Basic tectonic

8/12/2019 Basic tectonic

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Basic Wrench Tectonics

Abstract h lon structures which may trap oiland gas develop in a systematic pattern along wrenchzones in sedimentary basins. Laboratory clay modelssimulate the formation of n h lon folds and faultscaused by wrenching. Folds form early in the deformation and are accompanied or followed by conjugatestrike-slip reverse or normal faulting. Deformationmay cease at any stage or may continue until strikeslip along the wrench zone produces a wrench faultand separation of the severed parts of early structures. Oblique movements of fault blocks on oppositesides of a wrench fault cause divergence or convergence and enhancement respectively of extensionalor compressional structures Basins form in areas ofextension and are filled with sediment whereas up-thrust blocks emerge in areas of compression andbecome sediment SOurces The combined effects ofwrenching in a petroliferous basin are to increase itsprospectiveness for major hydrocarbon reserves.INTRODUCTION

Wrench faults (Kennedy, 1946; Anderson,1951) are high-angle strike-slip faults of greatlinear extent along which strike-slip may be tensof miles or considerably more. Basement invariably is involved in the deformation and a wrenchzone is a swath of terrane deformed by wrenching prior to and concurrently with strike-slipalong the throughgoing wrench fault. The termwrench fault has no genetic connotation.En echelon folds are the most important structures of potential value for trapping hydrocarbons in most wrench zones. They are also usefulfor recognition of wrench zones (Figs. 1-5). Asingle en echelon fold can be depicted as anellipse (Fig. 6), which represents the deformationof a circle in the wrench zone, with the longerellipse axis A-N) parallel with the fold axis.Other structural straps can be formed by faultingor a combination of faulting and folding. Fourtypes of fractures can form during wrench deformation, and if the wrenching continues, anyoneor all of these fractures can become faults. InFigure 6, the fracture directions are shown as xX (the strike of the primary wrench fault orwrench zone), C-C and D-D en echelon conjugate shear joints or strike-slip faults), and B-Ben echelon tension joints or normal faults). Thedevelopment and interrelations of these faultsand the en echelon folds are the main subjects ofthis paper. In a previous paper Moody and Hill

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The meflcan ASSOC 8f on of froleum Geoklgists SuIOtin 57 No.1 January 1973 P. 74-96, 16 Fig., 1Table

RONALD E. WILCOX . T. P. HARDING a and D. R. SEELYHouston Texas 77001

1956) have treated aspects of wrench tectonics,particularly as these pertain to proposed systemsand patterns of sets of wrench faults.Prolific reserves of hydrocarbons have beentrapped in wrench structures, mainly in en echelon folds and faulted folds. Some of the largestand best known of these structures are anticlinaltraps in the Los Angeles basin (Fig. 4) and thewest side of the San Joaquin Valley (Fig. 5),California see also Harding, 1973, the followingpaper in this issue).Clay models that illustrate the mechanics anddevelopment of this structural style representbroad basins filled with structurally homogeneous sediments whose total thickness is small compared with the size of the basin. The models alsoaid in prediction of traps by providing visualexamples of the three-dimensional relations between structural elements in wrench zones.MECH NICS OF WRENCHING

Wrench faults form in response to horizontalshear couples within the earth s crust, and theycan be simulated in clay models by moving tinsheets beneath a clay cake (Cloos, 1955). Simplewrenching results from the movements of thecrustal blocks or tin sheets in opposite directionsparallel with their adjacent edges. As a consequence of such parallel displacements, compressional and tensional stresses are generated in theoverlying sediments or clay. If, instead of movingexactly parallel with the wrench fault, the basement blocks or the tin sheets converge or divergeslightly, the compressional or tensional stresses,respectively, that result from the basic wrench areenhanced. These important special cases of convergent and divergent wrenching are discussedafter analysis of the more general case of simpleparallel wrenching.I Manuscript received, February 18, 1972; accepted, April I?,1972.EssoProduction Research Co.; present address: P.O. Box1230, Bellaire, Texas 77401. umble Oil Refining Co. Esso Production Research Co.We are grateful to E. Cloos, consultant to Esso ProductionResearch Co for contributing helpful suggestions during thecourse of this work. Our appreciation also is extended to J.Crowell for stimulating discussions.

tl 1973. The American Association of Petroleum Geologists. All rightsreserved

Intro Home

1973 American Association of Petroleum Geologists


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Basic Wrench Tectonics 75

_ WRENCH FAULT

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cFIG I n h lon folds along wrench faults. n h lon folds, some productive, northeast of Barisan Mountains Semangko)fault in Central and South Sumatra basins, Sumatra. Oblique convergent subduction along adjacent Java trench is additional factorin deformation here. B. El Pilar fault and associated faults and n h lon folds in eastern Venezuela and Tri nidad; not eproduction from folds near Los Bajos fault, southwestern Trinidad. Dead Sea rift Israel and Jordan; note location of Dead Seabetween overlapping ends of major wrenches. Some n h lon folds are bounded by thrusts and several are marginally productive.

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3/23

76 D R SeelyP Harding, and . .onald Wilcox, . .

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asic Wrench ectonics 77SIMPLE P R LLEL WREN HING

Simple parallel wrenching is a special case ofsimple shear, which is one kind of finite homogeneous strain Jaeger and Cook, 1969; Ramsay,1967 . The shear angle /t, Fig. 6 increases withincreasing simple shear. In some crustal deformation and in clay models the initial deformationsare plastic and involve folding. These are fol-lowed by a combination of plastic distortion andfracturing. As deformation proceeds, displacement along the wrench zone increases, and thezone of principal shear narrows. Finally, all ofthe slip occurs along a few closely spaced faultsor along one throughgoing wrench fault, and subsequent deformations within either fault blockare more or less independent of each other.On a wrench model Figs. 7, 8 , it is convenientto mark the clay surface with a circle and to notehow its shape changes during deformation. Pointsmoving closer together mark compression, andpoints moving apart denote extension. The original circles Fig. 7A on the clay are aligned alongthe edge of the underlying tin sheet and deforminto echelon ellipses during the plastic phaseof strain Fig. 7B . Straight lines on the clay Fig.7A, normal to the line of circles) are trowel marksthat become bent during deformation Figs. 7BC, 8 F). Maximum compression and extensionare parallel with the minor and major strain el-lipse axes, respectively, and neither of these directions is parallel with or perpendicular to the sheardirection imposed on the model, i.e. the strike ofthe wrench zone defined by the parallel edges ofthe tin sheets and the line of circles. I t followsfrom the echelon arrangement of ellipses Fig.7B that all structures associated with each ellipseFig. 6 may be repeated along the wrench zone.This echelon repetition of folds and faults isan important diagnostic feature of wrench zonesFigs. 1-5). The size and spacing of circles/ellipses on the models is arbitrary; the spacing offolds and faults in the model wrench zones isdetermined by various characteristics of eachmodel.)The clay models of wrenching are all basicallyalike. The model in Figures 7 and 8 has left-lateral displacement, whereas the models in Figures9 and 10 are right-lateral wrenches. By convention, the sense of fault displacement is describedby assuming that the block toward the observer isfixed, and the block across the wrench fault fromthe observer moves to his right or left.) Variousstructures form on each model, however, depending on the thickness and nature of the wet-claycake, on the rate of deformation, on any specialconditions built into the model, and to a certain

degree, on chance. Included in the chance as-pect that helps to determine the final modelstructures are, for example, slight inhomogeneities in the texture of the clay and the presence ofhidden bubbles beneath the clay surface.By analogy, the explorationist is faced with ahost of unknown chance) factors in interpretingwrench zones. Some of the more obvious factorsare the effects of nonuniform stratigraphy boththickness and composition), variable rates of deformation, and different directions of movementbetween crustal blocks during one stage of deformation or during succeeding stages. In spite ofthese inherent complexities in both nature andthe models, however, the overall pattern ofwrenching has key elements that are repeated,and the presence of anyone or more structures ofthe basic pattern serves as a clue for recognizingthis structural style and its associated prospectivestructures.The structures of the basic wrench-tectonicpatterns are echelon folds, echelon conjugate strike-slip faults, the main wrench fault orwrench-fault zone, and echelon normal faults.These are described below and are illustrated inthe models Figs. 7-10).

n helon ol sEn echelon folds are the most attractive prospective structures in wrench zones because theyform early and thus provide traps during earlyhydrocarbon migration, and because they commonly afford the largest closures that are genetically related to wrenching Harding, 1973 . Asthe amount of displacement on the wrench zoneincreases, the initial folds are broken first byfractures and then by faults. In later stages ofwrenching the folds may become shattered Fig.9C , and parts of the folds on either side of thewrench fault may be offset Fig. IOC . As movement of crustal blocks continues over long periods of geologic time, the half-folds on one blockcan be removed completely away from the area,and the wrench fault itself may provide updipclosure.The term en echelon refers to the arrangement of structures along a linear zone so thatindividual folds or faults of the same kind areparallel with each other and are inclined equallyto the strike of the zone. The nomenclature fordescribing echelon fold sets is similar to that

for wrench displacements. Right-lateral wrenchesproduce right-handed fold sets Fig. lIA), wherea traverse along the axis of any fold to its terminus would turn right to reach the next fold in the echelon set Campbell, 1958 . A left-handed

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78 Ronald E. Wilcox, T. P. Harding, and D. R. Seely

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asic Wrench ectonics 79set of e helon folds in eastern Panama Fig.lIB) is probably related to a left-lateral wrench.All e helon folds in one zone are usually ofsimilar shape and extent. The folds in Figure 9are more distinct and more uniform than is usualfor clay-wrench models, because a thin sheet ofplastic film 0.OOO5-in. thick was interlayered inthe clay 0.25 in. below the surface. Several larger e helon folds developed in the other two models Figs. 7, 8, 10 , which are homogeneous claycakes without plastic film. The folds in Figures 7and 8 are low and only faintly visible, whereasthose in Figure 10 are larger. This differenceprobably is explained by the rates of deformation; the model with distinct folds Fig. 10 wasdeformed 2.5 times faster than the other modelFigs. 7-8 .

A close examination of Figure 9A reveals asmall difference between the average fold trendand the trend of the longer axes of the ellipses.This difference probably is accentuated by thepresence of the thin plastic sheet, which has influenced strongly the folding. Other similar experiments have shown that the fold size and foldspacing in the wrench zone are related to thedepth of burial of the plastic film below the claysurface. Shallower plastic sheets produce smaller,more closely spaced folds. Another characteristicunique to models with plastic film layers is therapidity offolding after slow deformation begins.In the extreme case of the plastic sheet directlyon the clay surface, a very slight distortion bywrenching immediately causes folding in theplastic sheet and in the clay just below.In models without the plastic sheet e.g., Fig.lOB , the longer ellipse axes are nearly parallelwith the axes of the clay folds. This is similar tothe ellipse diagram Fig. 6 , but the model ellipsesare not so elongate as the ellipse in Figure 6.For a true simple shear the angle between thefold axis long axis of the ellipse and the strike ofthe wrench zone is always less than 45. For mostwrench-fault experiments with clay, the angle between e helon fold axes and the wrench faultapproximates 30. Folds that form later duringthe deformation have lower angles.Fortunately, in the early stages of explorationin an area where wrenching is suspected, therecognition of several typical wrench-zone structures will serve to define the trend of the zoneitself and probably also the sense of wrench displacement. By extrapolation from models, theaxes of en e helon folds, which may be subtle,low-relief closures, should lie at an angle of 30 15 to the wrench trend, either in a clockwisedirection left-handed folds or in a counterclockwise direction right-handed folds . If the

wrench-zone trend is known or suspected, andthe displacement sense is unknown, folds stillcould be anticipated along the wrench trend withtheir axes inclined about 30 to that trend.In nature Figs. 1-5 , fold orientations in awrench zone can be different for several foldsalong the same fault trend. Some folds, or partsof folds with irregular axial trends, may parallelthe wrench fault or cross the wrench zone at alow angle. Several factors that can influence theshape and trend of e helon folds include convergence of blocks during wrenching, changes instrike of the wrench fault, large components ofvertical displacement, differences in kind andthickness of sediments, and mobility of basementnear the folds.Conjugate Strike-Slip FaultsWrenching causes two sets of intersecting, vertical fractures to form in a predictable orientationalong the wrench zone. One set, the low-anglefractures e-e , Fig. 6 , makes an angle between10 and 30 with the wrench strike X-X),whereas the high-angle set D-D ) intersects thewrench at an angle between 70 and 90. Theseconjugate fractures can be either joints or faults,or both, depending on the magnitude of wrenching.The acute angle of intersection of the two fracture sets is dependent on the nature of the rocksand the deformation; it is usually in the range of60-70. This angle is bisected by the direction ofmaximum compression B-B , Fig. 6 . On the claymodel in Figure 7C, one fracture of each setforms an X cutting the center small ellipse. Thewedge in the acute angle of the intersection isdisplaced Fig. 8D toward the center of the ellipse as deformation continues. Two importantaspects of the deformation are illustrated by thiswedging: 1 the opposite senses of lateral dis

placement on the two intersecting strike-slipfaults; and 2 contemporaneous plastic deformation and faulting.The low-angle faults Fig. 7C intersect thewrench strike line of ellipse centers at 12 andhave the s me sense of displacement left as thatof the main wrench zone Figs. 7B-C, 8D-E) andthe final wrench fault Fig. 8F . These low-anglefaults are called synthetic strike-slip faults, orsimply synthetic faults. In contrast, the high-angle set of conjugate strike-slip faults has a displacement sense opposite that of the wrench;these are known as antithetic strike-slip faults,and they are right-lateral in this left-lateralwrench model. They form angles of 78 with thewrench and 66 with the synthetic fault in thecenter ellipse Fig. 7C . The low- and high-angle

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80 Ronald E. Wilcox T. P. Harding and D. R. Seely

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SA N GA BR IE L MOUNTAI NSI1S OO

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REVERSE F ULT

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NORM L F ULTILFIEL

FIG. Major wrench structures and oil fields. Los Angeles basin. California.

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asic Wrench ectonics

SOUTHERN S N JO QUIN V LLEYCONTOURS ON TOP OF LOWER PLIOCENE

V RI BLE CONTOUR INTERV L

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A F TE R H OO TS B EA R A N D K LEMP ELL 954

FIG 5 Major wrench structures and oil fields San Joaquin Valley California

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Ronald E. Wilcox T. P. arding and D. R. Seely

y -0+: :>-


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asic rench e toni s 8

IG 7 Clay model of parallelleft. ateral wrench fault (A-C _ three stages. -ertical views). e Figure 8 for three following nages.

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Ronald E Wilcox T P arding and R Seely

FIQ Oay Illodel ot parallc lleftl alnalwul lCb fault D F - three 11IIgn, vfttiQ. vieM).S Filure 1 for liQIIhr.., statts.

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asic rench e toni s

F Q. Cbymodel of parallel righi_lateral wrench fault wilh la) er of thin plaSlic film embedded 0.25 in below surface to enhanceen h on folds A C thr stages vertical views .

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86 Ronald E. Wilcox, T. P. Harding, and O. R. Seely

FI l. Il --..{:I.ay model ofparalltl ri ht-latt:ral wrnlC b f.ull A-C - t c P vMiaJ......., .

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asic Wrench ectonics sitory period in the long history of a majorwrench fault, but this early phase is of greatimportance in the process of hydrocarbon-trapformation.After a short interval of concurrent folding andconjugate faulting, the rocks (or clay) fracture ina relatively narrow zone within the overall deformational swath, and the master wrench fault iscreated. This process of rock failure begins atseveral points along the wrench zone e.g. seeFig. 8E between small circles 4 and 5 7 and 8 9and 10 . At some locations a synthetic fault deviates into the incipient wrench-fault trend, and atothers a new fracture forms more nearly parallelwith the strike of the wrench zone and at a smallangle to the nearby synthetic faults. As this process continues, the main wrench fault graduallyemerges as an interconnected series of these earlier fractures. (The plastic film prevented the for-mation of the single wrench fault in the model inFig. 9.A great variety of fault blocks is producedwithin the wrench zone. Some large blocks arecaught between early formed branches of themain wrench (fig. 8F, near large ellipse at left),and many smaller blocks are sliced and delormedinto horsts and grabens between the main wrenchfault and the conjugate faults. Once individualfault blocks are separated by faulting, they tendto deform somewhat independently; some rise,some sink, some are folded, and some are faultedagain.As displacement on the main wrench fault in-creases, slip diminishes on the other faults in thezone. fPe active fault plane, or a relativelythin, crush zone along the active part of the fault,commonly shifts from side to side of the wrenchzone. Distortion and faulting of the whole zonebecome complex, and this results in a braidedfault pattern that is typical of major wrenchzones (Fig. 12C .Changes in the strike of the active fault lead toadditional deformation of the wall rocks as strikeslip continues. TlJ.e parallel wrench becomes aconvergent or a divergent wrench, at least locally.The size and extent of the resulting compressional or extensional structures depend on theamount of change in fault strike and the amountof displacement along the curved fault surfacewithin the braided system (see Fig. 12 and accompanying text discussion of convergent anddivergent wrenching).

ension racturesThe orientation of tension joints or normalfaults parallels the short axis of the strain ellipse

(Fig. 6 B-B crosses the en echelon fold axes atright angles, and bisects the acute angle betweenthe conjugate shears. En echelon tension fractures may form along a wrench zone in the initialstage of deformation, but they easily are destroyed as wrench displacement increases andcompressive structures (folds and conjugatefaults) become more prominent. In clay modelsof wrenching, tension fractures are uncommonbecause of the strong cohesion within the clay.Water placed on the clay surface eliminates thIScohesion, and large, open, en echelon tensioncracks form to the exclusion of other fracturesand folds.Two examples of en echelon normal faults thatare presumed to lie above buried wrench faultsare the Lake Basin fault zone, Montana (Fig.3C), and the Cottage Grove fault zone, Illinois(Fig. 3D). In both these fault zones, the amountof wrench movement of the basement blocks af-ter sedimentation has been small-just enough tofracture the overlying sedimentary rocks withoutcausing significant lateral offset. Additional linear zones which may represent wrenching havebeen recognized near the Lake Basin zone(Smith, 1965 .The Cottage Grove zone displays two otherfeatures of wrenching. The northern block of themain east-west fault is downthrown in the western part of the zone and upthrown in the easternpart. This kind of change in the vertical displacement sense along strike is typical of wrenches.The tensional component ofwrenching is markedin the eastern area around the fault zone by maficdikes. Such intrusions and vein fillings in tensionfractures are well known in mineral deposits andplutonic terranes, and they fit the fracture patternfor wrenching along this zone.Antithetic fractures inherit some of the tensional component of a wrench deformation andcommonly become nearly vertical normal faultswith negligible lateral displacements. A downward displacement on either of the conjugatestrike-slip faults tends to be toward the acutewedge. This is well shown on one of the models(Fig. loe where there are many closely spacepantithetic faults at both ends of the wrench zone.Such concentrations of antithetic-normal fauhsimpart a pseudoplasticity to the clay (or rocks)that permits these zones to deform more or lessuniformly without being cut by one main wrenchfault. Thus, a wrench fault with measurable strikeslip can pass into one of these fracture zonesalong its strike where there is the same regionalshift across the zone but no single fault of largelateral displacement.ONVERGENT ND wERGENT WREN ffiNG

Opposed crustal blocks that do not move par-

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15/23

Ronald E Wilcox T P Harding and R Seely

l 1 [ XJ ~ 1 r1 II,1 1 r J--- ~ O L ' ...- 000 .. . ./ . ,


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asic rench TectonicsTable 1. O rie nt at io n o f an jugale Fractures

Ul l ing enter of Small llipse . ~ ~ ~ ~ ~ _ . _ _

Angle Between A ngle Between Angle etweenApproximate Wrench Strike and Wrench Strike and Synthetic andFigure Shear Angle Synthetic Fault ntithetic Fault ntithetic Faults~ _ . _ ~ ~

7c 2 12 78 66 8d 28 12 82 7 8e 3 6 14 87 73 8f 480 15 93 78

See figure

aileI with a wrench fault either converge or diverge as wrenching proceeds. These obliquemovements may be related to nonparallel dis-placements of crustal blocks on a regional scale,or they may be due to local changes in strike of agenerally parallel wrench. It is common for bothconvergence and divergence to develop locallyalong a wrench. Convergent wrenching, on whatever scale, tends to enhance compressive wrenchzone structures, namely, folds and conjugatestrike-slip faults, and strong convergence cancause reverse faulting and thrusting. The formation of tensional structures, mainly normal faults,is typical of divergent wrenching.A particularly good example of both convergence and divergence is seen north of Los Angeles California, in the San Andreas wrench systemFigs. 12 15 . A pod-shaped block, which isabout 100 mi long and 20 mi wide, lies southwest

of the San Andreas fault and northeast of thecurving San Gabriel fault Fig. 12A . Both faultsare well-documented, right-lateral wrenches.The pod-shaped block has moved southeastalong the curved San Gabriel fault and hascaused convergence on its southern and southeastern margin. Reverse faults with strike-slipcomponents characterize this margin and attestto the lateral wrenching combined with compression and high-angle thrusting.Concurrently, the northwestern part of the podwas under tension as it diverged from the curvingnorthern end of the San Gabriel fault, and theRidge basin was formed Fig. 12A . Sedimentsfilled this basin as faulting continued, and theyrecord the fault movements by preserving severalunique rock types whose source areas were dis-placed alongside the basin Fig. 12B .One such suite of gneissic rocks is preserved ascoarse blocks in the Violin Breccia Fig. 12A Bwhich accumulated along the northeast side ofthe San Gabriel fault scarp as wrenching continued from the late Miocene to the late PlioceneCrowell, I954a, b . The Ridge basin illustrates

how major wrench faults can influence basin development and sedimentation as well as the tectonic history and structural style of a region.n h lon folds in a clay model are enhancedby even a slight convergence of only 2 Fig. 13 .In the early stage of movement, the folds are welldeveloped throughout most of the central part ofthe model Fig. 13A and a few synthetic fractures have formed. At a later stage Fig. 13B thefolds have been offset along the synthetic faultsand the incipient throughgoing wrench. A fewantithetic faults also formed, but their importance in this deformation was minimal.A more intensive n h lon zone of compres

sion develops along a model wrench with a convergence of 15 Fig. 14A . Good n h lonfolds form in the narrow zone that later isuplifted, and both sets of conjugate shears arewell developed. Nearly all wrench displacementis concentrated on the synthetic faults, alongwhich the fold axes are offset. A side view of thesame model Fig. 14B reveals the complexthrusting of the wedges squeezed up and out ofthe wrench zone by the strong convergence. Asthese blocks rose, they were bounded by verticalor high-angle reverse synthetic faults, and theyresemble upthrust blocks.Just south of the San Gabriel fault in the LittleTujunga Canyon area, upthrusts out of the SanGabriel fault zone are exposed Fig. 15 . Reversefaulting in this area accompanied the San Fernando earthquake of February 9 1971; seePalmer and Henyey, 1971.Layered-sand models Emmons, 1969 are alsoinstructive in studying the cross-sectional characteristics of wrench faults. The fault zone widensas the wrench fault splays upward, and individualfaults have normal or reverse dip-slip separation,depending on how adjacent fault blocks are dis;placed within the wrench zone Fig. 16 .An important result of divergent wrenching isan overlay of extensional block faulting on the

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90 Ronald E Wilcox, T P. Harding, and D R Seely

. .. .. .. .. .. FAULT

NIo 1e

MILES

RIDGE BASIN SEDIMENTS PRE-RIDGE r I

BASIN ROCKS L---.JVIOLIN BRECCIA

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SAN ANDREAS FAULT ZONESAN GABRIEL FAULT

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1 0 0

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LITTLE ROCK

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o QUATERNARYr TERTIARYL.:..;.J SEDIMENTARY il MESOZOIC IGNEOUS METAMORPHIC= FAULT

cFlO Wrench structures along the San Andreas wrench-fault system, north of Los Angeles, California. A Pod-chaped majorslice between San Andreas and San Gabriel wrench faults. B Cross section of Ridge basin, formed and filled with sediments in the1orthern part of pod during wrenching. C Braiding of faults along San Andreas wrench fault zone on 1ortheastern side of''pod''; note right-lateral shift of Little Rock Creek and tilted fault blocks, evidenced by varied outcrop pattern.

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asic rench e toni s

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19/23

92 Ronald E Wilcox, T P. Harding, and D R SeelV

FlO I Cay modd of lS-JllYff (nt Ji&ht-latcn.l ....cnch r ull


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asic Wrench Tectonics 9

AASOUTHLOPEZ FAULT

Saugus

formationNORTH

2 5MILESSCALE :

FIG IS Upthrust structures caused by wrenching. A Map of upthrusts high-angle reverse faults) along San Gabriel fault zone,Little Tujunga Canyon area, north of Los Angeles, California. B Cross section of upthrusts, Little Tujunga Canyon area,

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21/23

Ronald E Wilcox T. P Harding and R Seely

AFTER EMMONS 969

=

T 1

FIG 16-Layered-sand model of a curved, right-lateral wrench fault (radius of curvature, 24 in.). A. Phot0 1 aph of cross sectionthrough center of model 15 in. high and II in. wide). B. Line drawing of faults in model; right-lateral wrencli movement shown by (away) and T(toward).

simple wrench pattern (Fig. 17A . Grabens formin preference to horsts, and nearly all fractureshave a tendency to develop into high-angle normal faults with oblique slip. n e helon folds arepoorly developed and have low relief along divergent wrenches, but warping of fault blocks toproduce closures between the faults is possible.The Fitzroy trough in northwestern Australia(Fig. 17B is probably a divergent wrench graben.It appears that wrenching formed the trough,which filled with sediments, and a final episode ofminor wrenching deformed the basin fill. En e -h lon folds in the trough and a zone of en e h-elon normal faults in the adjoining but shallowerNortheast Canning basin are properly orientedfor the inferred right-lateral wrench zone alongthe trend of the trough (Rattigan, 1967; Smith,1968 .CONCLUSIONSLarge quantities of oil and gas are trapped instructures caused by wrenching or influenced by

some aspect of wrench tectonics. Knowledge ofthe wrenching structural style is especially usefulin exploration because the basic structural patterns of wrenching are simple and consistent andare well documented from many areas. The struc-

tures and structural traps to be expected in awrench terrane generally can be predicted with ahigh degree of confidence.The principal elements of the basic wrenchpattern are 1 en e helon folds inclined at alow angle to the wrench zone; 2 conjugatestrike-slip faults, including synthetic faults inclined at a low angle to the wrench zone but inthe opposite direction from the folds, and antithetic faults nearly perpendicular to the wrenchzone; 3 the main wrench fault, parallel or subparallel with the wrench zone; and 4 normalfaults or tension joints oriented perpendicular tothe fold axes. Any combination of these structures may form within a given wrench zone, andthe recognition of anyone or a combination ofthem usually will serve to define the trend anddisplacement sense of the wrench zone.Three general styles of wrenching arerecognized: (I) simple parallel wrenching, inwhich crustal blocks move parallel with thewrench fault; 2 convergent wrenching, causedby blocks moving obliquely toward the wrench;and 3 divergent wrenching, resulting from oblique movements of the blocks away from thewrench. All three styles develop on both localand regional scales.

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Basic Wrench Tectonics 95

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Ronald E Wilcox T P Harding and D R SeelyREFEREN ES ITEDAharoni, E 1966, Oil and gas prospects of Kurnub GroupLower Cretaceous in southern Israel: Am. Assoc. Petroleum Geologists Bull., v. 50, no. II , p. 2388-2403.Anderson, E. M., 1951, The dynamics of faulting and dykeformation, with applications to Britain, 2d ed. : Edinburgh,Oliver and Boyd, 206 p,Bishop, D. G 1968, The geometric relationships of structuralfeatures associated with major strike-slip faults in New Zealand: New Zealand Jour. Geology and Geophysics, v 1I, no.2, p. 405-417.Campbell, J , D. , 1958, En echelon folding: Econ. Geology, v.53,no.4,p.448-472,Cloos, E., 1955, Experimental analysis of fracture patterns:eo Soc, America Bull., v 66, no, 3 p. 241-256.Crowell, J. C, I954a, Strike-slip displacement of the San Gabriel fault, southern California, pt. 6 Chap, 4, in R, H. Jahns,ed Geology southern California: California Div, MinesBull. 170 p, 49-52. 1954b, Geology of the Ridge bas in area, Los Angelesand Ventura Counties, in R. H, Jahns, ed Geology of southern California: California Div. Mines Bull. 170, Map Sheet 7Dibblee, T. W Jr., 1968, Displacements on the San Andreasfault system in the San Gabriel, San Bernardino, and SanJacinto Mountains, southern California, in W. R, Dickinsonand A, Grantz, eds., Proceedings of conference on geologicproblems, San Andreas fault sys tem: Stanford Univ, Pubs,GeoL Sci v II , p, 260-278,Dobbin, E., and C. E Erdmann, 1955, Structure contourmap of the Montana plains: U.S, GeoL Survey Oil and GasInv, Map OM 178A, scale 1:500,000,Emmons, R. 1969, Strike-slip rupture patterns in sand models: Tectonophysics, v 7, no. I p 71-87.Hamilton, W 1972, Preliminary tectonic map of the Indonesian region, scale 1:500,000: U,S, GeoL Survey Open FileRept.

Harding, T, P., 1973, The Newport-Inglewood trend, California-an example of wrenching style of deformation: Am.Assoc. Petroleum Geologists Bull., v. 57, no. I in press .Heyl, A. V., M. R. Brock, J. L Jolly, and E. Wells, 1966,Regional structure of the southeast Missouri and IIIinoisKentucky mineral districts: U.S. GeoL Survey Bull. 1202-B,p.1-20.Hoots, H. W., T. Bear, and W. D. Kleinpell, 1954, Geological summary of the San Joaquin Valley, California, pt. 8Chap. 2, in R. H. Jahns, ed., Geology of southern California:California Div. Mines Bull. 170, p. 113-129.Howell, B. F., Jr. , 1954, Geology of the Little Tujunga area, LosAngeles County, in R. H. Jahns, ed., Geology of southernCalifornia: California Div. Mines Bull. 170 Map Sheet 10.

Jaeger, J. and N. G. W. Cook, 1969, Fundamentals of rockmechanics: London, Methuen and Co. Ltd., 513 p.

Jennings, C W., and B. W. Troxel , 1954, Geologic guidethrough the Ventura basin and adjacent areas, southern California, in R. H. Jahns, ed., Geology of southern California:California Div. Mines Bull. 170, Geologic Guide No.2, 63 p.San Gabriel Mountains Section, p. 15-19 .Kennedy, W. Q., 1946, The Great Glen fault: GeoL Soc. London Quart. Jour., v. 102, pt. I, p. 41-76.Lowell, J. D., 1972, Spitsbergen Tertiary orogenic belt and theSpitsbergen fracture zone: GeoL Soc. America Bull., v. 83, in

press .Moody, J. D., and M. J. Hil l, 1956, Wrench-faul t tectonics:GeoL Soc. America Bull. , v. 67, no. 9 p. 1207-1246.Noble, F., 1954, The San Andreas fault zone from SoledadPass to Cajon Pass, California, pt. 5 Chap. 4 in R. H. Jahns,ed Geology of southern California: California Div. MinesBull. 170, p. 37 -48.Palmer, D. F., and T. L Henyey, 1971, San Fernando earthquake of 9 February 1971: pattern of faulting: Science, v.172, no. 3984, p. 712-715.

Quennell, A. M., 1959, Tectonics of the Dead Sea rift: Asociacion de Servicios Geologicos Africanos 20th Internat. Geol.Cong., Mexico, D.F., 1956, Actas y Tr., p. 385-405.Ramsay , J. G., 1967, Fo ld ing and fracturing of rocks: NewYork, McGraw-Hill, 568 p.Rattigan, J. H., 1967, Fold and fracture patterns resulting frombasement wrenching in the Fitzroy depression, Western Australia: Australasian Inst. Mining and Metallurgy Proc., no.223, p. 17-22.Salvador, A., and R. M. Stainforth, 1968, Clues in Venezuela to

the geology of Trinidad, and vice versa: 4th Caribbean GeoLConf. Trans., 1965, p. 31-40.Sigit, Soetarjo, 1962, Geologic map of Indonesia, scaleI:2,000,000: U.S. GeoL Survey, Misc. Geol. Inv. Map 1-414.Smith, J. G 1965, Fundamental transcurrent faulting in northern Rocky Mountains : Am. Assoc. Petroleum GeologistsBull., v. 49, no. 9 p. 1398-1409. 1968, Tectonics of the Fitzroy wrench trough, WesternAustralia: Am. Jour. Sci., v. 266, no. 9, p. 766-776.Tchalenko , J. S., 1970, Similarities be tween shear zones ofdifferent magnitudes: Geol. Soc. America Bull., v. 81, no. 6,

p. 1625-1640. and N. N. Ambraseys, 1970, Structural analysis of theDasht-e Bayaz Iran) earthquake fractures: GeoL Soc. America Bull., v. 81, no. I, p . 41-60.Yerkes, R. F., T. H. McCulloch, J. E. Schoel lhamer, and J. G.Vedder, 1965, Geology of the Los Angeles basin, California,an introduction: U.S. Geol. Survey Prof. Paper 420-A, 57 p.

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