made A14_1995_AAPG_folds httpsearthsci.stanford.eduresearch[geomechPeopleDavatzes_diss.pdf.pdf

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ABSTRACT

The majority of folds in extensional tectonic set-tings are associated with normal fault systems overa wide range of scales. The hinges of longitudinalfolds are subparallel to associated faults, whereasthose of transverse folds are oriented at a highangle to the related normal faults. Longitudinalfolds include drag folds (hanging-wall synclines andfootwall anticlines), reverse-drag folds (hanging-

wall anticlines and footwall synclines), and rolloverfolds (hanging-wall anticlines). Drag folds form as aresult of fault propagation into monoclinal warpspresent at the fault tips; some may form as a resultof frictional drag and differential compaction.Reverse-drag folds develop because displacementdecreases with distance from the fault surface.Rollover folds are produced by movement alonggravity-driven listric faults in thick sedimentary suc-cessions. Drag folds have a smaller wavelength thanreverse-drag or rollover folds and may be superim-posed on these larger structures. Basin-scale syn-clines are the largest type of transverse folds andare manifested in plan view by basin outlines thatare concave toward the border fault. These foldsform because displacement is greatest near the cen-ter of the map trace of the border fault and decreas-es toward its along-strike ends; this is a scale-invari-ant feature of most normal faults. Transverse foldsare also associated with segmented fault systems:hanging-wall synclines are located near the centersof fault segments, whereas hanging-wall anticlinesare found at segment boundaries where the fault

segments commonly overlap. Some transverse fomay be caused by movement along an undulatfault surface. Many folds in extensional tectonic tings form syndepositionally and control strathickness and facies relationships.

INTRODUCTION

Folds have long been recognized as import

structural traps. Most folds are associated with ctractional tectonic environments, transpressioregimes, and salt diapirism. However, many tyof folds and flexures are also present in extensiotectonic settings. Fault-parallel folds, such as dfolds and rollover folds (e.g., Hamblin, 19Harding, 1984), have received the most attentionthe literature, and form as a result of movementthe associated normal fault systems. Fault-perpdicular folds have been recognized since the wof Wheeler (1939) but have received consideraless attention. These folds also form in responsedisplacement on normal fault systems (eWithjack and Drickman Pollock, 1984; Schlisc1993) and, as shown in this paper, are as command important as fault-parallel folds.

This paper reviews the types, geometry, amechanisms of formation of folds and f lexureextensional settings; it also reviews the featuresnormal fault systems relevant to folding. Tgeometries of the various types of folds are illusted with examples from the Basin and RanMesozoic rift system of eastern North America, North Sea, and the Gulf of Suez. Emphasis is plaon fault-perpendicular folds. The significancethese folds to hydrocarbon exploration is abriefly addressed.

FEATURES OF NORMAL FAULTS

Recent research on normal fault systems has dumented that displacement is commonly greatesor near the center of the fault and decreases to zat the fault tips (Figure 1a; Chapman et al., 19Muraoka and Kamata, 1983; Barnett et al., 19

1AAPG Bulletin, V. 79, No. 11 (November 1995), P. 16611678.

Copyright 1995. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received December 27, 1994; revised manuscript receivedMay 10, 1995; final acceptance June 30, 1995.

2Department of Geological Sciences, Rutgers University, Busch Campus,Piscataway, New Jersey 08855-1179.

Research was supported by the National Science Foundation (EAR-9017785), a Henry Rutgers Research Fellowship, and Grants-in-Aid-of-Research from Sigma Xi and the Geological Society of America. My views offold geometries in extensional settings benefited from discussions with MarkAnders, Paul Olsen, Dave Reynolds, Alan Roberts, and Martha Withjack. Ithank Dave Reynolds for sharing unpublished data with me. Rolf Ackermann,Albert Bally, James Lowell, and John Shelton critically reviewed themanuscript.

Geometry and Origin of Fault-Related Folds inExtensional Settings1

Roy W. Schlische2


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Walsh and Watterson, 1987; Dawers et al. , 1993;Young et al., 1995). For isolated normal faults thatdo not intersect the Earths surface (blind faults),the zero-displacement contour (tip-line loop) of thefault plane has an elliptical geometry (Figure 1b),with the short axis of the ellipse parallel to the slipdirection. For large normal faults, the elliptical faultsurface geometry may be truncated by the Earthssurface and/or the base of the seismogenic crust.

The displacement field described in the previousparagraph is a scale-invariant feature of normalfaults. For the faults illustrated in Figure 1ac, thelengths of the faults vary from a few tens of metersto tens of kilometers. Similar displacement geome-tries have been observed on micro-normal faultswi th leng ths in the range of 0.5 cm to 100 cm(Young et al., 1995). Because of this displacementgeometry, faulted layers in the hanging wall areexpected to exhibit a synclinal geometry, and thosein the footwall exhibit an anticlinal geometry(Figure 1c). Given the scale independence of fault-displacement geometry, associated folds should besimilarly scale independent.

For large (kilometer-scale) faults, the synclinaldepression defines an elongate sedimentary basin.

The uplifted footwall may be an important sourceof sediment for the basin. The relative amounts offootwall uplift and hanging-wall subsidence areva riabl e; ra ti os of 1:1, 1:2, and 1:5 have beenreported in the literature (Stein and Barrientos,1985; Barrientos et al., 1987; Stein et al., 1988). Forisolated blind normal faults, the amounts of hang-ing-wall subsidence and footwall uplift are approxi-mately equal (Gibson et al., 1989). For faults that

intersect the Earths surface, the relative amount ofhanging-wall subsidence increases as the dip of thefault decreases (Gibson et al., 1989). For largefaults, footwall uplift is a consequence of displace-ment geometry plus isostatic effects (Jackson andMcKenzie, 1983). Isostasy is not likely to be respon-sible for the footwall uplift observed on meter-scaleand smaller normal faults.

In addition to variations in fault displacementalong strike, displacement also decreases with dis-tance normal to the fault surface, resulting in areverse-drag geometry in both the hanging wall andfootwall (Figure 1d) (Barnett et al., 1987). Thisgeometry is a manifestation of the elastic and flexu-ral (for large faults) response to faulting (see reviewin Roberts and Yielding, 1994). The distance fromthe fault at which fault displacement is negligible isknown as the reverse-drag radius, which increaseswith increasing displacement (Gibson et al., 1989).Along any given profile normal to the strike of thefault, displacement is greatest at the fault surface,generally near the center of the fault, and decreasestoward the fault tips and away from the fault itself.For isolated blind normal faults, this displacementgeometry results in deformation of units in the vol-

ume surrounding the fault (Muraoka and Kamata,1983; Barnett et al., 1987). There are four quad-rants of deformation, two of which are dilationaland two of which are contractional (Figure 1d).This deformational geometry probably affects per-meability and porosity within the faulted volume(Barnett et al., 1987).

Within the last decade, considerable attentionhas been focused on the scaling relationship

1662 Folds in Extensional Settings

0 10 20 30 40 50

1

Distance along fault (m)

Vertical

displacement(m)

10 km

FW cutoff

HW cutoff

Cross fault

Fault surface

No VE

(a)

500 m

1.5

1.9TWTT(sec.)

2.0

1.6

1.4

0

30

0

0.5

VE=12x

(b)

(c) (d)

+

+

-

-

1.3

1.8

Figure 1Displacement geometry associated with normal faults. (a) Vertical displacement derived from variation inscarp height on a normal fault within the Volcanic Tableland, California. Modified from Dawers et al. (1993). (b)Contours of equal fault displacement in milliseconds for normal fault imaged on a grid of closely spaced seismicreflection profiles from the North Sea. Modified from Barnett et al. (1987). TWTT = two-way traveltime. (c) Horizon-separation diagram for the Tiim Phonolite on the Saimo fault in the Kenya rift valley. Modified from Chapman et al.(1978). FW = footwall; HW = hanging wall. (d) Reverse-drag folds produced by decrease in displacement with dis-tance from a blind normal fault. Deformation quadrants are dilational (+) and contractional (). Modified from Bar-nett et al. (1987).


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between fault length and displacement. Building onthe work of Elliott (1976), Watterson (1986) docu-mented a positive relationship between fault lengthand displacement, and interpreted these results toindicate that faults grow in length as displacementaccrues. The general scaling relation is D = cLn,where D is displacement, c is a constant related torock properties, L is length, and n is some expo-nent. Considerable debate surrounds the value ofn; reported values range from 2 (Watterson, 1986;Walsh and Watterson, 1988) to 1.5 (Marrett andAllmendinger, 1991; Gillespie et al., 1992; Walshand Watterson, 1992) to 1 (Cowie and Scholz,1992a, b; Dawers et al., 1993; Young et al., 1995).Al though the va lue of n affects the manner inwhich faults grow through time (Schlische, 1991;Schlische and Anders, in press), controversy aboutits exact value, if there is a unique value, does notchange the fact that faults increase in lengththrough time and that associated folds must changethrough time as well.

Normal fault systems on a variety of scales com-monly consist of multiple fault segments (e.g.,Schwartz and Coppersmith, 1984; see recentreviews by Gawthorpe and Hurst, 1993; Andersand Schlische, 1994; Davison, 1994; Childs et al.,1995; Schlische and Anders, in press). Individualsegments of the fault system commonly exhibit thesame displacement geometry as isolated normalfaults (Dawers and Anders, 1995), although dis-placement gradients may be higher near segmentboundaries (Peacock and Sanderson, 1991).

Summing of displacements of individual faults cmonly results in a displacement profile for entire fault system that resembles the displacemprofile of an isolated fault (e.g., Walsh aWatterson, 1991; Trudgill and Cartwright, 19Dawers and Anders, 1995), suggesting that faults are kinematically linked and that the fault tem evolved through the growth and linkage

originally isolated fault segments (Anders aSchlische, 1994). A very common type of fault lage involves overlapping faults in which displament is transferred from one fault to another vzone of ductile deformation termed a relay ram(Larsen, 1988; Peacock and Sanderson, 19Trudgill and Car twright, 1994). Segment bouaries may be recognized by overlapping faults, foffsets, significant changes in fault strike, redudisplacement, and differences in the age of faulton either side of the segment boundary (eZhang et al., 1991).

Roberts and Yielding (1994) recognized thbroad types of normal faults. (1) Small normal fa

are confined entirely within the seismogenic laythese are mostly planar but may exhibit changegeometry as the faults pass through differlithologies (Peacock and Zhang, 1993). (2) Lanormal faults penetrate the entire sei smogelayer, are planar with moderate dip angles (Stand Barrientos, 1985; Jackson, 1987; Jackson aWhite, 1989), and are commonly associated whalf -grabentype sedimentary basins (eRosendahl, 1987); flexural effects of faulting, smentation within the hanging-wall basin, and esion of the uplifted footwall are important (eJackson and McKenzie, 1983; Stein et al., 1988).Gravity-driven normal faults typically form witthick, usually regressive sedimentary sequen(passive margins, deltas); are commonly stronlistric and detached in weak, commonly overpsured sedimentary intervals; and create growstructures (e.g., Shelton, 1984).

FAULT-RELATED FOLDS IN EXTENSIONALSETTINGS

The majority of folds in extensional tectoenvironments are associated with normal fault tems. Longitudinal folds have hinges that are palel or subparallel to the strike of the fault (Fig2a), and are best observed in sections perpendlar to the fault. Transverse folds have hinges tare perpendicular and subperpendicular to strike of the fault (Figure 2b), and are bobserved in sections parallel to the fault. Traverse folds are commonly elongated parallel to fault; the term transverse only strictly appliesthe orientation of the hinge with respect to

Schlische 1

(a)

(b)

Figure 2Idealized geometry of longitudinal (a) andtransverse (b) folds.


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fault. Other folds are possible in extensional set-tings, but these typically are associated with strike-slip movement or later compressional events.

Longitudinal Folds

Drag FoldsDrag folds are longitudinal folds that are gener-

ally restricted to the region immediately adjacentto the fault surface (Figure 2a). Synclines form inthe hanging walls of normal faults; anticlines arefound in the footwalls. Drag folds form as a resultof the lateral and upward propagation of faults (a

consequence of fault growth) into regions thathave been monoclinally flexed at the fault tips(Figure 3) (e.g., Hancock and Barka, 1987; Walshand Watterson, 1987). Thus, drag folds may beconsidered fault-propagation folds (see Mitra,1993). Drag folds may also form as a result of fric-tional drag along the fault surface (e.g., Hatcher,1994). Unfaulted folds at the tips of normal faultsare termed forced folds by Withjack et al.(1990). Drag and forced folds are common in theGulf of Suez (Figure 3a, b) (e.g., Robson, 1971;With jack et al ., 1990 ), th e Norwegi an marg in(Withjack et al., 1989), and the Rhine graben(Laubscher, 1982). Drag folds and related folds


(c)

(b)

1 km

pC

PKn

Kum

Kuc

Te

Te Kum

Tmp

pC

Kum

Kuc

Te

pC

PKn

Kum

Kuc

Te

pC

PKn

Kum

Kuc

Te

(a)

(1)

(2)

(3)

W E

PKn

Figure 3(a) Drag (forced)folds and associated basement-penetrating normal fault zonefrom the eastern El-Qa plain,Gulf of Suez. Box shows outcropdata. pC = Precambrian

basement; PKn = NubiaSandstone (CarboniferousUpperCretaceous); Kum = Upper

Cretaceous limestone and shale;Kuc = Upper CretaceousPaleocene shale and limestone;

Te = Eocene limestone; andTmp = MiocenePliocene units.Modified from Withjack et al.(1990). (b) Idealized crosssections illustrating evolutionof forced folds in the Gulf of Suezas a result of upward faultpropagation. Modified from

Withjack et al. (1990). (c)Fault-propagation fold associated

with lateral migration of fault tip.Modified from Walsh and

Watterson (1987).


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may form adjacent to all three major types of nor-mal faults previously discussed.

Reverse-Drag FoldsAs the name implies, reverse-drag folds have the

opposite geometry of drag folds in that they formhanging-wall anticlines and footwall synclines(Figure 4a). In contrast to drag folds, reverse-dragfolds are more areally extensive. As noted previous-ly, reverse-drag folds are a manifestation of thedecrease in displacement with distance from thefault surface (Figure 1d), which is an elastic and, insome cases, f lexural response to faulting. Reverse-drag radius increases with fault displacement. Asfaults grow in size through time, the associatedfolds should grow in width and amplitude as well.In general, the amplitude of hanging-wall reverse-drag anticlines is higher than the amplitude of foot-wall reverse-drag synclines (Figure 4a). Drag foldsmay be superimposed on reverse-drag folds, result-ing in composite folds. Reverse-drag folds are typi-cally associated with small and large faults. Thereverse-drag geometry is also produced by seismicslip on normal faults (Figure 4b; Stein andBarrientos, 1985).

Rollover Folds Associated with DetachedNormal Faults

Rollover folds (Figure 5) are common withinthick sedimentary successions associated with pas-sive margins, notably the Gulf Coast region and theNiger delta (see references and examples in Bally et

al., 1981; Shelton, 1984; and Roberts and Yieldi1994). Rollover folds form as a result of movemon concave-upward (listric) faults and are thus extensional equivalents of fault-bend folds (Fig5a, b). Movement on these faults creates a potenvoid between the hanging wall and the footwthe hanging wall then collapses into the v(Figure 5a). In reality, a void is never producbecause fault movement and hanging-wall defortion occur simultaneously. Because the faults asciated with rollover folds form in non-lithifmaterials, which have little strength, footwall fosimilar to those associated with reverse drag generally absent (Roberts and Yielding, 1994), footwall deformation may occur in responsedeformation of underlying shales (A. W. Bally, 19personal communication).

The geometry of a rollover fold is controlledthe shape of the fault surface and the mechanby which the hanging wall deforms to fill potential void. Deformation mechanisms incluvertical simple shear (e.g., Gibbs, 1983), inclinsimple shear (e.g., White et al., 1986), and flexuslip (Davison, 1986). The type of deformatmechanism may depend on the lithologies beaffected (Rowan and Kligfield, 1989). Kink-tyfolds are produced where the listric fault consof planar segments (Figure 5b; Groshong, 19Xiao and Suppe, 1992). This fault geometry mresult from differential compaction (Shelton, 19Davison, 1987) or as a result of refraction as fault passes through layers of differing pore-flpressures and mechanical properties (e.g., Murr1961; Bradshaw and Zoback, 1988).

Schlische 1

Changeinelevation(m)

1000 m

VE=4x(a)

0 10 20 30 40 50

Distance along survey line (km)

10 km

0.0

-1.0

+0.5

-0.5

-1.5

0.0

-1.0

+0.5

-0.5

-1.5

A'

B

B'

A

Fault splay

B AA'

B'

1983 Borah Peak

Earthquake

NVE=9660x

(b)

Figure 4(a) Reverse-dragfolds in both hanging walls anfootwalls of normal faults withcoalfields of the Dudley basin,

Australia. Modified from Barneet al. (1987). (b) Changes inground-surface elevationperpendicular and parallel tothe Lost River fault system, Ida

following the 1983 Borah Peakearthquake. Dots indicate statialong survey transects. Compathe geometry in cross section A

with that shown in Figure 1d, the geometry in cross-section B

with the hanging-wall cutoffin Figure 1c. Modified fromBarrientos et al. (1987).


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Numerous balancing models have been pro-posed in the literature to reconstruct fault geome-try based on the rollover shape or vice versa (seerecent reviews by Dula, 1991; Withjack andPeterson, 1993; and Groshong, 1994). Some recentmodels have incorporated the effects of com-paction (e.g., Rowan and Kligfield, 1989; Whiteand Yielding, 1991), involved multiple faults (deMatos, 1993), and extended the reconstruction tothree dimensions (e.g., Kerr et al., 1993). Robertsand Yielding (1994) have cautioned that such bal-ancing techniques should only be used in recon-structing the geometry of detached normal faults

and should not be applied to basement-penetratingfaults.

Given the geometric similarity, the termsrollover and reverse drag are used interchange-ably in the literature. If a distinction is drawn, it isthat rollover folds involve a reversal in dip direction,whereas reverse drag involves a steepening of dip(e.g., Shelton, 1984). Recall that reverse drag (as

used in this paper) is the elastic-flexural responseto tectonic faulting in lithified rock, whereasrollover folds are associated with gravity-drivenlistric faults. Given the fundamental mechanicaldifference between these two types of faulting, Irecommend that the terms not be used interchange-ably and that rollover only be applied to long-itudinal hanging-wall anticlines associated withgravity-driven listric faults, even if there is not a dipreversal.

The presence of rollover folds is a good indicatorthat the associated fault is listric (Figure 5c) (e.g.,Hamblin, 1965). However, as already noted,rollover and reverse-drag anticlines have nearly

identical geometries. Reverse-drag anticlines maybe mistakenly interpreted to be rollover folds, andthe associated fault may be erroneously inferred tobe listric. As originally noted by Barnett et al.(1987), the presence of reverse-drag folds does notimply the presence of listric faults. In fact, mostreverse-drag folds are associated with planar oronly mildly listric faults. An additional complicationis that the listric appearance of faults on seismicreflection profiles may be caused by velocity pull-ups (Withjack and Drickman Pollock, 1984).Distinguishing between rollover and reverse-dragfolds relies on the following: (1) rollover folds areto be expected in thick sedimentary successions inassociation with detached normal faults; (2)reverse-drag folds are present in both the footwalland hanging wall, whereas rollover folds are gener-ally found only in the hanging wall. It may be nec-essary to construct vertically exaggerated profilesto identify footwall reverse drag, which is of small-er amplitude than hanging-wall reverse drag forfaults that intersect the Earths surface.

In some cases, seismic data show folds superim-posed on rollover folds (Figure 5c, d). Gibson et al.(1989) suggested that these drag-like folds formedas a result of differential compaction. According toXiao and Suppe (1992), drag-foldlike structuresmay also be produced by movement along convex-upward normal faults.

Transverse Folds

All types of transverse folds result from along-strike variations in fault displacement, which isa scale-invariant feature of normal faults. Thus,


(b)

(a)

1

2

3

5 km

E

1000 m(c)

W

1s

(d)

Figure 5Rollover folds associated with movement on a

smoothly curved listric fault (a) and a kinked fault (b)(modified from Xiao and Suppe, 1992). (c) Rollover andnormal-drag folds in the hanging wall of the Big Springsfault, Arizona. Modified from Hamblin (1965). (d)Rollover fold associated with listric fault in the BohaiGulf of northern China; note the drag fold adjacent tothe normal fault. Modified from Zhang (1994).


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transverse folds also exist on a variety of scales. Thestyle of transverse folding depends largely on thegeometry of the normal fault system (isolated faultvs. segmented fault system).

Folds Associated with an Isolated FaultIn the hanging wall of a single isolated fault,

along-strike variations in fault displacement pro-duce a broad, elongated syncline that plungestoward the fault (Figure 2b); for a large isolatedbasin-bounding fault, the syncline defines a sedi-mentary basin (Figure 6a). A broad, elongated anti-cline, plunging away from the fault, is present inthe footwall (Figures 2b, 6a). The fold hinges forboth folds should be collinear and are located inthe region of maximum fault displacement (Figures2b, 6a). The amplitude of the syncline is commonlygreater than the amplitude of the anticline, as hang-ing-wall displacement is commonly greater thanfootwall displacement. As a result of fault and basingrowth, the amplitude of the folds and the width of

the folds increase through time, although the hingelines are likely to remain relatively fixed (Figure6a). Hinge-line migration may occur if the area ofmaximum displacement shifts position throughtime, which might be recognizable by the presenceof curved axial surfaces (Schlische and Anders, inpress).

Folds Associated with Segmented FaultSystems

Segmented fault systems are commonly associat-ed with multiple displacement minima and maxima(Figure 6be). For the hanging wall of the fault sys-tem, synclines form at local displacement maxima,typically located at or near the centers of the faultsegments; anticlines form at local displacementminima, typically located near fault segmentboundaries (Figure 7a). The synclines are consider-ably wider than the anticlines, which are common-ly associated with relay ramps between overlap-ping fault segments. The concept of relay rampsand displacement transfer between overlappingfaults can largely be traced to Larsens (1988) studyof the geometry of segmented normal fault systemsin Permian rifts of Greenland (Figure 7b), wheremultiple transverse folds are found in the hangingwall (Figure 7c).

In the footwall of a segmented fault system, anti-clines form at local displacement maxima, which,when present, are typically located at or near thecenters of fault segments; synclines form at localdisplacement minima, which, when present, aretypically located at fault segment boundaries. Theextent to which transverse folds are developed inthe footwall and the hanging wall depends on the

number of active fault segments, the stage in evolution of the fault system, and the nature of fault segment geometry. According to Anders Schlische (1994) and Schlische and Anders press), fault geometry for multiple-segment fasystems can be classified as (1) nonoverlappi(2) closely overlapping, and (3) widely overlping. For each broad class, additional geomet

are possible depending on whether the faults synthetically or antithetically. The evolutionmodels for the specific cases shown in Figurewhich are based on Sch lische and Anders press), apply to basin-scale fault systems. Howevas noted previously, the displacement geometrare scale invariant.

In the case of nonoverlapping synthetic fauisolated synclinal subbasins originally form in hanging wall (Figure 6b, stage 1). Footwall upprofiles define broad anticlines associated with isolated fault segments. As displacement accruthe fault tips propagate laterally, and the fault sments eventually link together. At this stage,

synclinal subbasins are separated by a transvehigh located in the region of low displacemwhere the faults linked together (Figure 6b, st2); the opposite fold geometry exists in the fowall. Eventually, displacement must increase in zone of linkage so that the maximum displacemon the fault system conforms to length-displament scaling relationships (Anders and Schlisc1994). Thus, the transverse high gradually subsideventually resulting in a single elongated synclin the hanging wall and a single elongated anticlin the footwall (Figure 6b, stage 3).

The initial geometry of the structures associawith closely overlapping synthetic faults is simto that of nonoverlapping synthetic faults (Fig6c, stage 1). As the fault tips propagate laterally begin to overlap, a transverse anticline forms aseparates two synclines in the hanging wall (Fig6c, stage 2); a transverse syncline forms in the fowal l in the area of overlap. According to Andand Schlische (1994), the transverse anticlinethe hanging wall will persist as long as multifault segments are coevally active so that slip is tributed on multiple faults, thus reducing amount of hanging-wall subsidence (Figure stage 3). If the overlapping faults physically ltogether, then the transverse high will eventusubside. Anders and Schlische (1994) suggesthat the footwall elevation low will gradually dispear, even if multiple overlapping fault segmeremain active (Figure 6c, stage 3). This is becathe footwall elevation profile reflects the total placement on the fault system (particularly displacement at depth), whereas the near-surfaceplacement on the nearest fault segment determithe depth of the hanging-wall basin.

Schlische 1


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(a) (b)

(c)

(d)

(e)

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1 23

1 23

1 23

12

3

123

Figure 6Evolution of extensional basins resulting from fault growth. Each panel shows three stages in the hang-ing-wall evolution of an extensional basin or subbasin system; ball-and-bar symbols indicate normal faults. Longitu-dinal sections shown for each stage in basin evolution [except stages 1 and 2 for (d) and (e)]. The lower diagram ineach panel shows geometry of footwall uplift after each of the three stages. (a) Single fault; (b) nonoverlapping syn-thetic fault segments; (c) closely spaced overlapping synthetic fault segments; (d) widely spaced overlapping syn-thetic fault segments; and (e) antithetic fault segments. Simplified from Schlische and Anders (in press).


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In the case of widely spaced overlapping syn-thetic faults, originally isolated synclinal subbasinsnever merge during fault-tip propagation (Figure6d). Each fault segment is associated with its ownanticlinal footwall uplift geometry. However, thesummed footwall uplift profile resembles that of asingle continuous fault.

The relay geometry of synthetic overlappingfaults may not persist indefinitely (Peacock andSanderson, 1991; Childs et al., 1995). Relay zonesare commonly breached by faults that connect themain fault segments. This may account for the dog-leg or zig-zag geometry of some normal fault sys-

tems (e.g., Harding, 1984).An example of the evolution of one type of anti-thetic fault system is shown in Figure 6e. Each faultsegment produces an originally isolated synclinalsubbasin. As the tips propagate, the faults curvetoward one another to prevent interference.Reduced displacement at the fault tips coupled withgreater strike slip on fault segments oblique to theextension direction result in reduced subsidence in

the area of fault overlap, forming a transverse acline. If the faults did not curve in this mannetransfer fault would need to form between theThe footwall fold pattern consists of isolated aclines; this geometry persists through timAdditional examples of the geometry of antithefault systems are provided by Morley et al. (1990

Examples of Basin-Scale Transverse FoldsThe Beaverhead and Lost River fault systems

Idaho (northern Basin and Range) each consist offault segments (Anders and Schlische, 1994; Fig8a). Segment boundaries marked by closely overping faults (shaded) are associated with hanging-whighs (intrabasin highs) manifested as Bouganomaly highs (indicating shallower depth to bment). In the regions of overlap, there is no assoced footwall elevation low. Rather, the anticlinal fwall uplift profiles resemble those of a single fasuggesting that all of the fault segments are kinemcally linked. The geometries depicted in Figure

Schlische 1

Rider block

Rela

yram

p

Relay

ramp

Footwall uplift

800

400

2000

0

-200

0400

0-

1000

-800

-200

-600

5 km

DU

D

DU

DU

U

DD U

U

DU

Relay ramp

Relay

ramp

0

1000

-1000A

A'

A A'

(a) (b)

(c)

Figure 7(a) Geometric relationships among a segmented normal fault system, relay ramps, transverse folds, rider blocks. Simplified from Schlische (1993). (b) Structure-contour map of the top of basement (datum is meanlevel) within a Permian rift basin, Greenland. Note relay ramps between offset fault segments, basement highs (tr

verse anticlines) at fault segment boundaries, and basement lows (transverse synclines) near the centers of faultments. Adapted from Larsen (1988). (c) Longitudinal profile of top of basement along section AA shown in (b).


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correspond to stage 3 in the evolution of a closelyoverlapping synthetic fault system (Figure 6c).

The Dixie ValleyPleasant Valley fault system ofNevada is an example of an overlapping antitheticfault system. Dixie Valley and Pleasant Valley aretwo synclinal subbasins associated with the south-east-dipping Dixie Valley fault and the northwest-dipping Pleasant Valley fault, respectively (Figure8b). The Sou Hills is an anticlinal high (accommo-dation zone) formed in the area where the twomajor fault systems overlap (Fonseca, 1988;Gawthorpe and Hurst, 1993; Schlische and Anders,in press). The Sou Hills area is also associated withfootwall elevation lows in the Stillwater and Tobinranges (Jackson and Leeder, 1994).

In the TriassicJurassic rift system in easternNorth America (Figure 9), the Newark andCulpeper basins are relatively simple basins, con-sisting of elongated synclines in longitudinal sec-tion, although this simple geometry is complicatedby intrabasinal normal faults and higher amplitudefolds along the border fault system. In the Newarkbasin, strata thicken and lacustrine facies deepentoward the hinge of the basin-scale syncline

(Schlische, 1992). The Hartford-Deerfield and DeepRiver basins consist of multiple synclinal subbasinsseparated by intrabasin highs, marking regionswhere originally isolated subbasins grew together(Schlische, 1993). In the Hartford-Deerfield basin,strata thicken toward the hinges of the synclinalsubbasins, and paleocurrents within fluvial unitsare directed away from the intrabasin high betweenthe two subbasins (Schlische, 1993).

Fault-Displacement FoldsFault-displacement folds are transverse folds of

smaller scale than those discussed in the previoussection. In some cases, these folds are associated

with segmented fault systems similar in geometrybut smaller in length of segments than thosedescribed (e.g., Figure 7b). In other cases, the foldsare not associated with an obvious or heretoforemapped segmented fault system. Nonetheless, thefolds have a geometry similar to those that are defi-nitely associated with segmented faults, and thusthey are also likely related to along-strike variationsin fault displacement.


Lost River

Range

10 km

1983 Borah

Peak

rupture

4

2

195

225

km

mgal

Challis

Warm

Springs

Thousand

Springs

Mackay

Pass

Creek

Arco

Beaverhead Mountains

10 km

4

2km

125

300

mgal

Lemhi

Mollie

Gulch

Leadore

Baldy

Mtn.

Nicholia

Blue

Dome

2

1

0

-2

-1

Dixie ValleyStillwater Range

Sou Hills

Pleasant Valley

AA'

BB'k

m

1

0

-1

-2

Pleasant Valley

Tobin RangeSou HillsDixie Valley

BB'

C

C'

km

SouHills

Pleasant Valley

Tobin Range

Dixie Valley

Stillwater Range

Clan Alpine Mountains

A

A'B

B'C

C'

N

10 km

Normal fault with pre-historical rupture

Normal fault with historical rupture

(b)

(a)

Figure 8(a) Transverse foldsassociated with recent normalfault systems in the Basin andRange. Range-crest longitudinalfootwall elevation profile andlongitudinal Bouguer gravityanomaly profile (a proxy ofhanging-wall subsidence) for theBeaverhead and Lost River fault

systems, Idaho; the names offault segments and segment

boundaries are indicated.Intrabasin highs are notassociated with footwall elevationlows at the shaded segment

boundaries. Modified fromAnders and Schlische (1994).(b) Upper panel: geologic map ofthe Dixie ValleyPleasant Valleyfault system. Modified fromZhang et al. (1991). Lowerpanel: longitudinal profiles offootwall uplift and hanging-wallsubsidence along transects

indicated in the upper panel.Modified from Jackson andLeeder (1994).


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Transverse folds are present along the borderfault system in the northeastern part of the Newarkbasin (Figure 9b). These folds decrease in amplitudeaway from the fault, clearly do not affect the entire

width of the basin, and are not associated with obvious or mapped fault segmentation. In the souwestern part of the basin, transverse folds are awell developed (cross section BB in Figure 9c),

Schlische 1

TA TA

TA

Bend in profileF F'

VE: x2

C C'U

E'

E

Culpeper basin

Deep Rivebasin

U

U

400 km

DRN

C

N

HD

F'

D=DeerfieldH=HartfordN=NewarkC=CulpeperDR=Deep Rive

E EU

A'

A

Hartfordsubbasin

Newarkbasin

C

C'Interbedded lava flowsand lacustrine strata

Diabase intrusions

Mostly deep-water

lacustrine strata

Mostly fluvial strata

Mostly shallow-waterlacustrine strata

Border fault

Other fault

Undifferentiated strataU

A A'U

N

25 km

F

B

B'

Fig. 10

Fig. 11

B B'

D'

D

D D'

U

U

Fig. 14

h

f

(b)

(c)

(a)

Deerfieldsubbasin

U

Coastal Plain cover

Figure 9Simplified geologic maps (b) and schematic longitudinal cross sections (c) of selected Mesozoic rift ba

of eastern North America (a). f = Flemington fault; h = Hopewell fault; A = motion away from reader; T = mottoward reader. Modified from Schlische (1992, 1993).

.

.


12/18

in this case there is a clear association between thefolds and a highly segmented border fault system(Figure 10a). The local and regional geometry of thefolds is most compatible with normal faulting (seeSchlische, 1992, for further discussion).

Outcrop and seismic reflection data from theJacksonwald-Sassamansville area in the southwest-ern Newark basin constrain the timing of deforma-tion. In the Jacksonwald syncline, stratigraphicmarker beds are thicker in the hinge than alongthe limbs (Figure 10a) (Schlische, 1992). Reynolds(1994) analyzed a grid of seismic reflection pro-files (provided by North-Central Oil and ExxonCorporation) in the southwestern Newark basin.Line NC 2-1 is oriented subparallel to the borderfault system (Figure 10b). Reynolds interpretationshows that synrift strata and the top of basementare concordantly folded. Although the border faultsystem is not planar, its irregularities do not sys-tematically correlate with the overlying folds. As

constrained by the seismic data and the Parestiswell, the Lockatong Formation is thinnest alongthe crest of the Sassamansville anticline and thick-ens into the flanking synclines (Figure 10b). Bothstudies clearly indicate syndepositional folding.

Transverse folds are also well developed along theFlemington and Hopewell intrabasinal normal faultsystems (f and h in Figure 9b). The amplitude of fold-ing decreases with distance from the Flemingtonfault system in the hanging wall (Figure 11a). Nofolds of a scale similar to those in the hanging wallare present in the footwall. Stratigraphic data frommarker beds that have been traced across the folds(Olsen et al., in press) indicate that units thicken andlacustrine facies deepen away from the anticlinalhinge (Figure 11b), strongly suggesting syndeposi-tional folding. A drag fold is present near the south-western end of the fault shown in Figure 11a.

As displacement increases on faults or fault seg-ments, transverse folds must increase in amplitude.


25

50

26 43

35

12

15

20

30

30

18

12

12

5 km

5.1

7.6

N

Sassamansvillesyncline anticline

Jacksonwaldsyncline

9.2

1 km

Passaic FormationDiabase

Basement

2-3Parestis

Well

Sassamansville anticline

NNESSW

1

0

2

2-4

Border fault system

Lockatong Fm.

Sassamansville syncline

(b)

Two-wayTraveltime(s)

(a)

NC 2-1

Stockton Fm.

Lockatong Fm.

Passaic Fm.

Jacksonwald Basalt

Diabase

Conglomerate Strike/dip of bedding30SynclineAnticline

Seismic lineNormal fault

Form line of bedding

Drill holeStrike-slip fault

(dashed where inferred)

Figure 10(a) Geologicmap of the southwesternNewark basin (see Figure 9bfor location) showingtransverse folds associated

with a segmented faultsystem and location ofseismic lines studied byReynolds (1994). Circled

numbers indicatethicknesses of marker

unit in meters. Modifiedfrom Schlische (1992).(b) Geologic interpretationof longitudinal seismic lineNC 2-1. Modified fromReynolds (1994).


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Consequently, bed lengths of units deformed by thefolding must increase through time (Figure 12a).Transverse folding of this type therefore leads to theformation of fold-parallel (fault-perpendicular) exten-sional structures, such as joints, normal faults, anddikes. Such relatively small-scale accommodationstructures have been imaged on a seismic reflectionprofile of a transverse fold in the North Sea (Figure12b, c) (Roberts et al., 1990). The normal faults forma conjugate system, dipping toward the locus of

greatest subsidence, and are restricted to older hzons that have been downwarped the most. Strthicken toward the center of this syndepositiostructure. [It should be noted that Roberts et(1990) ascribed this fold to strike slip, but its geotry is very similar to other transverse folds descriin this report, and is probably a fault-displacem

Schlische 1

5 kmN

3

2

1

L

S

P

OF

O

P

10m

Somersetcore

Flemington(1)

Copper Hill (2)

Muirheld(3)

Ukrainian

Member

Cedar

GroveMb.

Black laminated mudstoneGray thin-bedded mudstonePurple mudstoneRed massive mudstone

(a)

(b)

Figure 11(a) Geologic map of a part of the Flemingtonintrabasinal fault system of the central Newark basin (see

Figure 9b for location), with transverse folds in the hang-ing wall. F = Feltville Formation; L = Lockatong Forma-tion; O = Orange Mountain Basalt; P = Passaic Formation;S = Stockton Formation. Thin black lines are marker bedsconsisting of deeper water lacustrine strata. (b) Measuredstratigraphic sections [see (a) for locations] and paleo-magnetic polarity stratigraphy (black is normal, white isreversed) of top of the Cedar Grove Member and base ofthe Ukrainian Member, showing thinning toward theanticlinal hinge and thickening toward the synclinalhinges. Modified from Olsen et al. (in press).

Line x

(a)

2 km

Transverse syncline

1 s

3 s

WSW E

2

201800

1000

12001400

1600

1200

1400

1000

1200

1400

1600

2000

2200

1800 20

00 2200 2

400

2600

1600

1400

1200

1800

5830'N

245'W 230'W

5 km

U

D

D

U

U

D

U DD

U

D

U

D

U

D

U

D

U

DU

N

Structure contour mapTWT, milliseconds

Top Zechstein

Top Triassic

Seismi

cline

(b)

(c)

bK

tTr

tZtD

1000 m

VE=2x Bend in sectionA

6000

6500

N

A

(d)

6500

Figure 12(a) Changes in bed length during fault-placement folding accommodated by transverse norfaulting. (b) Structure contour map of a part of North Sea, with faulted transverse syncline imagedseismic reflection profile (c). tD = top Devonian; tZ =Zechstein; tTr = top Triassic; bK = base Cretaceous. Msimplified from and original seismic data presentedRoberts et al. (1990). (d) Footwall anticline associa

with normal fault system (ball-and-bar symbols) inBeatrice field, North Sea. Contours are depth in feethe upper surface of the reservoir unit. Modified frLinsley et al. (1980).


14/18

fold (A. M. Roberts, 1995, personal communica-tion).] In the southwestern Newark basin (Figure10a), a large northwest-striking subvertical dike sub-parallels the hinge of the Jacksonwald syncline, andis likely a fold-parallel extensional structure thataccommodated progressive transverse folding.

A hierarchy of fault-displacement folds is present

in the Newark basin and presumably other basins aswell. The largest scale fold is associated with thebasin itself (Figures 9b and 13); this suggests that onthe largest scale, the basin appears to be bounded bya single continuous border fault, although the borderfault is mapped as consisting of numerous fault seg-ments. Superimposed on the basin-scale fold aretransverse folds associated with the segmented bor-der fault (Figure 13). Their effects are relatively local-ized; the hinged margin is clearly not affected bythese folds (Figure 13). The distance of the hingedmargin from the border fault system (i.e., the widthof the basin) is a proxy of the total amount of exten-sion experienced on the border fault system and the

intrabasinal faults, both mapped and hidden. Asnoted by Marrett and Allmendinger (1992) andWalsh et al. (1991), hidden (i.e., small) faults com-monly account for significant amounts of extension.Although each fault segment has a local displace-ment minimum at a fault segment boundary, result-ing in the formation of an anticline in the immediatevicinity of the segment boundary, the sum of the dis-placements on the overlapping faults and any otherfaults balances for the localized deficits on any indi-vidual fault (Figure 13). To produce the overall syn-clinal geometry of the Newark basin, the centralfault segments must have greater displacement thanthe distal segments. In addition, intrabasinal faultsare concentrated in the central part of the Newarkbasin (Figure 9b) and may contribute to the greaterwidth of the basin in that area.

Fault-Line Deflection FoldsFault-line deflection folds were first described by

Wheeler (1939). These transverse folds are associated

with nonplanar fault surfaces. Syncl ines form atrecesses (convex toward the footwall) in the fault sur-face, whereas anticlines form at salients (convextoward the hanging wall) (Figure 14). The amplitudeof folding decreases away from the fault-line deflec-tion, which is attributable to an evening-out effectwith distance from the fault. The fault-segmentation

model for the formation of transverse folds and thefault-line deflection model are not necessarily mutual-ly exclusive because many segment boundaries (areasof displacement deficits) are marked by salients in thefault surface (e.g., Machette et al., 1991) and thustransverse anticlines. Dog-leg or zig-zag-type fault sys-tems resulting from breached relay zones of overlap-ping faults may also be associated with fault-linedeflection folds.

Fault-line deflections (undulations) have also beenreported from several detachment faults in thesouthwestern United States (e.g., Davis and Lister,1988). These folds have wavelengths of several tensof meters to hundreds of meters; the axes of these

folds are subparallel to the slip direction. Meter-scalestructures on neotectonic normal faults are referredto as corrugations by Stewart and Hancock (1991).

Interference Folds

The folds described previously account for thevast majority of folds found in extensional settings,but there are some additional examples that are noteasily classified as transverse or longitudinal.Interference folds form when one type of fold issuperimposed on another; for example, longitudinalfolds superimposed on transverse folds. Depending

on the geometry of the superimposed folds, domes,basins, saddles, and culminations (Figure 12d) maybe produced.

DISCUSSION

The majority of the folds described previouslyare related to (1) movement on normal faults, (2)


Minimum

segment

andBFS

displacement

Minimum

segment

andBFS

displacement

Maximumfault segment

displacement

Maximum BFSand fault

segmentdisplacement

Localminimum

Maximumfault segment

displacementLocal

minimum

Hinged marginunaffected byBFS segmentation

Hinged margin

affected byBFS segmentation

(not observed)

"Folded"marker bed

Figure 13Hierarchy of transversefolds associated with extensional

basins. BFS refers to border faultsystem. Ball-and-bar symbolsdenote normal faults. See text fordiscussion.


15/18

growth and propagation of faults, and (3) displace-ment variations on normal faults and in the volumeof rock affected by the faulting. Fault-related foldstherefore form at the same time as the faults or, inthe case of drag folds, immediately prior to faulting.Folds related to faults that intersect the free surfaceof the Earth are therefore expected to grow synde-positionally. Consequently, sedimentary units influ-enced by the folds will be thicker in the hinges ofsynclines and thinner or absent on the crests ofanticlines. Sedimentary facies are expected to havebeen deposited in deeper water in the hinges ofsynclines; shallower water deposits are expected inthe hinges of anticlines. Emergent footwalls may besites of nondeposition or erosion. As faults or faultsegments grow longer through time, most folds will

also increase in size. During the growth and evotion of segmented basins, some structures migradually be replaced by others (e.g., intrabahighs gradually subsiding as multiple depocentevolve into a single depocenter).

Within the hanging walls of normal fault stems, anticlines (intrabasin highs) form at fault sment boundaries. In the case of overlapping fau

these segment boundaries are commonly assoced with relay ramps and, at least in the initial staof fault linkage, zones of reduced footwall up(Figure 7a). Streams flowing off the uplifted fowalls commonly exploit these low points, and trelay ramps serve as important conduits for traporting sediment from the footwall to the hangwall (e.g., Leeder and Gawthorpe, 1987; Robertal., 1991; Gawthorpe and Hurst, 1993). At thinput zones, sediments are expected to be somwhat coarser grained and may serve as potenreservoir rocks; e.g., sand bodies within the SoViking graben (Jackson and White, 198 9) aCentral graben (Roberts et al., 1991) of the No

Sea. Such relay ramps are therefore importexploration targets; however, relay ramps theselves may be difficult to detect seismically, but associated transverse anticlines may be easieridentify. As the fault system evolves, footwall eletion lows at some segment boundaries are gradueliminated (see Figure 6c). In such cases, the sply of coarser clastics is shut off, and the oldcoarser rocks may be buried by finer grained sments that may serve as seals. This would form ideal trap: coarser grained clastic rocks sealedfine-grained rocks on an anticlinal structure, whydrocarbons derived from the deep-water shaof adjacent synclines.

The wealth of folds and faults in extensional ttonic settings provides numerous structural traFootwall structural traps include drag folds, forfolds, and fault-propagation folds, which are assated with productive fields in, for example, Sirte basin, Gulf of Suez, and the Viking grab(Harding, 1984). Footwall folds provide effectstructural closure because displacement (uplift) maximum at the center of an associated fault sment and decreases toward the fault tip, and updecreases with distance from the fault. The faitself or impermeable layers in the hanging wmay provide seals along the fault-bounded sidethe trap. The Beatrice field in the North Sea isexcellent example of this type of structural t(Figure 12d). Closure may be enhanced by superposition of transverse anticlines and longdinal drag folds. Folds associated with intrabasifaults containing synextensional sedimentdeposits in the footwall are preferred to large bder fault systems that typically lack synextensiodeposits in the footwall.

Schlische 1

Recess

Salient

Recess

(b)

(a)

Basalt

Figure 14Transverse folds associated with fault-line

deflections along the border fault system of the Hartfordsubbasin, Connecticut (see Figure 9b for location). Syn-clines form at recesses; anticlines form at salients. (a)Present-day geometry; (b) geometry with upper synrift

units removed. Modified from Wheeler (1939).


16/18

Structural traps in the hanging walls of normalfaults include the aforementioned fault-displace-ment anticlines associated with segmented faultsystems and relay ramps and rollover folds (e.g.,Shelton, 1984). Hanging-wall reverse-drag anti-clines affecting units with regional subhorizontalorientation may not provide closure, especially ifsuperposed transverse synclines create saddle

structures. Closure may be possible if the regionaldip is in the same direction as the dip direction ofthe fault. Interference folds, particularly domes andculminations, also may form structural traps.

In regions in which strata lack primary porosityand permeability, secondary fracture porosity isextremely important. Secondary structures associ-ated with folds that may contribute to fracture per-meability include (1) fold-parallel extensional struc-tures that accommodate downwarping andupwarping associated with fault-displacement folds(Figure 12a), (2) fold-parallel faults and shear zonesthat accommodate reverse-drag and rollover fold-ing, and (3) extensional structures in the lower

footwall and upper hanging-wall quadrants associ-ated with displacement fields on blin d faults(Figure 1d).

Longitudinal folds, such as drag folds androllover folds, have long been recognized as beingassociated with normal faults. One of the goals ofthis paper is to show that transverse folds are equal-ly as common as longitudinal folds. Hopefully, theidealized models and examples of fault-associatedfolds presented in this paper will allow for aug-mented interpretations of seismic reflection pro-files in areas of poor data quality.

SUMMARY AND CONCLUSIONS

(1) Characteristics of normal faults that influ-ence associated folds include the following: maxi-mum displacement occurs at the center of the faultand decreases toward the fault tips; displacementdecreases with distance from the fault surface;faults increase in length as displacement increases;and fault systems are commonly segmented.

(2) Folds associated with normal faults canbroadly be classified as fault-parallel (longitudinalfolds) and fault-normal (transverse folds).

(3) Drag folds (including forced folds and fault-propagation folds) are longitudinal folds that formlargely as a result of the growth of faults intoregions of monoclinal folding at the tips of faults;synclines are present in the hanging wall; anticlinesare present in the footwall. Such folds may also beproduced by frictional drag and differential com-paction.

(4) As defined in this paper, reverse-drag foldsare longitudinal folds that are a manifestation of the

elastic (and in some cases flexural) response tofaulting that results in a decrease in displacementwith distance from the fault; these folds consist ofhanging-wall anticlines and footwall synclines.

(5) As defined in this paper, rollover folds arelongitudinal folds that form as a result of movementon nonplanar (typically listric) faults in unlithifiedsediments in which movement is driven by gravity

sliding. Anticlines form in the hanging wall; thefootwall is generally unfolded. Although rolloveranticlines are geometrically similar to reverse-draganticlines, the causal mechanism is different. Thisgeometry by itself cannot be used to infer the exis-tence of listric faults. Balancing techniques used toinfer fault geometry can only be used for rolloverfolds associated with gravity-driven faults.

(6) The majority of transverse folds form as aresult of along-strike variations in fault displacement.In the hanging wall, synclines form near displace-ment maxima, typically at or near the centers of faultsegments. Anticlines form near displacement mini-ma, typically near fault segment boundaries.

Synclines tend to be broader than anticlines. In thefootwall, anticlines form at displacement maxima,and synclines form at displacement minima. Thegeometry of transverse folds is strongly influencedby the growth and linkage of fault segments.

(7) Fault-line deflection folds are transverse foldsrelated to undulations on the fault surfaces, withsynclines forming at recesses and anticlines form-ing at salients.

(8) Interference folds result from the superposi-tion of longitudinal and transverse folds, andinclude domes, basins, culminations, and saddles.

(9) All of the folds described form at the sametime as or just prior to faulting. For faults that inter-sect the Earths surface, the associated folds devel-op syndepositionally. Synfolding sedimentarydeposits are thicker in the hinges of synclines andthinner in the hinges of anticlines; water-depthdependent facies are deeper in synclines than anti-clines. Coarser grained facies tend to accumulate atanticlines (intrabasin highs) associated with faultsegment boundaries and relay ramps.

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Roy W. Schlische

Roy W. Schlische is an assistantprofessor of geology at RutgersUniversity. He received his B.A.degree from Rutgers and his M.A.degree and Ph.D. from ColumbiaUniversity. His research interestsinclude the structural and strati-

graphic evolution of extensionalbasins, the geometry and causes ofrift basin inversion, length-dis-placement scaling relations for nor-mal faults, the mechanics of fault growth, analysis offracture populations, and tectonic geomorphology.

ABOUT THE AUTHOR

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