1he American Association of Petroleum Geokigists Bultetin
V, 57, No, 1 (Jatiuary 1973), P, 74-96,16 Figs,, 1 Table
RONALD E. WILCOX,' T. P. HARDING,' and D. R. SEELY'
Houston, Texas 77001
Abstract En Schelon structures which may trap oil and gas develop in a systematic pattern along wrench zones-in sedimentai^ basins. Laboratory clay models simulate the formation of en Echelon folds and faults caused by wrenching. Folds form early in the deformation and are accompanied or followed by conjugate strike-slip, reverse, or normal faulting. Deformation may cease at any stage or may continue until strike slip along the wrench zone produces a wrench fault and separation of the severed parts of early structures. Oblique movements of fault blocks on opposite sides of a wrench fault cause divergence or convergence and enhancement, respectively, of extensional or compressional structures. Basins form in areas of extension and are filled with sediment, whereas upthrust blocks emerge in areas of compression and become sediment sources. The combined effects of wrenching in a petroliferous basin are to increase its prospectiveness for major hydrocartwn reserves.
INTRODUCTION
Wrench faults (Kennedy, 1946; Anderson,
1951) are high-angle strike-slip faults of great
Hnear extent along which strike-slip may be tens of miles or considerably more. Basement invariably is involved in the deformation and a wrench zone is a swath of terrane deformed by wrenching prior to and concurrently with strike-slip along the throughgoing wrench fault. The term
"wrench 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 useful for recognition of wrench zones (Figs. 1-5). A single en echelon fold can be depicted as an ellipse (Fig. 6), which represents the deformation of a circle in the wrench zone, with the longer ellipse axis (A-A') parallel with the fold axis.
Other structural straps can be formed by faulting or a combination of faulting and folding. Four types of fractures can form during wrench deformation, and if the wrenching continues, any one or all of these fractures can become faults. In
Figure 6, the fracture directions are shown as X-
X' (the strike of the primary wrench fault or wrench zone), C-C and D-D' {en echelon conjugate shear joints or strike-slip faults), and B-B'
(en echelon tension joints or normal faults). The development and interrelations of these faults and the en echelon folds are the main subjects of this paper. In a previous paper Moody and Hill
(1956) have treated aspects of wrench tectonics, particularly as these jjertain to proposed systems and patterns of sets of wrench faults.
Prolific reserves of hydrocarbons have been trapped in wrench structures, mainly in en ech-
elon folds and faulted folds. Some of the largest and best known of these structures are anticlinal traps in the Los Angeles basin (Fig. 4) and the west side of the San Joaquin VaUey (Fig. 5),
California (see also Harding, 1973, the following paper in this issue).
Clay models that illustrate the mechanics and development of this structural style represent broad basins filled with structurally homogeneous sediments whose total thickness is small compared with the size of the basin. The models also aid in prediction of traps by providing visual examples of the three-dimensional relations between structural elements in wrench zones.
MECHANICS OF WRENCHING
Wrench faults form in response to horizontal shear couples within the earth's crust, and they can be simulated in clay models by moving tin sheets beneath a clay cake (Cloos, 1955). Simple wrenching results from the movements of the crustal blocks or tin sheets in opposite directions parallel with their adjacent edges. As a consequence of such parallel displacements, compressional and tensional stresses are generated in the overlying sediments or clay. If, instead of moving exactly parallel with the wrench fault, the basement blocks or the tin sheets converge or diverge sHghtly, the compressional or tensional stresses, respectively, that result from the basic wrench are enhanced. These important special cases of convergent and divergent wrenching are discussed after analysis of the more general case of simple parallel wrenching.
'Manuscript received, February 18, 1972; accepted, April 17,
1972.
^Esso Production Research Co.; present address: P.O. Box
1230, Bellaire, Texas 77401.
'Humble Oil & Refining Co.
' Esso Production Research Co.
We are grateful to E. Cloos, consultant to Esso Production
Research Co., for contributing helpful suggestions during the course of this work. Our appreciation also is extended to J.
Crowell for stimulating discussions.
® 1973. The American Association of Petroleum Geologists. All rights reserved.
74

Basic Wrench Tectonics
, WRENCH FAUL T
l A F T E R S I G I T . 1 9 6 2 ; H A M I L T O N . 197 2)
MARGARITA f N D I A jv
O C E A N
( F I E L D S A P P R O X . F R O M C A M P B E L L ET A L . 197
FAULTS: 1.URICA 2, SAN F RANCISCO
3 SOLDADO 4 LOS BAJOS
0 TOO 200
I I I
KILOMETERS
W — E L PILAR FAULT
75
( A F T E R S A L V A D O R A N D S T A I N F O R T H . 19«S)
• DEADSEA WRENCH FAULTS
OTHER FAULTS
OIL AND GAS \ \
FIELDS
^^f--^iin:RRA,\tA\StA
( A F T E R Q U E N N E L L . t B 5 » ; A H A R O N I , I 9 « « )
FIG. \~~En echelon folds along wrench faults. A. En echelon 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 factor in deformation here. B. El Pilar fault and associated faults and en echelon folds in eastern Venezuela and Trinidad; note production from folds near Los Bajos fault, southwestern Trinidad. C. Dead Sea rift, Israel and Jordan; note location of Dead Sea between overlapping ends of major wrenches. Some en echelon folds are bounded by thrusts and several are marginally productive.

76 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
1 ALPINE FAULT
2 A W A T E R E F A U L T
3 CLARENCE FAULT
4 HOPE FAULT
(FROM BISHOP, 1968)
(FROM BISHOP, 19681
» WRENCH FAULT
OTHER FAULT
• FOLD
• DIKE A N D SILL TRENDS
/
&
• Tapuaenuku
CLARENCE
^ ^ ' ^ ^ ^ : ^ ^
(FROM BISHOP. 1968)
FIG. 2—Wrench-fault structures, New Zealand. A, Index map. B. En echelon folds along Alpine fault. C. Awatere and Clarence wrench faults and associated en echelon folds, dikes and sills, and subsidiary faults.

Basic Wrench Tectonics 77
SIMPLE PARALLEL WRENCHING
Simple parallel wrenching is a special case of simple shear, which is one kind of finite homogeneous strain (Jaeger and Cook, 1969; Ramsay,
1967). The shear angle (;*, Fig. 6) increases with increasing simple shear. In some crustal deformation and in clay models the initial deformations are plastic and involve folding. These are followed by a combination of plastic distortion and fracturing. As deformation proceeds, displacement along the wrench zone increases, and the zone of principal shear narrows. Finally, all of the slip occurs along a few closely spaced faults or along one throughgoing wrench fault, and subsequent deformations within either fault block are more or less independent of each other.
On a wrench model (Figs. 7, 8), it is convenient to mark the clay surface with a circle and to note how its shape changes during deformation. Points moving closer together mark compression, and points moving apart denote extension. The original circles (Fig. 7A) on the clay are aligned along the edge of the underlying tin sheet and deform into eji echelon ellipses during the plastic phase of strain (Fig. 7B). Straight lines on the clay (Fig.
7A, normal to the line of circles) are trowel marks that become bent during deformation (Figs. 7B-
C, 8D-F). Maximum compression and extension are parallel with the minor and major strain ellipse axes, respectively, and neither of these directions is parallel with or perpendicular to the shear direction imposed on the model, i.e., the strike of the wrench zone defined by the parallel edges of the tin sheets and the line of circles. It follows from the en echelon arrangement of ellipses (Fig.
7B) that all structures associated with each ellipse
(Fig. 6) may be repeated along the wrench zone.
This en echelon repetition of folds and faults is an important diagnostic feature of wrench zones
(Figs. 1-5). (The size and spacing of circles/ellipses on the models is arbitrary; the spacing of folds and faults in the model wrench zones is determined by various characteristics of each model.)
The clay models of wrenching are all basically alike. The model in Figures 7 and 8 has left-lateral displacement, whereas the models in Figures
9 and 10 are right-lateral wrenches. (By convention, the sense of fault displacement is described by assuming that the block toward the observer is fixed, and the block across the wrench fault from the observer moves to his right or left.) Various structures form on each model, however, depending on the thickness and nature of the wet-clay cake, on the rate of deformation, on any special conditions built into the model, and to a certain degree, on chance. Included in the "chance" aspect that helps to determine the final model structures are, for example, slight inhomogeneities in the texture of the clay and the presence of hidden bubbles beneath the clay surface.
By analogy, the explorationist is faced with a host of unknown (chance) factors in interpreting wrench zones. Some of the more obvious factors are the effects of nonuniform stratigraphy (both thickness and composition), variable rates of deformation, and different directions of movement between crustal blocks during one stage of deformation or during succeeding stages. In spite of these inherent complexities in both nature and the models, however, the overall pattern of wrenching has key elements that are repeated, and the presence of any one or more structures of the basic pattern serves as a clue for recognizing this structural style and its associated prospective structures.
The structures of the basic wrench-tectonic patterns are en echelon folds, en echelon conjugate strike-slip faults, the main wrench fault or wrench-fault zone, and en echelon normal faults.
These are described below and are illustrated in the models (Figs. 7-10).
En Echelon Folds
En echelon folds are the most attractive prospective structures in wrench zones because they form early and thus provide traps during early hydrocarbon migration, and because they commonly afford the largest closures that are genetically related to wrenching (Harding, 1973). As the amount of displacement on the wrench zone increases, the initial folds are broken first by fractures and then by faults. In later stages of wrenching the folds may become shattered (Fig.
9C), and parts of the folds on either side of the wrench fault may be offset (Fig. IOC). As movement of crustal blocks continues over long periods of geologic time, the half-folds on one block can be removed completely away from the area, and the wrench fault itself may provide updip closure.
The term "en echelon" refers to the arrangement of structures along a hnear zone so that individual folds or faults of the same kind are parallel with each other and are inclined equally to the strike of the zone. The nomenclature for describing en echelon fold sets is similar to that for wrench displacements. Right-lateral wrenches produce right-handed fold sets (Fig. 11 A), where a traverse along the axis of any fold to its terminus would turn right to reach the next fold in the
en echelon set (Campbell, 1958). A left-handed

78 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
Chnh Khjnflri
' T O M ' A * ^
100 200 300 400 METERS
^o^>9.
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( F R O M T C H A L . E N K O A N D A M B R A S E Y S , 1970) ^
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108 W
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( A F T E R H E Y L ET A l . . , i g 6 6 i ) O
FIG. 3—Fracture patterns in wrench zones.
A. Part of Dasht-e Bayaz (Iran) earthquake fracture zone along left-lateral wrench. Western part shows development of synthetic en
echelon faults; eastern end shows both antithetic and synthetic en echelon faults. B. Synthetic and antithetic en echelon fractures
(enlargement of east end of wrench zone in A above). C. Lake Basin fault zone, Montana, showing en echelon normal faults along indicated wrench zone. D. Cottage Grove fault zone, Illinois; note en echelon normal faults, parallel mafic dikes, and reversal of vertical separation sense on throughgoing strike-slip fault. Vertical components indicated in feet.

Basic Wrench Tectonics 79 set of en echelon folds in eastern Panama (Fig.
1 IB) is probably related to a left-lateral wrench.
All en echelon folds in one zone are usually of similar shape and extent. The folds in Figure 9 are more distinct and more uniform than is usual for clay-wrench models, because a thin sheet of plastic film (0.0005-in. thick) was interlayered in the clay 0.25 in. below the surface. Several larger
en echelon folds developed in the other two models (Figs. 7, 8, 10), which are homogeneous clay cakes without plastic film. The folds in Figures 7 and 8 are low and only faintly visible, whereas those in Figure 10 are larger. This difference probably is explained by the rates of deformation; the model with distinct folds (Fig. 10) was deformed 2.5 times faster than the other model
(Figs. 7-8). wrench-zone trend is known or suspected, and the displacement sense is unknown, folds still could be anticipated along the wrench trend with their axes inchned about 30° to that trend.
In nature (Figs. 1-5), fold orientations in a wrench zone can be different for several folds along the same fault trend. Some folds, or parts of folds with irregular axial trends, may parallel the wrench fault or cross the wrench zone at a low angle. Several factors that can influence the shape and trend of en echelon folds include convergence of blocks during wrenching, changes in strike of the wrench fault, large components of vertical displacement, differences in kind and thickness of sediments, and mobility of basement near the folds.
A close examination of Figure 9A reveals a small difference between the average fold trend and the trend of the longer axes of the ellipses.
This difference probably is accentuated by the presence of the thin plastic sheet, which has influenced strongly the folding. Other similar experiments have shown that the fold size and fold spacing in the wrench zone are related to the depth of burial of the plastic film below the clay surface. Shallower plastic sheets produce smaller, more closely spaced folds. Another characteristic unique to models with plastic film layers is the rapidity of folding after slow deformation begins.
In the extreme case of the plastic sheet directly on the clay surface, a very slight distortion by wrenching immediately causes folding in the plastic sheet and in the clay just below.
In models without the plastic sheet (e.g.. Fig. lOB), the longer ellipse axes are nearly parallel with the axes of the clay folds. This is similar to the ellipse diagram (Fig. 6), but the model ellipses are not so elongate as the ellipse in Figure 6.
For a true simple shear the angle between the fold axis (long axis of the ellipse) and the strike of the wrench zone is always less than 45°. For most wrench-fault experiments with clay, the angle between en echelon fold axes and the wrench fault approximates 30°. Folds that form later during the deformation have lower angles.
Fortunately, in the early stages of exploration in an area where wrenching is suspected, the recognition of several typical wrench-zone structures will serve to define the trend of the zone itself and probably also the sense of wrench displacement. By extrapolation from models, the axes of en echelon folds, which may be subtle, low-relief closures, should lie at an angle of 30°
± 15° to the wrench trend, either in a clockwise direction (left-handed folds) or in a counterclockwise direction (right-handed folds). If the
Conjugate Strike-Slip Faults
Wrenching causes two sets of intersecting, vertical fractures to form in a predictable orientation along the wrench zone. One set, the low-angle fractures (C-C, Fig. 6), makes an angle between
10 and 30° with the wrench strike (X-X9, whereas the high-angle set (D-D') intersects the wrench at an angle between 70 and 90°. These conjugate 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 rocks and the deformation; it is usually in the range of
60-70°. This angle is bisected by the direction of maximum compression (B-B', Fig. 6). On the clay model in Figure 7C, one fracture of each set forms an "X" cutting the center small ellipse. The wedge in the acute angle of the intersection is displaced (Fig. 8D) toward the center of the ellipse as deformation continues. Two important aspects of the deformation are illustrated by this wedging: (1) the opposite senses of lateral displacement on the two intersecting strike-slip faults; and (2) contemporaneous plastic deformation and faulting.
The low-angle faults (Fig. 7C) intersect the wrench strike (line of ellipse centers) at 12° and have the same sense of displacement (left) as that of the main wrench zone (Figs. 7B-C, 8D-E) and the flnal wrench fault (Fig. 8F). These low-angle faults are called synthetic strike-shp faults, or simply 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-lateral wrench model. They form angles of 78° with the wrench and 66° with the synthetic fault in the center ellipse (Fig. 7C). The low- and high-angle

80 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
'\ V .
SAN GABRIEL M O U N T A I N S
(AFTER Y E R K E 5 ET A C , 1965)
STRUCTURE CONTOURS
Drawn on basement rock surface.
EXPLANATION
FAULT RE VERSE FA UL T NORMAL FA UL T
ANTICLINE SYNCLINE Q^^ F / ^ L D
Showing direction of plunge Showing direction of plunge
FIG. 4—Major wrench structures and oil fields, Los Angeles basin, California.

Basic Wrench Tectonics 81
SOUTHERN SAN JOAQUIN VALLEY
CONTOURS ON TOP OF LOWER PLIOCENE
VARIABLE CONTOUR INTERVAL
^
MAJOR SURFACE
STRUCTURES
GENERALIZED POROSITY
TERMINATIONS
( A F T E R H O O T S . B E A R . A N D K U E M P E L L , I 9 5 4 |
FIG. 5—Major wrench structures and oil fields, San Joaquin Valley, California.

82 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
FIG. 6—Strain ellipse. conjugate fractures have been termed Riedel shears and conjugate Riedel shears, respectively, by Tchalenko and Ambraseys (1970).
Continuing deformation after the conjugate fractures have developed proceeds as a combination of strike-slip faulting and plastic distortion.
The acute angle between the two faults enlarges as the two faults rotate away from each other.
The supplementary obtuse angles decrease as the larger wedges bounded by them move outward along the long-ellipse axis (A-A' Fig. 6), which marks the direction of extension (or minimum compression).
The rotation of the conjugate faults is an internal (local) rotation caused by compressive deformations and is not related uniquely to wrenching.
The same conjugate fault pattern, wedging, and internal fault rotation are possible when rocks (or clay) are subjected to straight external compression, that is, when the compressive forces are opposed on a straight line (Ramsay, 1967, p. 60).
Wrenching, however, also produces external
(regional) rotational deformation. The wrenching forces, which result from regional simple shear, act in opposite directions as if on separate, parallel lines so as to form a couple. The resulting deformation generally is restricted to a linear wrench zone parallel with the couple and to the edges of the moving crustal blocks. A left-lateral wrench has an external sense of rotation that is counterclockwise (Figs. 7, 8), whereas right-lateral wrenches have clockwise external rotation
(Figs. 9, 10). This can be seen in the models by noting the rotation of the ellipse axes as wrenching proceeds.
The effects of both the internal rotation due to wedging and the external rotation due to wrenching further distinguish synthetic and antithetic faults. For a left-lateral wrench (Figs. 7, 8), external rotation tends to move the synthetic fault counterclockwise away from the wrench trend as the internal rotation tends to move the fault clockwise toward the main wrench. The result is
Uttle rotation of the synthetic fault in either direction. It originally formed nearly parallel with the strike of the main wrench zone and, therefore, remains in this favorable orientation to accommodate additional wrench displacements.
The antithetic faults, however, formed at a high angle to the wrench, and the continuing deformation cause both the external and the internal rotations to be counterclockwise (Figs. 7,
8). This tends to increase further the original high angle to around 90° to the wrench zone. As a consequence, lateral displacements on antithetic faults are generally small compared with those on either their synthetic counterparts or the main wrench fault. In some cases, the high-angle position of the antithetics is so poorly favored for displacements as to preclude their formation. In all the clay models (Figs. 7-10) synthethics are much better developed and account for much more wrench displacement than the antithetics.
The combined effects of external and internal rotation on the fault sets are compared in Table 1 for the left-lateral wrench model (Figs. 7, 8). Note a second set of conjugate shears, nearly parallel with the first set, cutting the center ellipse (Fig.
8D-F).
A useful clue to interpretation is provided by the antithetic faults that have been rotated. Their original planar attitude becomes bent by the combined internal and external rotations acting in opposite directions on either side of the wrench zone. The map view of the twisted faults is a flat S with the arcs of the S pointing toward the direction of displacement, i.e., S for left wrenches (Fig. 8D-F), and a reverse S for right wrenches (Fig. IOC).
Wrench Faults
The development of the main, throughgoing wrench fault is the last stage in the early phase of wrench-zone deformation. The entire early phase of wrenching usually constitutes a brief and tran-

Basic Wrench Tectonics 83
FIG. 7—Clay model of parallel left-lateral wrench fault (A-C = three stages, vertical views). See Figure 8 for three following stages.

84 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
FIG. 8—Clay model of parallel left-lateral wrench fault (D-F = three stages, vertical views). See Figure 7 for first three stages.

Basic Wrench Tectonics
85
FIG. 9 - C l a y model of parallel right-lateral wrench fault with layer of thin plastic film embedded 0.25 in. below surface to enhance
en echelon folds (A-C = three stages, vertical views).

86
Ronald E. Wilcox, T. P. Harding, and D. R. Seely
FIG. 10—Clay model of parallel right-lateral wrench fault (A-C = three stages, vertical views).

Basic Wrench Tectonics 87 sitory period in the long history of a major wrench fault, but this early phase is of great importance in the process of hydrocarbon-trap formation.
After a short interval of concurrent folding and conjugate faulting, the rocks (or clay) fracture in a relatively narrow zone within the overall deformational swath, and the master wrench fault is created. TTiis process of rock failure begins at several points along the wrench zone {e.g., see
Fig. 8E, between small circles 4 and 5, 7 and 8, 9 and 10). At some locations a synthetic fault deviates into the incipient wrench-fault trend, and at others a new fracture forms more nearly parallel with the strike of the wrench zone and at a small angle to the nearby synthetic faults. As this process continues, the main wrench fault gradually emerges as an interconnected series of these earlier fractures. (The plastic film prevented the formation of the single wrench fault in the model in
Fig. 9.)
A great variety of fault blocks is produced within the wrench zone. Some large blocks are caught between early formed branches of the main wrench (Fig. 8F, near large ellipse at left), and many smaller blocks are sliced and delbrmed into horsts and grabens between the main wrench fault and the conjugate faults. Once individual fault blocks are separated by faulting, they tend to deform somewhat independently; some rise, some sink, some are folded, and some are faulted again.
As displacement on the main wrench fault increases, slip diminishes on the other faults in the zone. The active fault "plane," or a relatively thin, crush zone along the active part of the fault, commonly shifts from side to side of the wrench zone. Distortion and faulting of the whole zone become complex, and this results in a braided fault pattern that is typical of major wrench zones (Fig. 12C).
Changes in the strike of the active fault lead to additional deformation of the wall rocks as strike slip continues. The parallel wrench becomes a convergent or a divergent wrench, at least locally.
The size and extent of the resulting compressional or extensional structures depend on the amount of change in fault strike and the amount of displacement along the curved fault surface within the braided system (see Fig. 12 and accompanying text discussion of convergent and divergent wrenching).
(Fig. 6, B-B"), crosses the en echelon fold axes at right angles, and bisects the acute angle between the conjugate shears. En echelon tension fractures may form along a wrench zone in the initial stage of deformation, but they easily are destroyed as wrench displacement increases and compressive structures (folds and conjugate faults) become more prominent. In clay models of wrenching, tension fractures are uncommon because of the strong cohesion within the clay.
Water placed on the clay surface eliminates this cohesion, and large, open, en echelon tension cracks form to the exclusion of other fractures and folds.
Two examples of en echelon normal faults that are presumed to lie above buried wrench faults are the Lake Basin fault zone, Montana (Fig.
3C), and the Cottage Grove fault zone, Illinois
(Fig. 3D). In both these fault zones, the amount of wrench movement of the basement blocks after sedimentation has been small—just enough to fracture the overlying sedimentary rocks without causing significant lateral offset. Additional linear zones which may represent wrenching have been recognized near the Lake Basin zone
(Smith, 1965).
The Cottage Grove zone displays two other features of wrenching. The northern block of the main east-west fault is downthrown in the westem part of the zone and upthrown in the eastern part. This kind of change in the vertical displacement sense along strike is typical of wrenches.
The tensional component of wrenching is marked in the eastern area around the fault zone by mafic dikes. Such intrusions and vein fillings in tension fractures are well known in mineral deposits and plutonic terranes, and they fit the fracture pattern for wrenching along this zone.
Antithetic fractures inherit some of the tensional component of a wrench deformation and commonly become nearly vertical normal faults with negligible lateral displacements. A downward displacement on either of the conjugate strike-slip faults tends to be toward the acute wedge. This is well shown on one of the models
(Fig. IOC), where there are many closely spaced antithetic faults at both ends of the wrench zone.
Such concentrations of "antithetic-normal" faults impart a pseudoplasticity to the clay (or rocks) that permits these zones to deform more or less uniformly without being cut by one main wrench fault. Thus, a wrench fault with measurable strike slip can pass into One of these fracture zones along its strike where there is the same regional shift across the zone but no single fault of large lateral displacement.
Tension Fractures
The orientation of tension joints or normal faults parallels the short axis of the strain ellipse
CONVERGENT AND DIVERGENT WRENCHING
Opposed crustal blocks that do not move par-

88 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
L
L
Mtn H'COTHi-iiraL WRENCH ZO'
/^ O a i E M T A T I O N 0 1 l ) E " I V E C > COMFiRE
^
X
- . . ^ ' ^ ^ V ^ ^ ^ ^ K ^ ^ ^ - * ' ^ ' ' ^ - ' - ^ ^ ' ' ••<f- •^'"
•^S^^^sS^^I^^^
10
\ ,
\
FIG. II—En echelon folds. A. Diagrammatic right- and left-handed fold sets (caused by right- and left-lateral wrenching, respectively). B. Radar image of surface folds, Darien basin, eastern Panama, caused by left-lateral wrenching. (Imagery acquired by
Westinghouse Electric Corporation, under contract from U. S. Army TOPOCOM. Ft. Belvoir, Virginia.)

Basic Wrench Tectonics 89
Figure
T a b l e I . O r i e n t a t i o n o t C o n j u g a t e F r a c t u r e s
C u t t i n g C e n t e r o f S m a l l E l l i p s e
A p p r o x i m a t e
Shear A n g l e *
Angle Between
Wrench S t r i k e , and
S y n t h e t i c Fault
Angle Between
Wrench Strike, and
A n t i t h e t i c F a u l t
Angle Between
S y n t h e t i c and
A n t i t h e t i c Faults
7c
Sd
2 0 °
2 8 °
8e • 3 6 °
8f 4 8 °
' S e e f i g u r e 6.
1 2 °
1 2 °
1 4 °
1 5 °
7 8 °
8 2 °
8 7 °
9 3 °
6 6 °
7 0 °
7 3 °
7 8 ° allel with a wrench fault either converge or dihow major wrench faults can influence basin deverge as wrenching proceeds. These oblique velopment and sedimentation as well as the tecmovements may be related to nonparallel distonic history and structural style of a region. placements of crustal blocks on a regional scale, or they may be due to local changes in strike of a generally parallel wrench. It is common for both
En echelon folds in a clay model are enhanced by even a slight convergence of only 2° (Fig. 13).
In the early stage of movement, the folds are well convergence and divergence to develop locally developed throughout most of the central part of along a wrench. Convergent wrenching, on whatever scale, tends to enhance compressive wrenchzone structures, namely, folds and conjugate strike-slip faults, and strong convergence can filled this basin as faulting continued, and they the model (Fig. 13A), and a few synthetic fractures have formed. At a later stage (Fig. 13B), the folds have been offset along the synthetic faults and the incipient throughgoing wrench. A few cause reverse faulting and thrusting. The formation of tensional structures, mainly normal faults, is typical of divergent wrenching. antithetic faults also formed, but their importance in this deformation was minimal.
A particularly good example of both convergence and divergence is seen north of Los Ange-
A more intensive en echelon zone of compression develops along a model wrench with a conles, California, in the San Andreas wrench system
(Figs. 12, 15). A pod-shaped block, which is about 100 mi long and 20 mi wide, lies southwest of the San Andreas fault and northeast of the curving San Gabriel fault (Fig. 12A). Both faults are well-documented, right-lateral wrenches. which the fold axes are offset. A side view of the
The {xxl-shaped block has moved southeast along the curved San Gabriel fault and has caused convergence on its southern and southeastern margin. Reverse faults with strike-slip same model (Fig. 14B) reveals the complex thrusting of the wedges squeezed up and out of the wrench zone by the strong convergence. As these blocks rose, they were bounded by vertical components characterize this margin and attest or high-angle reverse synthetic faults, and they to the lateral wrenching combined with compression and high-angle thrusting. vergence of 15° (Fig. 14A). Good en echelon folds form in the narrow zone that later is uplifted, and both sets of conjugate shears are well developed. Nearly all wrench displacement is concentrated on the synthetic faults, along resemble upthrust blocks.
Just south of the San Gabriel fault in the Little
Concurrently, the northwestern part of the pod
Tujunga Canyon area, upthrusts out of the San was under tension as it diverged from the curving
Gabriel fault zone are exposed (Fig. 15). (Reverse northern end of the San Gabriel fault, and the
Ridge basin was formed (Fig. 12A). Sediments faulting in this area accompanied the San Fernando earthquake of February 9, 1971; see
Palmer and Henyey, 1971.) record the fault movements by preserving several unique rock types whose source areas were displaced alongside the basin (Fig. 12B).
Layered-sand models (Emmons, 1969) are also instructive in studying the cross-sectional characteristics of wrench faults. The fault zone widens as the wrench fault splays upward, and individual One such suite of gneissic rocks is preserved as coarse blocks in the Violin Breccia (Fig. 12A, B), which accumulated along the northeast side of the San Gabriel fault scarp as wrenching continfaults have normal or reverse dip-slip separation, depending on how adjacent fault blocks are displaced within the wrench zone (Fig. 16). ued from the late Miocene to the late Pliocene
(Crowell, 1954a, b). The Ridge basin illustrates
An important result of divergent wrenching is an overlay of extensional block faulting on the

90 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
"•ss,^ A'
\
N
1
1
RIDGE BASIN C-!-!-!-|:U|
SEDIMENTS t-g.'-iwi
VIOLIN BRECCIA
HKb-HIDGE 1 1
BASIN ROCKS 1 ] ^ ^ il*.''.'":-.'-:
^n^ ^--'
^ ^
^ L O C A T I O N OF
I T FIGURE 11c
.^r^
LOCATION OF P S j /
FIGURE M A - ' ^ ' - i ' - ' ^ ' " * ^ ^ ^ ^ ^
FAULT
0 10
1 . 1
MILES
^ ^ < ^
Pasadena • ^^* '^'^^<:i^
San Bernardino
(AFTER DIBBLEE. I«68)
VIOLIN BRECCIA
(PROM CROWELL, 19S4b|
RIDGE BASIN V//A PRE-HIOGE BASIN
B
SAN ANOREAS-
F A U L T
"LIT-I^ L E ' a i . ' R O ' ^ ' ^ ^ y ' ^ ^ ' ^ I
L/'rr/« Rock Creak- T
LITTLE ROCK
QUATERNARY
I
TERTIARY
SEDIMENTARY i l f I MESOZOIC IGNEOUS t ' i t ' i i a METAMORPHIC
(FROM NOaLE, l«S4| 34- 30'
Fio. 12—Wrench structures along the San Andreas wrench-fault system, north of Los Angeles, California. A. Pod-shaped major slice between San AndreiiB and San Gabriel wrench faults. B. Cross section of Ridge basin, formed and filled with sediments in the northern part of "pod" during wrenching. C. Braiding of faults along San Andreas wrench-fault zone on northeastern side of
"pod"; note right-lateral shift of Little Rock Creek and tilted fault blocks, evidenced by varied outcrop pattern.

Basic Wrench Tectonics 91
FIG. 13—Clay model of 2''-convergent right-lateral wrench fault (A, B = two stages, oblique views). (From unpublished work by P.
G. Temple.)

92 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
FIG. 14—Clay model of 15°-convergent right-lateral wrench fault (Lowell, 1972). A. Vertical view. B. Side view. Note reversals of vertical separation on synthetic faults in foreground and dominant strike-slip offset of fold axes.

Basic Wrench Tectonics 93
NORTH
SAN GABR
F A U L T ZONE
Saugus formation
LOPEZ F A U L T
t ^ Saugus formatii
SOUTH
2.5 MILES
SCALE 1:1
( A F T E R J E N N I N G S A N D T R O X E i . ,
1 9 5 4 ; H O W E L I . , 1 9 5 4 )
FIG. 15—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.
B

94 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
-rr Y" —'
Kv
W
\i
V\
\ ^
IT
(AFTER EMMONS, 1969| B
FIG. 16—Layered-sand model of a curved, right-lateral wrench fault (radius of curvature, 24 in.). A. Photo^aph of cross section through center of model (15 in. high and 11 in. wide). B. Line drawing of faults in model; right-lateral wrench movement shown by
A (away) and T (toward). simple wrench pattern (Fig. 17A). Grabens form in preference to horsts, and nearly all fractures have a tendency to develop into high-angle normal faults with oblique slip. En echelon folds are poorly developed and have low relief along divergent wrenches, but warping of fault blocks to produce closures between the faults is possible.
The Fitzroy trough in northwestern Australia
(Fig. I7B) is probably a divergent wrench graben.
It appears that wrenching formed the trough, which filled with sediments, and a final episode of minor wrenching deformed the basin fill. En 6c-
belon folds in the trough and a zone of en 6ch-
elon normal fauhs in the adjoining but shallower
Northeast Canning basin are properly oriented for the inferred right-lateral wrench zone along the trend of the trough (Rattigan, 1967; Smith,
1968).
CONCLUSIONS
Large quantities of oil and gas are trapped in structures caused by wrenching or influenced by some aspect of wrench tectonics. Knowledge of the wrenching structural style is especially useful in exploration because the basic structural patterns of wrenching are simple and consistent and are well documented from many areas. The structures and structural traps to be expected in a wrench terrane generally can be predicted with a high degree of confidence.
The principal elements of the basic wrench pattern are (1) en Schelon folds inclined at a low angle to the wrench zone; (2) conjugate strike-slip faults, including synthetic faults incUned at a low angle to the wrench zone but in the opposite direction from the folds, and antithetic faults nearly perpendicular to the wrench zone; (3) the main wrench fault, parallel or subparallel with the wrench zone; and (4) normal faults or tension joints oriented perpendicular to the fold axes. Any combination of these structures may form within a given wrench zone, and the recognition of any one or a combination of them usually will serve to define the trend and displacement sense of the wrench zone.
Three general styles of wrenching are recognized: (1) simple parallel wrenching, in which crustal blocks move parallel with the wrench fault; (2) convergent wrenching, caused by blocks moving obliquely toward the wrench; and (3) divergent wrenching, resulting from oblique movements of the blocks away from the wrench. All three styles develop on both local and regional scales.

Basic Wrench Tectonics 95
PRECAMBRIAN
CANNING BASIN
" ^ - - v
\
PRECAMBRIAN ^ ^ ^
MESOZOIC AND PALEOZOIC
N
PILBARA BLOCK
L_
100
I
PROVINCE
BOUNDARY
B lAFTKR SMITH. I 06I)
FIG. 17—Divergent wrenching. A. Clay model of 15°-divergent right-lateral wrench fault. B. En echelon folds and faults in the
Fitzroy trough, western Australia.

96 Ronald E. Wilcox, T. P. Harding, and D. R. Seely
REFERENCES CITED
Aharoni, E., 1966, Oil and gas prospects of Kurnub Group
(Lower Cretaceous) in southern Israel: Am. Assoc. Petroleum Geologists Bull., v. 50, no. 11, p. 2388-2403.
Anderson, E. M., 1951, The dynamics of faulting and dyke formation, with applications to Britain. 2d ed.: Edinburgh,
Oliver and Boyd, 206 p.
Bishop, D. G., 1968, The geometric relationships of structural features associated with major strike-slip faults in New Zealand: New Zealand Jour. Geology and Geophysics, v. 11, 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:
Geol. Soc. America Bull., v. 66, no. 3, p. 241-256.
Crowell, J. C , 1954a. Strike-slip displacement of the San Gabriel fault, southern Califoriiia, pt. 6, Chap. 4, in R. H. Jahns, ed.. Geology of southern California: Cahfornia Div. Mines
Bull. 170, p. 49-52.
1954b, Geology of the Ridge basin area, Los Angeles and Ventura Counties, in R. H. Jahns, ed.. Geology of southern California: California Div. Mines Bull. 170, Map Sheet 7.
Dibblee, T. W., Jr., 1968, Displacements on the San Andreas fault system in the San Gabriel, San Bernardino, and San
Jacinto Mountains, southern California, in W. R. Dickinson and A. Grantz, eds.. Proceedings of conference on geologic problems, San Andreas fault system: Stanford Univ. Pubs.
GeoLSci., V. 11, p. 260-278.
Dobbin, C. E., and C. E. Erdmann, 1955, Structure contour map of the Montana plains: U.S. Geol. Survey Oil and Gas
Inv. Map OM 178A, scale 1:500,000.
Emmons, R. C , 1969, Strike-slip rupture patterns in sand models: Tectonophysics, v. 7, no. 1, p. 71-87.
Hamilton, W., 1972, Preliminary tectonic map of the Indonesian region, scale 1:500,000: U.S. Geol. Survey Open File
Rept.
Harding, T. P., 1973, The Newport-Inglewood trend, California—an example of wrenching style of deformation: Am.
Assoc. Petroleum Geologists Bull., v. 57, no. 1 (in press).
Heyl, A. V., M. R. Brock, J. L. Jolly, and C. E. Wells, 1966,
Regional structure of the southeast Missouri and lUinois-
Kentucky mineral districts: U.S. Geol. Survey Bull. 1202-B, p. 1-20.
Hoots, H. W., T. L. Bear, and W. D. Kleinpell, 1954, Geological summary of the San Joaquin Valley, California, pt. 8,
Chap. 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, Los
Angeles County, in R. H. Jahns, ed.. Geology of southern
California: California Div. Mines Bull. 170, Map Sheet 10.
Jaeger, J. C , and N. G. W, Cook, 1969, Fundamentals of rock mechanics: London, Methuen and Co. Ltd., 513 p.
Jennings, C. W., and B. W. Troxel, 1954, Geologic guide through 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. 1, p. 41-76.
Lowell, J. D., 1972, Spitsbergen Tertiary orogenic belt and the
Spitsbergen fracture zone: Geol. Soc. America Bull., v. 83, (in press).
Moody, J. D., and M. J. Hill, 1956, Wrench-fault tectonics:
Geol. Soc. America Bull., v. 67, no. 9, p. 1207-1246.
Noble, L. F., 1954, The San Andreas fault zone from Soledad
Pass to Cajon Pass, California, pt. 5, Chap. 4, in R. H. Jahns, ed.. Geology of southern California: California Div. Mines
Bull. 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: Asociaci6n de Servicios Geologicos Africanos 20th Internat. Geol.
Cong., Mexico, D.F., 1956, Actas y Tr., p. 385-405.
Ramsay, J. G., 1967, Folding and fracturing of rocks: New
York, McGraw-Hill, 568 p.
Rattigan, J. H., 1967, Fold and fracture patterns resulting from basement 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 Geol.
Conf Trans., 1965, p. 31-40.
Sigit, Soetarjo, 1962. Geologic map of Indonesia, scale
1:2,000,000: U.S. Geol. Survey, Misc. Geol. Inv. Map 1-414.
Smith, J. G., 1965, Fundamental transcurrent faulting in northern Rocky Mountains: Am. Assoc. Petroleum Geologists
Bull., V. 49, no. 9, p. 1398-1409.
1968, Tectonics of the Fitzroy wrench trough. Western
Australia: Am. Jour. Sci., v. 266, no. 9, p. 766-776.
Tchalenko, J. S., 1970, Similarities between shear zones of different magnitudes: Geol. Soc. America Bull., v. 81, no. 6, p. 1625-1640. and N. N. Ambraseys, 1970, Structural analysis of the
Dasht-e Bayaz (Iran) earthquake fractures: Geol. Soc. America Bull., v. 81, no. 1, p. 41-60.
Yerkes, R. F., T. H. McCuUoch, J. E. Schoellhamer, and J. G.
Vedder, 1965, Geology of the Los Angeles basin, California, an introduction: U.S. Geol. Survey Prof. Paper 420-A, 57 p.