding in the middle part (pink arrows), and detachment folding in the more internal zone of the fold belt (green arrow). Also notice onlapping pattern and thinning upward of sub-horizontal sedimentary packages deposited upon the frontal (right) part of the GFTB, and the thinning upward and deformed (folded) nature of the depth-equivalent packages on the back (left) portion of the GFTB.

GFTBs with Growth Folding When the process of gravity sliding/contraction is long-lasting (20–50 m.y.) and takes place in areas with low rates of sedimentation, growth folding occurs. Since GFTBs develop in exclusively submarine environments, they are never subaerially exposed, sedimentation takes place concomitantly with the compressional deformation leading to the deposition of syntectonic growth strata that thin up onto the upper parts of the fold belt, in the same way the growth wedges develop in the downthrown sides of the normal faults in the extensional domains. Medwedeff (1989) unraveled complex growth stratigraphic relationships between coeval sediments deposited in the forelimb and backlimb of a fault-bend fold in California. Numerous wells and seismic data allowed the author to deduce that syntectonic sedimentary strata onlap a time-transgressive unconformity on the forelimb but are folded below the unconformity on the backlimb. The same mechanism seems to be applicable to GFTBs in the Brazilian Equatorial Atlantic margin, such as the Pará-Maranhão and Barreirinhas GFTBs. Figure 3 shows a depth-migrated seismic section from the ParáMaranhão Basin, where a complete, fully developed linked extensionalcompressional system can be seen.

Figure 5: Geological interpretation of section displayed in Figure 4. Detachment zone and reverse/thrust faults are shown. Two reflectors (aorange and b-light green) were tracked within the interpreted pre-tectonic section, and three reflectors (c-blue, d-purple, and edark green) were tracked within the interpreted growth section, which is also highlighted by a gray transparent mask. Stepping- and youngingupward unconformities (U) are displayed in yellow.



2-25: GFTBs in the deep waters of Brazil Within the GFTB fault-bend folds are concentrated in the frontal part while fault-propagation folds occur in the middle part (Figure 4). Detachment folds occur in the innermost part of the fold belt. Three reflectors can be tentatively correlated across four time-transgressive unconformities (dark yellow) (Figure 5). These reflectors (dark green, purple, and blue) are interpreted to encompass the growth strata associated to this GFTB. They thin onto the highest topography of the fold belt and thicken away into the lower surrounding areas. In contrast, lower beds involved in the compressional deformation (light green and orange reflectors) show constant thickness throughout the fold belt and are interpreted as pregrowth strata.

Figure 6: Time-migrated seismic section from the Barreirinhas GFTB. Two major open folds associated to thrust faults (red arrows) can be clearly seen and are covered by sub-horizontal onlapping sedimentary packages (yellow arrows).

The geometry of the deformation in this GFTB suggests that the folding and uplift of the thrust strata was a long-lived process. Since there is a major time-transgressive unconformity (as well as three minor ones) that separates the non-deformed strata above from the deformed strata below (Figure 5) it is plausible that the rates of sedimentation were low. We estimate that the time span involved in the sedimentation of the growth strata is around 45 m.y. Deformation started slightly below reflector c (Figure 5) (roughly Late Maastrichtian) and ended slightly above reflector e (Figure 5) (roughly Late Oligocene) (sedimentation rate about 35–40 m/m.y). Thus, the deformed strata were left exposed at the sea bottom several times and submarine erosion (currents) could take place.

Figure 7: Geological interpretation of section displayed in Figure 6. Detachment zone and two major thrust faults are shown in the center and right portions (external portion of foldbelt) of the section. The two major anticlines are interpreted as fault-bend folds. Active (blue) and inactive (pink) axial surfaces are shown in each fold. Towards the more internal parts of the fold belt, deformation is more complex, consisting of tight higher-angle reverse faults and associated folding (suggestive of fault-propagation folds). Deformed strata are topped by a major and a secondary upward climbing unconformity (yellow). Interpreted growth section is highlighted by a gray transparent mask. Depth scale is valid for the central portion of the seismic line.

In a less spectacular manner, the Barreirinhas GFTB display faultbend folds covered by a major unconformity (there is also a minor associated stepping-upward unconformity), upon which sub-horizontal beds onlap and thin upwards (Figures 6 and 7). The geometry suggests that the same growth fault-bend folding mechanism described in the Pará-Maranhão GFTB may work here. The same pattern of a series of younging- and stepping-upward timetransgressive unconformities separating non-deformed onlapping strata above from thrusted and folded strata below can be seen in several other GFTBs in Brazil (e.g. Touros, in the Potiguar Basin; Zalán, 2001) and elsewhere in the world (for instance, in the Krishna-Godavari Basin, in India, Stuart and Hickman, 2001). We suggest that these patterns are diagnostic of gravity sliding/contraction accompanied by growth folding in areas dominated by low rates of deformation and sedimentation.



2-25: GFTBs in the deep waters of Brazil GFTBs Without Growth Folding Some GFTBs do not display the complicated pattern of time-transgressive unconformities as described above. They involve thick packages of sediments that are folded and thrusted harmonically. Syntectonic sedimentation seems to follow a simpler pattern of being confined to intervening synclines between anticlines. In this case, the syncline packages typically thicken downward towards the depocenter and thin upward towards the anticlines. Such is the case in the GFTBs related to the Amazon Mouth (Figures 8, 9, and 10), to the Rio Grande Cone (Figures 11, 12, and 13) and to the Niger delta, where all allochtonous sediments are harmonically folded and thrusted up out in the sea bottom. They are situated in front of young (Miocene) and huge deltas where enormous piles of sediments accumulated very quickly while gravity sliding was taking place during the same short time.

Figure 9: Detailed view of the GFTB shown in Figure 8, displaying the internal architecture and structural styles of the compressional domain. Tight fault-propagation folds associated with reverse faults dominate the external (right) part of the fold belt, while detachment folds nucleated by shale diapirs constitute the dominant style in the internal (left) part of the fold belt.

The pattern of harmonically folded and thrusted sediments, with thick syn-tectonic packages in the synclines (Figures 10 and 13) and thinner correlative packages on the anticlines, and more importantly, the absence of time-transgressive unconformities, are here interpreted as being diagnostic of gravity sliding/contraction in areas dominated by high rates of deformation and sedimentation.

Figure 10: Geological interpretation of section displayed in Figure 9. Detachment zone, reverse faults, and shale diapirs are shown. Growth strata (highlighted by a gray transparent mask) are ponded in synclines, thinning upward towards the flanks of anticlines. Notice the remarkable absence of extensive time-transgressive unconformities throughout the whole sedimentary section, in clear contrast with the sections illustrated in Figures 5 and 7. Only a very minor unconformity (yellow) can be seen in the easternmost flank of the fold belt.

Figure 8: Time-migrated seismic section from the Amazon Mouth GFTB. Extensional domain is only partly shown. Translational and compressional domains are fully displayed.



2-25: GFTBs in the deep waters of Brazil Conclusions:

Figure 12: Detailed view of the GFTB shown in Figure 11, displaying the internal architecture and structural styles of the compressional domain. A train of harmonically folded anticlines dominate the fold belt. Fault-propagation folds associated to reverse faults are preponderant in the external (right) part of the fold belt, while a detachment anticline, partly ruptured by reverse faults, can be seen in the internal (center) part of the fold belt.

• Gravitational fold and thrust belts associated to extensional-compressional systems linked via detachment zones nucleated in shales, in the Brazilian deep and ultra-deep waters, show two end members as related to structural styles and syntectonic sedimentation. • GFTBs developed in continental margins dominated by low rates of sedimentation/deformation display a zonation of fault-bend folding in the more external parts passing through fault-propagation folding and to detachment folding as one moves backwards into the internal zones. Stepping-upward time-transgressive unconformities cover the folded/thrusted assemblages and are onlapped in the frontal parts of the folds by sub-horizontal growth strata, whose time-equivalent packages are involved in the compressional deformation below the unconformities in the back limbs of the innermost folds.

Figure 13: Geological interpretation of section displayed in Figure 12. Detachment zone and reverse faults are shown. Growth strata (highlighted by a gray transparent mask) are ponded in synclines, thinning upward towards the flanks of anticlines. Notice the absence of extensive time-transgressive unconformities throughout the whole sedimentary section, in clear contrast with the sections illustrated in Figures 5 and 7. Only a very minor unconformity (yellow) can be seen in the easternmost flank of the fold belt, similarly to the Amazon Mouth example (Figure 10).

Figure 11: Time-migrated seismic section from the Rio Grande Cone. Extensional domain is only partly shown. Translational domain is practically non-existent. Compressional domain is fully illustrated.



2-25: GFTBs in the deep waters of Brazil Conclusions:

(cont.)

• GFTBs developed in areas dominated by high rates of sedimentation/deformation, usually associated to major deltas (as is the case in the examples shown in the Amazon Mouth and Rio Grande Cone), display intense folding, dominated by detachment and fault-propagation folding. These structures typically have very high structural relief, and seafloor expression. The intense, rapid, and continuous process of sedimentary loading/sliding/ contraction does not allow the development of unconformities chiseling the higher parts of the foldbelt. Consequently, there are practically no sub-horizontal strata covering the deformed rocks. Syntectonic sediments are concentrated in the synclines, situated between the intervening trains of anticlines. The major implications for such differences in the depositional/structural styles of the growth strata are in the location of the turbidite beds and the related hydrocarbon traps. In the first case, syntectonic turbidites should be present as onlapping strata (stratigraphic traps) above unconformities in the frontal parts of the GFTB and in the folds (structural traps) in the internal parts of the GFTB, below unconformities. In the second case, all syntectonic turbidite deposits will tend to be channelized bodies that are thicker in the synclines and on the flanks of the anticlines (mixed stratigraphic/structural traps), and thinner or absent up on the crests. Eventually, inverted depocenters due to shifting of deformation locus, from the outer to the inner parts, may uplift such turbidite channels into the core of younger anticlines. Pretectonic turbidites may be present in the core of folds anywhere in the GFTB.

Acknowledgments I would like to thank Petrobras for the permission to publish this work and my colleagues Haroldo M. Ramos, Sergio Rogerio P. da Silva, Alvaro Henrique A. de Castro, Desiderio P. Silveira, Sergio de O. Guimarães, and Marcia de B. Pimentel for their help in the processing, interpretation, and drawing of the seismic sections. 153 Part 2: Case Studies


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