BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 67, PP. 1207-1246. 26 FIGS. SEPTEMBER 1966 ' WRENCH-FAULT TECTONICS Bv J. D. MOODY AND M. J. HILL ABSTRACT Extending the work of E. M. Anderson, M. K. Hubbert, and W. Hafner on faulting, the authors develop the hypothesis that anticlinal folds, thrust faults, and wrench faults can be generated as a result of movement on a large wrench fault such as the San Andreas of California. Extension of this concept leads to the conclusion that for any given tec- tonic area, at least eight directions of wrench faulting and four directions of anticlinal folding and/or thrusting should accommodate the structural elements of that region; these directions should have a more or less symmetrical disposition relative to the direc- tion of the primary compressive stress. The angles a, (3, and y are defined to describe the geometry of such a wrench-fault tectonic system relatively completely. The authors' interpretations of tectonics in various areas indicate that wrench-fault tectonic systems do exist and are aligned systematically over large portions of the earth's crust as indicated by Hobbs, Vening Meinesz, Bonder, and others. Eight principal wrench directions are denned in terms of major elements of the earth's crust such as the Alpine fault of New Zealand. Structural elements aligned in these eight directions constitute major features of the regmatic shear pattern of Sender. The authors conclude that the shear pattern may have resulted from stresses which are oriented essentially meridionally and have been acting in nearly the same direction throughout much of crustal history. It is concluded that major wrench faults, which penetrate the entire outer crust of the earth and result in wholesale segmentation of the outer crust into polygonal blocks, constitute a fundamental type of yielding in the crust. Possible origins of the stresses involved, formation of geosynclines, island arcs, vol- canism, and crustal evolution are discussed in terms of these ideas. Some possible objec- tions and weak points in the argument are pointed out, and suggestions for further study are included. CONTENTS TEXT Page Pacific Northwest examples 1223 Page Midcontinent examples 1223 Introduction 1208 Other examples 1225 Purpose 1208 Geotectonics 1227 Principal references 1208 Regmatic shear pattern 1227 Definition of wrench fault 1208 Major wrench directions 1228 Wrench-fault tectonics 1208 Primary-stress orientation 1230 Acknowledgments 1209 Analysis of fault pattern of France 1233 Theory 1209 Possible origin of the regmatic shear Stress ellipsoid and the angle /3 1209 pattern 1234 Orientation of stress ellipsoid 1210 Periodic versus aperiodic orogeny 1236 Normal, thrust, and wrench faults 1210 Consequences of a segmented crust 1238 Second-order effects and the angle y 1211 "Island arcs" 1240 The angle a 1212 Igneous activity 1240 Wrench-fault tectonics 1212 Crustal evolution 1240 Field observations 1214 Conclusion 1241 Recognition of wrench faults 1214 Summary 1241 Dating of wrench faults 1215 Suggestions for further study 1242 California examples 1215 Possible objections to the thesis 1243 Basin and Range examples 1221 References cited 1243 1207 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/67/9/1207/3431718/i0016-7606-67-9-1207.pdf by guest on 29 September 2021 1208 MOODY AND HILL—WRENCH-FAULT TECTONICS ILLUSTRATIONS Figure Page 13. Thomas fault, Wichita Mountains, Figure Page Oklahoma 1225 1. Axes of the stress ellipsoid 1209 14. Ti Valley fault, Ouachita Mountains, 2. Shear directions in homogeneous media. 1210 Oklahoma 1226 15. New Zealand direction 1229 3. Theoretical fault orientations 1211 16. Oca direction 1230 4. Second-order wrench faults 1212 17. Great Glen direction 1231 5. Plan of wrench system under north- 18. Bartlett direction 1232 south simple compression 1213 19. Major tectonic directions, a = 0° 1233 6. Major fault trends of California 1216 20. Major tectonic directions, a = 345° 1234 7. Drag folds along San Andreas fault, 21. Relation of a to latitude, for 0 = 30°.... 1235 22. Structural pattern of the Dead Sea 1236 California 1217 23. Strain rosette for 176 faults in France... 1237 8. Right-lateral stream offsets along San 24. Diagrammatic sketch of geosyncline and Andreas fault 1218 geosynclinal sedimentary suite 1239 9. Inglewood fault trend, Los Angeles area, 25. Cloos' Silesian shear zone 1241 California 1219 TABLES 10. Calaveras fault, San Ramon area, Table Page California 1220 1. Theoretical wrench- and thrust-fault 11. Major lineaments of western United directions 1212 States 1222 2. Angle of drag folding, 7, along Inglewood 12. Hillside fault, Hudspeth County, Texas.. 1224 trend, California 1219 INTRODUCTION Definition of Wrench Fault Purpose The term wrench fault is adopted from Kennedy (1946) and Anderson (1951) to de- New concepts of fault dynamics, evolved scribe ruptures in the earth's crust in which through re-evaluation of published data on the dominant relative motion of one block crustal strain, mechanics of faulting, and field to the other is horizontal and the fault planes observations, are presented. The purpose of essentially vertical. The term is translated this paper is to develop an over-all hypothesis, from the German "blatt", originally used by based on these concepts, of the stress and Suess (1885), and is synonymous with strike- strain mechanisms in the earth's crust (ex- slip fault and transcurrent fault. The authors cluding gravity tectonics and salt tectonics). favor using wrench in deference to Anderson's and Kennedy's pioneer work. Wrench fault is Principal References interchangeable with lateral fault where that expression means actual rather than apparent There are three references in the English horizontal movement. Right lateral and left literature on fault mechanics and faults which lateral refer to the apparent relative movement are fundamental to the development of prin- of the two blocks viewed in plan; right lateral ciples: (1) Anderson (1942; 1951) outlined the indicates clockwise and left lateral indicates fundamental concepts of rock fracture. (2) counterclockwise separation, as described by Hubbert (1951) corroborated Anderson's views Hill (1947). The authors extend the use of and presented experimental and theoretical right lateral and left lateral by adding wrench, data for the value of the angle between com- so that actual horizontal slips are implied. pressional stress directions and resultant shear planes. (3) Hafner (1951) analyzed stress dis- tribution and faulting and emphasized the Wrench-Fault Tectonics necessity for considering stress distributions The writers propose that large-scale wrench in attempting to understand any strain situa- faults may be a dominant type of failure in tion. The integration of the various principles the earth's crust. Large areas, probably con- and theories of fault mechanics by Billings tinental in dimensions, appear to have been (1954) has also been most helpful. subjected to rather uniform stresses for ex- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/67/9/1207/3431718/i0016-7606-67-9-1207.pdf by guest on 29 September 2021 INTRODUCTION 1209 tended periods. Possible orientations and These three stress axes are of unequal length origins of these regional stresses and strains and describe an ellipsoid which long has been are discussed. The application of these con- termed the stress ellipsoid. cepts to the interpretation of local and regional If a material of sufficient rigidity to react tectonics is considered, and many geotectonic elastically rather than plastically is stressed hypotheses are re-examined in the light of wrench-fault tectonics. ACKNOWLEDGMENTS This paper represents a synthesis of many Maximum previous workers' published material and count- ompressiv less conversations with fellow workers and Stress friends. Particular mention is made of the many discussions and constructive criticisms by C. D. Cordry, H. D. Hedberg, and G. W. Ledingham. J. B. Currie, P. A. Grant, B. F. Hake, R. T. Hazzard, R. L. Johnston, and G. B. Lamb have contributed particularly valuable ideas and Minimum Compressive Intermediate suggestions. The following have critically Compressive Stress reviewed the manuscript, for which the writers Stress are most appreciative: M. P. Billings, B. W. Blanpied, M. A. Hanna, R. T. Hazzard, K. C. Heald, H. D. Hedberg, M. L. Hill, G. B. Lamb, L. V. Lombardi, C. R. Longwell, C. L. Moody, H. M. Nielsen, and G. Pardo; no responsibility for any of the views presented herein is assumed by these gentlemen. The authors are indebted to the Gulf Oil Corporation and the Western Gulf Oil Company for permission to publish FIGURE 1.—AXES OF THE STRESS ELLIPSOID this paper and wish to express their appreciation to the officials of the Gulf Companies for the beyond its strength, it will rupture. In situa- creation of a climate favorable to interregional tions such as those described above, the planes exchange of ideas on a broad scale. of maximum shearing stress are parallel to the intermediate stress axis and lie at angles of 45° THEORY on either side of the maximum compressive stress. The planes of actual shear do not Stress Ellipsoid and the Angle ft coincide with the planes of maximum shearing stress but lie closer to the axis of maximum A brief review of Anderson's (1951) work compressional stress and form an angle with it provides a basis for understanding fault which is here called ft, the angle of shear mechanics. Fundamental to his exposition is the (Fig. 2). The factors which contribute to the expression of stresses in terms of a set of three deviation in direction between actual strain mutually perpendicular axes. In a homogeneous and theoretical strain are included in the "angle isotropic material under compression, the of internal friction" which is one of the con- maximum compressive stress can be represented trolling parameters for the value of the angle ft. as acting in a given direction (Fig. 1, Y).