Earth Structure: An Introduction to Structural Geology and Tectonics PDF

2917-fm2.pdf 11/25/03 3:55 PM Page i SECOND EDITION EARTH STRUCTURE AN INTRODUCTION TO STRUCTURAL GEOLO...

2917-fm2.pdf 11/25/03 3:55 PM Page i SECOND EDITION EARTH STRUCTURE AN INTRODUCTION TO STRUCTURAL GEOLOGY AND TECTONICS 2917-fm2.pdf 11/25/03 3:55 PM Page ii Ben A. van der Pluijm Stephen Marshak UNIVERSIT Y OF MICHIGAN UNIVERSIT Y OF ILLINOIS With contributions by RICHARD W. ALLMENDINGER TERES A E. JORDAN MARK T. BRANDON ELIZ ABETH L. MILLER B. CL ARK BUR CHFIEL BORIS A. NATAL’IN FREDERICK A. COOK KE VIN T. PICKERING DAVID A. FOSTER LEIGH H. ROYDEN DAVID R. GRAY STEFAN M. SCHMID JAMES P. HIBBARD A. M. CÊ L AL ŞENGÖR PAUL F. HOFFMAN AL AN G. SMITH M. SCOT T WILKERSON 2917-fm2.pdf 11/25/03 3:55 PM Page iii SECOND EDITION EARTH STRUCTURE AN INTRODUCTION TO STRUCTURAL GEOLOGY AND TECTONICS B W W NORTON & COMPANY NEW YORK LONDON 2917-fm2.pdf 11/25/03 3:55 PM Page iv W. W. Norton & Company has been independent since its founding in 1923, when William Warder Norton and Mary D. Herter Norton first published lectures delivered at the People’s Institute, the adult education division of New York City’s Cooper Union. The Nortons soon expanded their program beyond the Institute, publishing books by celebrated academics from America and abroad. By mid-century, the two major pillars of Norton’s publishing program—trade books and college texts—were firmly established. In the 1950s, the Norton family transferred control of the company to its employees, and today—with a staff of four hundred and a comparable number of trade, college, and professional titles published each year—W. W. Norton & Company stands as the largest and oldest publishing house owned wholly by its employees. Copyright © 2004 by W. W. Norton & Company, Inc. All rights reserved. Printed in the United States of America. Second Edition The text of this book is composed in Times, with the display set in Conduit ITC. Composition by Shepherd Incorporated Manufacturing by Courier, Westford Editor: Leo A. W. Wiegman Project editor: Thomas Foley Director of manufacturing: Roy Tedoff Copy editor: Philippa Solomon Photography editors: Nathan Odell and Erin O’Brien Layout artists: Shepherd Incorporated Editorial assistants: Erin O’Brien and Rob Bellinger Book Designer: Rubina Yeh Library of Congress Cataloging-in-Publication Data Van der Pluijm, Ben A., 1955- Earth structure : an introduction to structural geology and tectonics / Ben A. van der Pluijm, Stephen Marshak ; with contributions by Richard W. Allmendinger... [et al.]-- 2nd ed. p. cm. Includes bibliographical references and index. ISBN 0-393-92467-X 1. Geology, Structural. 2. Plate tectonics. I. Marshak, Stephen, 1955- II. Title. QE601.V363 2003 551.8--dc22 2003063957 W. W. Norton & Company, Inc., 500 Fifth Avenue, New York, N.Y. 10110 www.wwnorton.com W. W. Norton & Company Ltd., Castle House, 75/76 Wells Street, London W1T 3QT 1 2 3 4 5 6 7 8 9 0 2917-fm2.pdf 11/25/03 3:55 PM Page v Brief Contents xv Preface 1 PART A FUNDAMENTALS 2 Chapter 1 Overview 14 Chapter 2 Primary and Nontectonic Structures 40 Chapter 3 Force and Stress 62 Chapter 4 Deformation and Strain 90 Chapter 5 Rheology 113 PART B BRIT TLE STRUCTURES 114 Chapter 6 Brittle Deformation 138 Chapter 7 Joints and Veins 166 Chapter 8 Faults and Faulting 203 PART C DUCTILE STRUCTURES 204 Chapter 9 Ductile Deformation Processes 238 Chapter 10 Folds and Folding 270 Chapter 11 Fabrics: Foliations and Lineations 294 Chapter 12 Ductile Shear Zones, Textures, and Transposition 316 Chapter 13 Deformation, Metamorphism, and Time 335 PART D TECTONICS 336 Chapter 14 Whole-Earth Structure and Plate Tectonics 368 Chapter 15 Geophysical Imaging of the Continental Lithosphere—An Essay by Frederick A. Cook 382 Chapter 16 Rifting, Seafloor Spreading, and Extensional Tectonics 412 Chapter 17 Convergence and Collision 444 Chapter 18 Fold-Thrust Belts—An Essay by Stephen Marshak and M. Scott Wilkerson 476 Chapter 19 Strike-Slip Tectonics 501 PART E REGIONAL PERSPECTIVES 502 Chapter 20 A Global View 509 Chapter 21 Eastern Hemisphere 556 Chapter 22 Western Hemisphere 628 Appendix 1 Spherical Projections 631 Appendix 2 Geologic Timescale 633 Credits 641 Index v 2917-fm2.pdf 11/25/03 3:55 PM Page vi 2917-fm2.pdf 11/25/03 3:55 PM Page vii Contents Preface xv 2.5 Impact Structures 35 2.6 Closing Remarks 38 Additional Reading 38 PART A FUNDAMENTALS 1 3 Force and Stress 40 1 Overview 2 3.1 Introduction 40 3.2 Units and Fundamental Quantities 42 1.1 Introduction 2 3.3 Force 43 1.2 Classification of Geologic Structures 4 3.4 Stress 44 1.3 Stress, Strain, and Deformation 6 3.5 Two-Dimensional Stress: Normal Stress and 1.4 Structural Analysis and Scales of Observation 8 Shear Stress 44 1.5 Some Guidelines for Structural Interpretation 10 3.6 Three-Dimensional Stress: Principal Planes 1.6 Closing Remarks 12 and Principal Stresses 45 Additional Reading 12 3.6.1 Stress at a Point 46 3.6.2 The Components of Stress 46 2 Primary and Nontectonic 3.6.3 Stress States 47 Structures 14 3.7 Deriving Some Stress Relationships 48 3.8 Mohr Diagram for Stress 49 2.1 Introduction 14 3.8.1 Constructing the Mohr Diagram 50 2.2 Sedimentary Structures 14 3.8.2 Some Common Stress States 51 2.2.1 The Use of Bedding in Structural Analysis 16 3.9 Mean Stress and Deviation Stress 52 2.2.2 Graded Beds and Cross Beds 17 3.10 The Stress Tensor 53 2.2.3 Surface Markings 19 3.11 A Brief Summary of Stress 54 2.2.4 Disrupted Bedding 19 3.12 Stress Trajectories and Stress Fields 55 2.2.5 Conformable and Unconformable Contacts 19 3.13 Methods of Stress Measurement 56 2.2.6 Compaction and Diagenetic Structures 23 3.13.1 Present-Day Stress 56 2.2.7 Penecontemporaneous Structures 24 3.13.2 Paleostress 57 2.3 Salt Structures 26 3.13.3 Stress in Earth 57 2.3.1 Why Halokinesis Occurs 26 3.14 Closing Remarks 60 2.3.2 Geometry of Salt Structures and Associated Additional Reading 60 Processes 27 2.3.3 Gravity-Driven Faulting and Folding 29 2.3.4 Practical Importance of Salt Structures 30 4 Deformation and Strain 62 2.4 Igneous Structures 30 4.1 Introduction 62 2.4.1 Structures Associated with Sheet Intrusions 31 4.2 Deformation and Strain 63 2.4.2 Structures Associated with Plutons 32 4.3 Homogenous Strain and the Strain Ellipsoid 65 2.4.3 Structures Associated with Extrusion 33 4.4 Strain Path 66 2.4.4 Cooling Fractures 35 4.5 Coaxial and Non-Coaxial Strain Accumulation 67 vii 2917-fm2.pdf 11/25/03 3:55 PM Page viii 4.6 Superimposed Strain 69 PART B BRITTLE STRUCTURES 113 4.7 Strain Quantities 70 4.7.1 Longitudinal Strain 70 6 Brittle Deformation 114 4.7.2 Volumetric Strain 71 4.7.3 Angular Strain 71 6.1 Introduction 114 4.7.4 Other Strain Quantities 71 6.2 Vocabulary of Brittle Deformation 114 4.8 The Mohr Circle for Strain 73 6.3 What is Brittle Deformation? 117 4.9 Strain States 75 6.4 Tensile Cracking 118 4.10 Representation of Strain 75 6.4.1 Stress Concentration and Griffith Cracks 118 4.10.1 Orientation 75 6.4.2 Exploring Tensile Crack Development 121 4.10.2 Shape and Intensity 76 6.4.3 Modes of Crack-Surface Displacement 122 4.11 Finite Strain Measurement 78 6.5 Processes of Brittle Faulting 123 4.11.1 What Are We Really Measuring in Strain Analysis 79 6.5.1 Slip by Growth of Fault-Parallel Veins 123 4.11.2 Initially Spherical Objects 81 6.5.2 Cataclasis and Cataclastic Flow 123 4.11.3 Initially Nonspherical Objects 82 6.6 Formation of Shear Fractures 124 4.11.3.1 Center-to-Center Method 83 6.7 Predicting Initiation of Brittle Deformation 126 4.11.3.2 Rf/Φmethod 83 6.7.1 Tensile Cracking Criteria 126 4.11.4 Objects with Known Angular Relationships or 6.7.2 Shear-Fracture Criteria and Failure Envelopes 127 Lengths 84 6.8 Frictional Sliding 132 4.11.4.1 Angular Changes 84 6.8.1 Frictional Sliding Criteria 132 4.11.4.2 Length Changes 85 6.8.2 Will New Fractures Form or Will Existing 4.11.5 Rock Textures and Other Strain Guages 86 Fractures Slide? 133 4.11.6 What Do We Learn from Strain Analysis? 87 6.9 Effect of Environmental Factors in Failure 134 4.12 Closing Remarks 89 6.9.1 Effect of Fluids on Tensile Crack Growth 134 Additional Reading 89 6.9.2 Effect of Dimensions on Tensile Strength 136 6.9.3 Effect of Pore Pressure on Shear Failure and 5 Rheology 90 Frictional Sliding 136 6.9.4 Effect of Intermediate Principal Stress 5.1 Introduction 90 on Shear Rupture 136 5.1.1 Strain Rate 91 6.10 Closing Remarks 136 5.2 General Behavior: The Creep Curve 92 Additional Reading 137 5.3 Rheologic Relationships 93 5.3.1 Elastic Behavior 93 5.3.2 Viscous Behavior 96 7 Joints and Veins 138 5.3.3 Visoelastic Behavior 97 7.1 Introduction 138 5.3.4 Elastico-Viscous Behavior 97 7.2 Surface Morphology of Joints 140 5.3.5 General Linear Behavior 98 7.2.1 Plumose Structure 140 5.3.6 Nonlinear Behavior 98 7.2.2 Why Does Plumose Structure Form? 141 5.4 Adventures with Natural Rocks 100 7.2.3 Twist Hackle 144 5.4.1 The Deformation Apparatus 101 7.3 Joint Arrays 144 5.4.2 Confining Pressure 102 7.3.1 Systematic versus Nonsystematic Joints 144 5.4.3 Temperature 103 7.3.2 Joint Sets and Joint Systems 145 5.4.4 Strain Rate 104 7.3.3 Cross-Cutting Relations Between Joints 146 5.4.5 Pore-Fluid Pressure 105 7.3.4 Joint Spacing in Sedimentary Rocks 147 5.4.6 Work Hardening—Work Softening 106 7.4 Joint Studies in the Field 149 5.4.7 Significance of Experiments to 7.4.1 Dealing with Field Data About Joints 150 Natural Conditions 107 7.5 Origin and Interpretation of Joints 152 5.5 Confused by the Terminology? 108 7.5.1 Joints Related to Uplift and Unroofing 152 5.6 Closing Remarks 111 7.5.2 Formation of Sheeting Joints 153 Additional Reading 112 7.5.3 Natural Hydraulic Fracturing 154 viii CONTENTS 2917-fm2.pdf 11/25/03 3:55 PM Page ix 7.5.4 Joints Related to Regional Deformation 155 PART C DUCTILE STRUCTURES 203 7.5.5 Orthogonal Joint Systems 156 7.5.6 Conjugate Joint Systems 157 9 Ductile Deformation Processes 204 7.5.7 Joint Trend as Paleostress Trajectory 158 7.6 Limits on Joint Growth 158 9.1 Introduction 204 7.7 Veins and Vein Arrays 159 9.2 Cataclastic Flow 206 7.7.1 Formation of Vein Arrays 160 9.3 Crystal Defects 207 7.7.2 Vein Fill: Blocky and Fibrous Veins 160 9.3.1 Point Defects 207 7.7.3 Interpretation of Fibrous Veins 162 9.3.2 Line Defects or Dislocations 207 7.8 Lineaments 163 9.4 Crystal Plasticity 210 7.9 Closing Remarks 163 9.4.1 Dislocation Glide 210 Additional Reading 165 9.4.2 Cross-Slip and Climb 210 9.4.3 Mechanical Twinning 213 8 Faults and Faulting 166 9.4.4 Strain-Producing versus Rate-Controlling Mechanisms 216 8.1 Introduction 166 9.4.5 Where Do Dislocations Come 8.2 Fault Geometry and Displacement 169 From? 216 8.2.1 Basic Vocabulary 169 9.5 Diffusional Mass Transfer 217 8.2.2 Representation of Faults on Maps and Cross 9.5.1 Volume Diffusion and Grain-Boundary Sections 172 Diffusion 218 8.2.3 Fault Separation and Determination 9.5.2 Pressure Solution 218 of Net Slip 174 9.6 Constitutive Equations or Flow Laws 219 8.2.4 Fault Bends 176 9.7 A Microstructural View of Laboratory 8.2.5 Fault Terminations and Fault Length 177 Behavior 220 8.3 Characteristics of Faults and Fault Zones 179 9.8 Imaging Dislocations 221 8.3.1 Brittle Fault Rocks 179 9.9 Deformation Microstructures 222 8.3.2 Slickensides and Slip Lineations 182 9.9.1 Recovery 222 8.3.3 Subsidiary Fault and Fracture Geometries 184 9.9.2 Recrystallization 225 8.3.4 Fault-Related Folding 184 9.9.3 Mechanisms of Recrystallization 226 8.3.5 Shear-Sense Indicators of Brittle Faults— 9.9.4 Superplastic Creep 228 A Summary 187 9.10 Deformation Mechanism Maps 229 8.4 Recognizing and Interpreting Faults 187 9.10.1 How to Construct a Deformation Mechanism 8.4.1 Recognition of Faults from Subsurface Data 189 Map 232 8.4.2 Changes in Fault Character with Depth 190 9.10.2 A Note of Caution 233 8.5 Relation of Faulting to Stress 191 9.11 Closing Remarks 234 8.5.1 Formation of Listric Faults 192 Additional Reading 234 8.5.2 Fluids and Faulting 192 Appendix: Dislocation Decoration 236 8.5.3 Stress and Faulting—A Continuing Debate 193 8.6 Fault Systems 195 8.6.1 Geometric Classification of Fault Arrays 195 10 Folds and Folding 238 8.6.2 Normal Fault systems 196 10.1 Introduction 238 8.6.3 Reverse Fault Systems 196 10.2 Anatomy of a Folded Surface 239 8.6.4 Strike-Slip Fault Systems 197 10.2.1 Fold Facing: Antiform, Synform, Anticline, 8.6.5 Inversion of Fault Systems 197 and Syncline 241 8.6.6 Fault Systems and Paleostress 197 10.3 Fold Classification 243 8.7 Faulting and Society 198 10.3.1 Fold Orientation 244 8.7.1 Faulting and Resources 199 10.3.2 Fold Shape in Profile 245 8.7.2 Faulting and Earthquakes 199 10.4 Fold Systems 246 8.8 Closing Remarks 201 10.4.1 The Enveloping Surface 247 Additional Reading 201 10.4.2 Folds Symmetry and Fold Vergence 248 CONTENTS ix 2917-fm2.pdf 11/25/03 3:55 PM Page x 10.5 Some Special Fold Geometries 250 12.3 Shear-Sense Indicators 298 10.6 Superposed Folding 252 12.3.1 Plane of Observation 298 10.6.1 The Priciple of Fold Superposition 252 12.3.2 Grain-Tail Complexes 299 10.6.2 Fold Interference Patterns 254 12.3.3 Fractured Grains and Mica Fish 299 10.6.3 Fold Style 255 12.3.4 Foliations: C-S and C-C′ 10.6.4 A Few Philosophical Points 257 Structures 302 10.7 The Mechanics of Folding 257 12.3.5 A Summary of Shear-Sense 10.7.1 Passive Folding and Active Folding 257 Indicators 303 10.7.2 Buckle Folds 259 12.4 Strain in Shear Zones 304 10.7.3 Folded Multilayers 262 12.4.1 Rotated Grains 304 10.8 Kinematic Models of Folding 262 12.4.2 Deflected Foliations 305 10.8.1 Flexural Slip/Flow Folding 262 12.5 Textures or Crystallographic-Preferred Fabrics 307 10.8.2 Neutral-Surface Folding 263 12.5.1 The Symmetry Principle 308 10.8.3 Shear Folding 264 12.5.2 Textures as Shear-Sense Indicators 310 10.8.4 Fold Shape Modification 265 12.6 Fold Transposition 311 10.8.5 A Natural Example 265 12.6.1 Sheath Folds 313 10.9 A Possible Sequence of Events 266 12.7 Closing Remarks 313 10.10 Closing Remarks 268 Additional Reading 315 Additional Reading 269 13 Deformation, Metamorphism, 11 Fabrics: Foliations and Time 316 and Lineations 270 13.1 Introduction 316 11.1 Introduction 270 13.2 Field Observations and Study Goals 316 11.2 Fabric Terminology 270 13.3 Pressure and Temperature 319 11.3 Foliations 272 13.3.1 Status Report I 321 11.3.1 What is Cleavage? 273 13.4 Deformation and Metamorphism 322 11.3.2 Disjunctive Cleavage 274 13.4.1 Status Report II 324 11.3.3 Pencil Cleavage 277 13.5 Time 325 11.3.4 Slaty Cleavage 278 13.5.1 The Isochron Equation 325 11.3.5 Phyllitic Cleavage and Schistosity 278 13.5.2 The Isotopic Closure Temperature 327 11.3.6 Crenulation Cleavage 280 13.5.3 Dating Deformation 328 11.3.7 Gneissic Layering and Migmatization 282 13.5.4 Status Report III 329 11.3.8 Mylonitic Foliation 284 13.6 D-P-T-t Paths 329 11.4 Cleavage and Strain 284 13.6.1 Temperature-Time (T-t) History 331 11.5 Foliations in Folds and Fault Zones 285 13.6.2 Pressure-Temperature (P-T) History 331 11.6 Lineations 288 13.6.3 Pressure-Time (P-t) History 331 11.6.1 Form Lineations 288 13.6.4 The Geothermal Gradient 331 11.6.2 Surface Lineations 289 13.6.5 The Deformational Setting 333 11.6.3 Mineral Lineations 290 13.7 Closing Remarks 333 11.6.4 Tectonic Interpretation of Lineations 290 Additional Reading 333 11.7 Other Physical Properties of Fabrics 292 11.8 Closing Remarks 292 Additional Reading 293 PART D TECTONICS 335 12 Ductile Shear Zones, Textures, 14 Whole-Earth Structure and and Transposition 294 Plate Tectonics 336 12.1 Introduction 294 14.1 Introduction 336 12.2 Mylonites 296 14.2 Studying Earth’s Internal Layering 337 12.2.1 Type Mylonites 297 14.3 Seismically Defined Layers of the Earth 337 x CONTENTS 2917-fm2.pdf 11/25/03 3:55 PM Page xi 14.4 The Crust 342 16.3 Cordilleran Metamorphic Core Complexes 390 14.4.1 Oceanic Crust 342 16.4 Formation of a Rift System 394 14.4.2 Continental Crust 342 16.5 Controls on Rift Orientation 396 14.4.3 The Moho 348 16.6 Rocks and Topographic Features of Rifts 397 14.5 The Mantle 348 16.6.1 Sedimentary-Rock Assemblages in Rifts 397 14.5.1 Internal Structure of the Mantle 348 16.6.2 Igneous-Rock Assemblage of Rifts 397 14.5.2 Mantle Plumes 350 16.6.3 Active Rift Topography and Rift-Margin Uplifts 399 14.6 The Core 350 16.7 Tectonics of Midocean Ridges 402 14.7 Defining Earth Layers Based on Rheologic 16.8 Passive Margins 405 Behavior 350 16.9 Causes of Rifting 408 14.7.1 The Lithosphere 351 16.10 Closing Remarks 410 14.7.2 The Asthenosphere 353 Additional Reading 410 14.7.3 Isostasy 353 14.8 The Tenets of Plate Tectonics Theory 355 17 Convergence and Collision 412 14.9 Basic Plate Kinematics 359 17.1 Introduction 412 14.9.1 Absolute Plate Velocity 359 17.2 Convergent Plate Margins 414 14.9.2 Relative Plate Velocity 360 17.2.1 The Downgoing Slab 415 14.9.3 Using Vectors to Describe Relative Plate 17.2.2 The Trench 418 Velocity 361 17.2.3 The Accretionary Prism 420 14.9.4 Triple Junctions 364 17.2.4 The Forearc Basin and 14.10 Plate-Driving Forces 364 the Volcanic Arc 424 14.11 The Supercontinent Cycle 366 17.2.5 The Backarc Region 425 14.12 Closing Remarks 367 17.2.6 Curvature of Island Arcs 428 Additional Reading 367 17.2.7 Coupled versus Uncoupled Convergent Margins 428 15 Geophysical Imaging of the 17.3 Basic Stages of Collisional Tectonics 429 Continental Lithosphere— 17.3.1 Stage 1: Precollision and Initial Interaction 431 17.3.2 Stage 2: Abortive Subduction and Suturing 433 An Essay by Frederick A. Cook 368 17.3.3 Stage 3: Crustal Thickening and Extensional 15.1 Introduction 368 Collapse 435 15.2 What is Seismic Imaging? 368 17.4 Other Consequences of Collisional Tectonics 436 15.3 How are Data Interpreted? 370 17.4.1 Regional Strike-Slip Faulting 15.4 Some Examples 370 and Lateral Escape 436 15.5 The Crust—Mantle Transition 372 17.4.2 Plateau Uplift 438 15.6 The Importance of Regional Profiles— 17.4.3 Continental Interior Fault-and-Fold Zones 438 Longer, Deeper, More Detailed 374 17.4.4 Crustal Accretion (Accetionary Tectonics) 440 15.7 An Example from Northwestern Canada 375 17.4.5 Deep Structure of Collisional 15.8 Other Geophysical Techniques 379 Orogens 442 15.9 Closing Remarks 381 17.5 Insights from Modeling Studies 442 Additional Reading 381 17.6 Closing Remarks 443 Additional Reading 443 16 Rifting, Seafloor Spreading, and Extensional Tectonics 382 18 Fold-Thrust Belts—An Essay by Stephen Marshak and 16.1 Introduction 382 16.2 Cross-Sectional Structure of a Rift 385 M. Scott Wilkerson 444 16.2.1 Normal Fault Systems 385 18.1 Introduction 444 16.2.2 Pure-Shear versus Simple-Shear Models 18.2 Fold-Thrust Belts in a Regional Context 448 of Rifting 389 18.2.1 Tectonic Settings of Fold-Thrust Belts 448 16.2.3 Examples of Rift Structure in Cross Section 389 18.2.2 Mechanical Stratigraphy 452 CONTENTS xi 2917-fm2.pdf 11/25/03 3:55 PM Page xii 18.3 Geometry of Thrusts and Thrust Systems 452 21 Eastern Hemisphere 509 18.3.1 A Cross-Sectional Image of a Thrust Fault 452 18.3.2 Thrust Systems 455 21.1 The Tectonic Evolution of the European Alps 18.3.3 Overall Fold-Thrust Belt Architecture 457 and Forelands—An Essay by Stefan M. Schmid 510 18.4 Thrust-Related Folding 459 21.1.1 Introduction 510 18.5 Mesoscopic- and Microscopic-Scale Strain 21.1.2 The Major Tectonic Units of the European Alps 510 in Thrust Sheets 465 21.1.3 The Major Paleogeographic Units of the Alps 512 18.6 Fold-Thrust Belts in Map View 465 21.1.4 Three Alpine Transects and Their Deep Structure 514 18.7 Balanced Cross Sections 468 21.1.5 Inferences Concerning Rheologic Behavior 517 18.8 Mechanics of Fold-Thrust Belts 470 21.1.6 Evolution of the Alpine System and Its 18.9 Closing Remarks 474 Forelands in Time Slices 517 Additional Reading 474 21.1.7 Recent Movements in the Upper Rhine Graben 522 21.1.8 Closing Remarks 523 Additional Reading 524 19 Strike-Slip Tectonics 476 21.2 The Tibetan Plateau and Surrounding Regions— 19.1 Introduction 476 An Essay by Leigh H. Royden and B. Clark Burchfiel 525 19.2 Transform versus Transcurrent Faults 479 21.2.1 Introduction 525 19.2.1 Transform Faults 479 21.2.2 Precollisional History 525 19.2.2 Transcurrent Faults 481 21.2.3 Postcollisional Convergent Deformation 527 19.3 Structural Features of Major Continental 21.2.4 Crustal Shortening and Strike-Slip Faulting 530 Strike-Slip Faults 482 21.2.5 Extension of the Tibetan 19.3.1 Description of Distributed Deformation Plateau 532 in Strike-Slip Zones 482 21.2.6 Closing Remarks 533 19.3.2 The Causes of Structural Complexity Additional Reading 533 in Strike-Slip Zones 484 21.3 Tectonics of the Altaids: An Example of 19.3.3 Map-View Block Rotation in Strike-Slip Zones 487 a Turkic-type Orogen—An Essay By 19.3.4 Transpression and Transtension 487 A. M. Cêlal Şengör and Boris A. Natal’in 535 19.3.5 Restraining and Releasing Bends 490 21.3.1 Introduction 535 19.3.6 Strike-Slip Duplexes 492 21.3.2 The Present Structure 19.3.7 Deep-Crustal Strike-Slip Fault Geometry 492 of the Altaids 538 19.4 Tectonic Setting of Continental Strike-Slip Faults 493 21.3.3 Evolution of the Altaids 539 19.4.1 Oblique Convergence and Collision 493 21.3.4 Implications for Continental 19.4.2 Strike-Slip Faulting in Fold-Thrust Belts 493 Growth 545 19.4.3 Strike-Slip Faulting in Rifts 493 21.3.5 Closing Remarks 545 19.4.4 Continental Transform Faults 495 Additional Reading 545 19.5 Oceanic Transforms and Fracture Zones 497 21.4 The Tasman Orogenic Belt, Eastern Australia: 19.6 Closing Remarks 498 An Example of Paleozoic Tectonic Accretion— Additional Reading 498 An Essay by David R. Gray and David A. Foster 547 21.4.1 Introduction 547 21.4.2 Crustal Structure and Main Tectonic Elements 548 PART E 21.4.3 Timing of Deformation and Regional Events 551 REGIONAL PERSPECTIVES 501 21.4.4 Mechanics of Deformation in Accretionary Orogens 554 20 A Global View 502 Additional Reading 555 20.1 Introduction 502 20.2 Global Deformation Patterns 503 22 Western Hemisphere 556 20.3 What Can We Learn from Regional Perspectives? 504 22.1 The North American Cordillera—An Essay by 20.4 Some Speculation on Contrasting Orogenic Styles 506 Elizabeth L. Miller 557 20.5 Closing Remarks and Outline 507 22.1.1 Introduction 557 Additional Reading 508 22.1.2 Precambrian and Paleozoic History 558 xii CONTENTS 2917-fm2.pdf 11/25/03 3:55 PM Page xiii 22.1.3 Mesozoic History 559 22.5.5 Early Ordovician Breakup of the Northwest 22.1.4 Cenozoic History 560 Margin of Gondwana 599 22.1.5 Closing Remarks 564 22.5.6 Middle-Late Ordovician Subduction, Additional Reading 565 Continental Fragmentation, and Collisions 600 22.2 The Cascadia Subduction Wedge: The Role of 22.5.7 Middle Ordovician—Silurian Closure of the Accretion, Uplift, and Erosion—An Essay Eastern Iapetus Ocean 601 by Mark T. Brandon 566 22.5.8 Late Ordovician Icehouse 603 22.2.1 Introduction 566 22.5.9 Ordovician-Silurian Magmatic Arcs Elsewhere 22.2.2 Accretionary Flux 566 in Europe 604 22.2.3 Wedges, Taper, and Stability 567 22.5.10 Postorogenic Continental Sedimentation and 22.2.4 Double-Sided Wedges 567 Igneous Activity 605 22.2.5 Subduction Polarity and Pro-Side Accretion 568 22.5.11 Closing Remarks 605 22.2.6 The Cascadia Subduction Zone 569 Additional Reading 606 22.2.7 Comparison between the Cascadia and Alpine 22.6 Tectonic Genealogy of North America— Wedges 574 An Essay by Paul F. Hoffman 607 Additional Reading 574 22.6.1 Introduction 607 22.3 The Central Andes: A Natural Laboratory 22.6.2 Phanerozoic (545-0 Ma) Orogens and Pangea 608 for Noncollisional Mountain Building— 22.6.3 Neoproterozoic (1000-545 Ma) Orogens An Essay by Richard W. Allmendinger and Gondwanaland 608 and Teresa E. Jordan 575 22.6.4 Mesoproterozoic (1600-1000 Ma) Orogens 22.3.1 Introduction 575 and Rodinia 609 22.3.2 The Andean Orogeny 575 22.6.5 Paleoproterozoic (2500-1600 Ma) Collisional 22.3.3 Late Cenozoic Tectonics of the Andes 577 Orogens and Nuna 610 22.3.4 Crustal Thickening and Lithospheric Thinning 580 22.6.6 Paleoproterozoic Accretionary Orogens Add 22.3.5 Closing Remarks 581 to Nuna 611 Additional Reading 581 22.6.7 Archean Cratons and Kenorland 612 22.4 The Appalachian Orogen—An Essay by 22.6.8 Closing Remarks 613 James P. Hibbard 582 Additional Reading 613 22.4.1 Introduction 582 22.7 Phanerozoic Tectonics of the United States 22.4.2 Overview 582 Midcontinent 615 22.4.3 Tectonic Components 583 22.7.1 Introduction 615 22.4.4 Assembly 587 22.7.2 Classes of Structures in the Midcontinent 616 22.4.5 Closing Remarks 591 22.7.3 Some Causes of Epeirogeny 623 Additional Reading 591 22.7.4 Speculations on Midcontinent 22.5 The Caledonides—An Essay by Kevin T. Pickering Fault-and-Fold Zones 625 and Alan G. Smith 593 22.7.5 Closing Remarks 626 22.5.1 Introduction 593 Additional Reading 627 22.5.2 Late Precambrian—Cambrian Extension and Passive Margins 597 APPENDIX 1 Spherical Projections 628 22.5.3 Late Precambrian—Cambrian Arcs, Northern APPENDIX 2 Geologic Timescale 631 and Northwestern Gondwana 597 Credits 633 22.5.4 Early-Middle Ordovician Arcs, Marginal Basins, Index 641 and Ophiolites 598 CONTENTS xiii 2917-fm2.pdf 11/25/03 3:55 PM Page xiv 2917-fm2.pdf 11/25/03 3:55 PM Page xv Preface T his book is concerned with the deformation of rock in the Earth’s lithosphere, as viewed from the atomic scale, through the grain scale, the hand specimen scale, the outcrop scale, the mountain range scale, and the tectonic plate scale. A deforma- concepts discussed, which will not only stimulate the mind but also aid in absorbing the material. Concepts are remembered better when their interrelationships are recognized, rather than being presented as just a series of definitions. In some cases we may have tional feature observed on one scale typically reflects advanced a controversial position and perhaps future processes occurring on other scales. For example, we readers will be the ones to prove some of our view- can’t understand continental deformation without points either right or wrong. understanding mountains, we can’t understand moun- Structural geology and tectonics are a lot of fun once tains without understanding folding and faulting, and one has waded through the initial terminology morass. we can’t understand folding and faulting without Our personal approach to teaching structural geology understanding ductile and brittle deformation mecha- and tectonics is reflected in the fairly informal writing nisms at the atomic scale. This book attempts to inte- style of this text. Whenever possible, we use familiar grate topics pertaining to all scales of rock deforma- analogies such as rubber bands, syrup, and pool balls. tion, emphasizing the linkages between structural Similarly, we have kept illustrations simple in the early geology and tectonics. chapters so that the point of the figure is obvious. Terms Every month, perhaps a thousand pages of new and definitions related to topics that we do not intro- ideas and observations relevant to structural geology duce in the main body of the text are included in tables and tectonics are published in the major scholarly as a reference. The subject index will direct you to the journals. The amount of material on structural geology appropriate location in the text for any specific term. and tectonics that has appeared over the past 150 years There’s no single right way to teach structural geol- is staggering. We have purposely decided to write this ogy and tectonics. Moreover, we increasingly see that book with a novice to the field in mind. We, as instruc- structural geology and tectonics is one of the first tors, face a massive challenge when trying to distill an classes for students who plan to major in geological introductory course out of this ever-changing and sciences. We decided to write this book because we ever-growing mountain of information. We want stu- found both that existing books did not suit the chang- dents to be comfortable with certain basic concepts ing needs of the courses that we teach ourselves and (say, fault terminology or stress theory), and at the that many other instructors shared our views. Some same time, we want them to experience the excitement books try to be a lab manual and a lecture text at the of discovery and to build their own “big picture” of same time, while others are slanted too much toward how the Earth works. And all this must be done in a the research interests of the particular writer(s). Some few short months! Rather than loading the text with books are organized in such a way that a reading excessive detail and peppering it with extensive refer- assignment on a single topic must include splices from encing, we opted instead to present a distillation that all over the book, and others provide more detail than offers a perspective on most aspects of the field. The can possibly be covered in a single semester course so reason for this approach is to highlight the “guts” of that students are, frankly, overwhelmed. We have structural geology and tectonics, thereby providing a deleted topics that are generally taught in laboratory foundation for future study and a platform for further sections because these topics cannot be treated ade- discussion. When reading the text, the reader should quately within the framework of a lecture textbook. We maintain a critical and questioning attitude toward the also do not burden the narrative with references, but xv 2917-fm2.pdf 11/25/03 3:55 PM Page xvi rather provide introductory reading lists at the end of THANKS! each chapter. In order to provide instructors with optimal freedom This book could not have been written without the to develop their own course outlines, we’ve made sure help of the students in our classes, who, through their that most chapters are self-contained modules that can successes and mistakes, have shown us which expla- be presented in various sequences. Ben, for example, nations work and which do not. We are indebted to the starts his course with a description of rocks, via primary following colleagues for their expert contributions to structures, faults and fractures, folds, and fabrics, this new edition: Rick Allmendinger, Mark Brandon, before introducing stress, strain, rheology, and defor- Clark Burchfiel, A. M. Cêlal Şengör, Fred Cook, David mation mechanisms. Steve, by contrast, teaches stress, Foster, David Gray, Jim Hibbard, Paul Hoffman, Teresa strain, and rheology immediately after primary struc- Jordan, Elizabeth Miller, Boris Natal’in, Kevin Picker- tures and presents brittle deformation theory before ing, Leigh Royden, Stefan Schmid, Alan Smith, and discussing faults and fractures. We both concentrate on Scott Wilkerson. tectonics at the end of our courses, but tectonic impli- We are also grateful to our many colleagues who cations are typically interwoven with the discussion of have provided generous dollops of advice and from different classes of structures earlier in the course. In whom we have borrowed data and interpretations. the end, instructors work hard to make their lectures Colleagues who have commented on and/or con- comprehensive yet comprehensible, accurate yet tributed to one or more chapters include (in alphabeti- enjoyable. We have tried to do the same with this book. cal order): Mark Fisher, Jerry Magloughlin, Klaus Mezger, Carl Richter, Mike Sandiford, and John Sta- matakos. Formal reviews of chapters in the First Edi- tion were given by David Anastasio, Stanley Cebull, CHANGES IN THE SECOND EDITION Bill Dunne, Terry Engelder, Karl Karlstrom, Win Means, Jim Talbot, Adolph Yonkee, and Vincent All chapters were revised for the Second Edition, but Cronic. The Second Edition was revised based on our the general organization remains the same. New sec- own experiences with the First Edition, a better appre- tions have been added, while some old ones have been ciation of some of the topics, and the informal feedback removed or combined. The new edition also includes a from many users of the First Edition, which received chapter on “Geophysical Imaging” and four new essays formal reviews from Roy Schlische and Bill Dunne. in Chapters 21 and 22 on the European Alps, the The editorial and production staff for W. W. Norton, Altaids, the Appalachians, and the Cascadia wedge. particularly copy editor Philippa Solomon and editor The remaining essays were updated and revised. The Leo Wiegman, as well as Erin O’Brien, Thom Foley, new and revised art offers an even more informative Rubina Yeh, and Jack Repcheck, have been most help- illustration of concepts and topics and will give ful and accommodating. Stan Maddock and Dale instructors the opportunity for modern classroom use Austin produced the artwork, most of which has been (see ancillaries). redrafted and updated from the First Edition. We also thank our graduate advisors (Paul Williams, Henk Zwart, and Terry Engelder, respectively) for helping us enter this business and for guiding our first uncertain ANCILL ARIES steps. We thank all of our graduate students for many lively and interesting discussions. And finally, but Earth Structure is supported by a Norton Resource foremost, we thank our wives, Lies and Kathy, and our Library offering teachers hundreds of digital copies of children. Wouter and Robbie, and David and Emma, the figures from the new edition. The Norton Resource respectively, for not grumbling too much about the Library images may be used in classrooms as overhead absences in body and spirit that writing this book has transparencies, computer presentations, and student required. To them we thankfully dedicate this book worksheets incorporated in exams, or course websites. and hope that one day they may even read it. Instructors may either download figures by chapter from the Norton Resource Library, after obtaining a Ben van der Pluijm, Ann Arbor, Michigan password from Norton, or request the images on a CD- Stephen Marshak, Urbana, Illinois ROM. Both password and CD-ROM requests are September 12, 2003 located at the Norton Resource Library web address: www.wwnorton.com/college/nrl/welcome.htm. xvi PREFACE 2917-CH01.pdf 11/20/03 5:07 PM Page 1 PART A FUNDAMENTALS 2917-CH01.pdf 11/20/03 5:07 PM Page 2 CHAPTER ONE Overview 1.1 Introduction 2 1.5 Some Guidelines for Structural Interpretation 10 1.2 Classification of Geologic Structures 4 1.6 Closing Remarks 12 1.3 Stress, Strain, and Deformation 6 Additional Reading 12 1.4 Structural Analysis and Scales of Observation 8 1.1 IN T RODUCTION the true shapes of rock bodies in sketches to under- stand the natural shape of the Earth (Figure 1.1). In the Did you ever take a cross-country drive? Hour after seventeenth century came the first description of rock hour of tedious driving, as the highway climbed hills deformation. Nicholas Steno (1631–1686) examined and dropped into valleys? The monotonous gray rocks outcrops where the bedding of rock was not horizontal, exposed in road cuts largely went unnoticed, right? and speculated that strata that do not presently lie in You passed pretty scenery, but it was static and seemed horizontal layers must have in some way been dislo- to tell no story simply because you did not have a basis cated (the term he used for deformed). Perhaps Steno’s in your mind with which to interpret your natural sur- establishment of the principle of original horizontal- roundings. It was much the same for scholars of gen- ity can be viewed as the birth of structural geology. By erations past, before the establishment of modern the beginning of the eighteenth century, the structural science. The Earth was a closed book, hiding its complexity of rocks in mountain ranges like the Alps secrets in a language that no one could translate. was widely recognized (Figure 1.2), and it became Certainly, ancient observers marveled at the enormity clear that such features demanded explanation. of mountains and oceans, but with the knowledge they The pace of discovery quickened during the latter had at hand they could do little more than dream of half of the eighteenth century and through the nine- supernatural processes to explain the origin of these teenth century. In his “Theory of the Earth with Proofs features. Gods and monsters contorted the Earth and and Illustrations,” James Hutton (1726–1797) proposed spit flaming rock; and giant turtles and catfish shook the concept of uniformitarianism and provided an the ground. Then, in fifteenth-century Europe, an intel- explanation for the nature of unconformities. Since the lectual renaissance spawned an age of discovery, during publication of this book in 1785 there has been a group which the Earth was systematically charted, and the of scientists who recognize themselves as geologists. pioneers of science cast aside dogmatic views of our These new geologists defined the geometry of struc- universe that had closed peoples’ minds for the previ- tures in mountain ranges, learned how to make geologic ous millennia and began to systematically observe their maps, discovered the processes involved in the forma- surroundings and carry out experiments to create new tion of rocks, and speculated on the origins of specific knowledge. The scientific method was born. structures and on mountain ranges in general. In geology, the stirrings of discovery are evident in Ideas about the origin of mountains have evolved the ink sketches of the great artist and inventor gradually. At first, mountain ranges were thought to be Leonardo da Vinci (1452–1519), who carefully drew a consequence of a vertical push from below, perhaps 2 2917-CH01.pdf 11/20/03 5:07 PM Page 3 F I G U R E 1. 1 Sketch by Leonardo da Vinci showing details of folded strata in the mountains of Italy (ca. 1500 AD). In recent years it was discovered that in addition to his careful observations of the natural world, da Vinci also completed insightful friction experiments. FIGURE 1.2 Aerial view of the European Alps (France). 1.1 INTRODUCTION 3 2917-CH01.pdf 11/20/03 5:07 PM Page 4 F I G U R E 1. 3 Model of mountain building and associated deformation as represented by G. P. Scrope (1825). The uplift is caused by intrusion of an igneous core, and the folds are generated by down-slope movement. associated with intrusion of molten rock along preex- to see and discuss roadside outcrops. The rocks are no isting zones of weakness, and folds and faults in strata longer gray masses to you, but they contain recogniz- were attributed to gravity sliding down the flanks of able patterns and shapes and fabrics. The purpose of these uplifts (Figure 1.3). Subsequently, the signifi- this book is to increase your ability to interpret these cance of horizontal forces was emphasized, and geolo- features, and particularly to use them as clues to under- gists speculated that mountain ranges and their standing the processes that have shaped and continue component structures reflected the contraction of the to change the outer layers of the Earth. Earth that resulted from progressive cooling. In this model, the shrinking of the Earth led to wrinkling of the surface. One of the more notable discoveries (about 1850) was the recognition by James Hall (1811–1898) 1.2 CL A SSIFIC ATION OF that Paleozoic strata in the Appalachian Mountains of GEOLOGIC STRUCTURES North America were much thicker than correlative strata in the interior of the continent. This discovery When you finished your introductory geology course, led to the development of the geosyncline theory, a you probably had a general concept of what a geologic model in which deep sedimentary basins, called geo- structure is. The term probably brings to mind images synclines, evolved into mountain ranges. Contraction of folds and faults. Perhaps you had the opportunity to theory and geosynclinal theory, or various combina- take a field trip where you saw some of these structures tions of the two, were widely accepted until the 1960s, in the wild. These features are formed in response to when the views of Alfred Wegener (1880–1930), pushes and pulls associated with the forces that arise Arthur Holmes (1898–1965), and Harry Hess from the movement of tectonic plates or as a conse- (1906–1969) led to the formulation of a very different quence of differential buoyancy between parts of the model. Building on the work of Alfred Wegener’s con- lithosphere. But what about bedding in a sedimentary tinental drift theory and Arthur Holmes’s mantle con- rock and flow banding in a rhyolite flow; are these vection model, Harry Hess proposed the revolutionary structures? And what about slump folds in a debris idea of a mobile seafloor (seafloor spreading hypoth- flow; are they structures? Well... yes, but the link esis) that lead to the formulation of plate tectonic the- between their formation and plate motion is less obvi- ory. In this theory, the Earth consists of several, rigid ous. So, maybe we need to have a more general con- plates that change in space and time. The interaction cept of a geologic structure. between these plates offers a unifying explanation for The most fundamental definition of a geologic the occurrence of mountain ranges, ocean basins, structure is a geometric feature in rock whose shape, earthquakes, volcanoes, and other previously disparate form, and distribution can be described. From this def- geologic phenomena. inition it is obvious that there are several ways in As the foundations of geology grew, diverse fea- which geologic structures can be subdivided into tures of rocks and mountains gained names, and the groups. In other words, by necessity there are several once amorphous, nondescript masses of rock exposed different, yet equally valid classification schemes that on our planet became history books that preserve the can be used in organizing the description of geologic Earth’s biography. Perhaps your concept of the planet structures. Different schemes are relevant for different has evolved rapidly as well, because of the courses in purposes, so we will briefly look at various classifica- geology and other sciences that you have taken thus tion schemes for geologic structures that will return in far. Now, as you drive across the countryside, you subsequent chapters. At first, these various classifica- scare the daylights out of your passengers as you twist tion schemes may seem very confusing. Thus, we rec- 4 OVERVIEW 2917-CH01.pdf 11/20/03 5:07 PM Page 5 ommend that you start by recognizing the basic geo- Post-formational: formed after the rock has metric classes as the foundation of your understanding. fully formed, as a consequence of phenomena As you learn about these classes, refer back to the lists not related to the immediate environment of below, and see how a particular geometric class fits rock formation into one or more of the classification schemes. IV. Classification based on the process of formation, that is, the deformation mechanism I. Classification based on geometry, that is, on the Fracturing: related to development or coales- shape and form of a particular structure cence of cracks in rock Planar (or subplanar) surface Frictional sliding: related to the slip of one Curviplanar surface body of rock past another, or of grains past one Linear feature another, both of which are resisted by friction This subdivision represents perhaps the most Plasticity: resulting from deformation by the basic classification scheme. In this scheme we internal flow of crystals without loss of cohe- include the following classes of structures: sion, or by non-frictional sliding of crystals past joint, vein, fault, fold, shear zone, foliation, and one another lineation. Diffusion: resulting from material transport II. Classification based on geologic significance either solid-state or assisted by a fluid Primary: formed as a consequence of the for- (dissolution) mation process of the rock itself V. Classification based on the mesoscopic cohesive- Local gravity-driven: formed due to slip down ness during deformation an inclined surface; slumping at any scale dri- Brittle: formed by loss of cohesion across a ven by local excess gravitational potential mesoscopic discrete surface Local density-inversion driven: formed due to Ductile: formed without loss of cohesion across local lateral variations in rock density, causing a a mesoscopic discrete surface local buoyancy force Brittle/ductile: involving both brittle and ductile Fluid-pressure driven: formed by injection of aspects unconsolidated material due to sudden release Note that the scale of observation (in this case, of pressure mesoscopic) is critical in the distinction between Tectonic: formed due to lithospheric plate inter- brittle and ductile deformation, because ductile actions, due to regional interaction between the deformation can involve microscopic-scale frac- asthenosphere and the lithosphere, due to turing and frictional sliding. crustal-scale or lithosphere-scale gravitational VI. Classification based on the strain significance, in potential energy and the tendency of crust to which a reference frame, usually the Earth’s sur- achieve isostatic compensation face, is defined The first four items in this scheme can be grouped Contractional: resulting in shortening of a as primary and nontectonic structures, meaning region that they are not directly related to the forces asso- Extensional: resulting in extension of a region ciated with moving plates. We purposely say Strike-slip: resulting from movement without “can” because in many circumstances these cate- either shortening or extension gories of structures do form in association with Note that shortening in one direction can be, but tectonic activity. For example, gravity sliding may does not have to be, accompanied by extension be triggered by tectonically generated seismicity, in a different direction, and vice versa. Also, and salt domes may be localized by movement of regional deformation usually results in the vertical tectonic normal faults. These first four categories displacement of the Earth’s surface, a component will be discussed in Chapter 3. The fifth category of deformation that is commonly overlooked. of structures is very large and forms the primary VII. Classification based on the distribution of defor- focus of this book. mation in a volume of rock III. Classification based on timing of formation Continuous: occurs through the rock body at all Syn-formational: formed at the same time as the scales material that will ultimately form the rock Penetrative: occurs throughout the rock body, at Penecontemporaneous: formed before full lithi- the scale of observation; up close, there may be fication, but after initial deposition spaces between the structures 1.2 CLASSIFICATION OF GEOLOGIC STRUCTURES 5 2917-CH01.pdf 11/20/03 5:07 PM Page 6 C D C L C Lithosphere Lithosphere Asthenosphere Mesosphere F I G U R E 1. 4 The principal features of plate tectonics. Three types of plate boundaries arise from the relative movement (arrows) of lithospheric plates: C—convergent boundary, D—divergent boundary, and L—lateral slip (or transform) boundary. Localized: continuous or penetrative structure Ultimately, most crustal structures are a conse- occurs only within a definable region quence of plate tectonic activity, which is the slow (on Discrete: structure occurs as an isolated feature the order of centimeters per year) but steady motion of segments of the outer, stiff layer of the Earth, called the We can conveniently consider the basic geometric lithosphere, over the weaker asthenosphere. The classes of structures to be a manifestation of the meso- forces that this motion generates, especially those from scopic cohesiveness of deformation. Joints, veins, and interactions at plate boundaries, produce the structures certain types of faults are manifestations of primarily we study in the field and in the laboratory. The three brittle deformation, whereas cleavage, foliation, and types of plate motions are convergence, divergence, folding are largely manifestations of ductile deforma- and lateral slip (Figure 1.4). Without the activity that tion processes. Thus, in this book we subdivide our dis- arises from these plate motions, such as deformation, cussions of specific structures into two parts: “Brittle volcanism, and earthquakes, the Earth would be as dead Structures” (Part B) and “Ductile Structures” (Part C). as the Moon. In other words, plate tectonics provides As a first approximation, brittle deformation is more the global framework to examine the significance of common in the upper part of the crust, where tempera- structures that occur on local and regional scales. tures and pressures are relatively low, and ductile defor- mation is more common in the deeper part of the crust, because it is favored under conditions of greater pres- sure and temperature. Also, ductile deformation is com- 1.3 STRESS, STRAIN, monly a manifestation of plastic deformation and AND DEFORMATION diffusion, whereas brittle deformation is a consequence of fracturing and frictional sliding. However, it is We have already used the words stress, strain, and important to emphasize right from the start that differ- deformation without definition, because these are ent processes can act in the same places in the Earth. common English words and most people have an intu- The processes that occur at any given time may reflect itive grasp of what they mean. Stress presumably has geologic variables such as strain rate, which is the rate something to do with pushing and pulling, and strain of displacement in the rock body (Chapter 5). For and deformation have something to do with bending, example, a sudden increase in strain rate may cause breaking, stretching, or squashing. But in standard rock that is deforming in a ductile manner (by folding) English, stress and strain are often used interchange- to suddenly behave in a brittle manner (by fracturing). ably; for example, advertisements for aspirin talk You can see this remarkable effect by, respectively, about “the stress and strain of everyday life.” In struc- slow and quick pulling of a piece of SillyPutty®. tural geology, however, these terms have more exact 6 OVERVIEW 2917-CH01.pdf 11/20/03 5:07 PM Page 7 Before After (a) Rotation (a) (b) (b) Translation F I G U R E 1. 6 Strain in the real world. (a) A car approaches a brick wall in a crash test; (b) the same car after impact. Note the extreme distortion of the front (i.e., inhomogeneous strain distribution). (c) Strain one point in the body is the same as the strain at all F I G U R E 1. 5 The three components of deformation: other points in the body. Cars are designed so that (a) rotation, (b) translation, and (c) strain. strain is heterogeneous, meaning that the strain is not equal throughout the body, and the passengers are pro- tected from some of the impact. meanings, so right from the start we want to clarify What about translation and rotation? These compo- their usage (and avoid headaches). nents of deformation are a bit harder to recognize, but The stress (σ) acting on a plane is the force per unit they do occur. For example, a rigid body of rock that has area of the plane (σ = F/area). We will see in Chap- moved along a fault plane clearly has been translated ter 3 that when referring to the stress at a point in a relative to the opposing side of the fault (Figure 1.7a), body, a more complicated definition is needed. and a fault block in which strata are inclined relative to Deformation refers to changes in shape, position, or horizontal strata on the opposing wall of the fault has orientation of a body resulting from the application of clearly been rotated (Figure 1.7b). Such rotations occur a differential stress (i.e., a state in which the magnitude at all scales, as emphasized by work in paleomagnetism, of stress is not the same in all directions). More specif- which demonstrates that continental blocks have been ically, deformation consists of three components rotated around a vertical axis as a consequence of shear (Figure 1.5): (1) a rotation, which is the pivoting of a along major strike-slip faults and plate boundaries. body around a fixed axis, (2) a translation, which is a In order to describe deformation, it is necessary to change in the position of a body, and (3) a strain, define a reference frame. The reference frame used in which is a distortion or change in shape of a body structural geology is loosely called the undeformed (Chapter 3). To visualize a strain, consider the test state. We can’t know whether a rock body has been crash of a car that is rapidly approaching a brick wall moved or distorted unless we know where it origi- (Figure 1.6a). In Figure 1.6b, the car and the wall have nally was and what its original shape was. Ideally, if attempted to occupy the same space at the same time, we know both the original and final positions of an with variable success. Since the structural integrity of array of points in a body of rock, we can describe a the car is less than that of the wall, the push between deformation with mathematical precision by defining car and wall squashed the car, thereby resulting in a a coordinate transformation. For example, in Fig- strain. In homogeneous strain, the strain exhibited at ure 1.8a, four points (labeled m, n, o, and p) define a 1.3 STRESS, STRAIN, AND DEFORMATION 7 2917-CH01.pdf 11/20/03 5:07 PM Page 8 may be able to describe strain—say, because of the presence of deformed fossils—but we have no absolute record of translations or rotations. Then we may talk about relative displacement and relative rotation. A flat-lying bed of Paleozoic limestone in the Midcontinent region of the United States was at one time below sea level and, because of plate motion, it Translation was formed at a different latitude than today, but we (a) can’t immediately characterize these movements.1 If, however, we see a fault offset a limestone bed by 2 meters, we say that one side of the fault has moved 2 m relative to the other side. 1.4 STRUCTURAL ANALYSIS Rotation AND SC ALES OF (b) F I G U R E 1. 7 The translational and rotational components OBSERVATION of deformation shown schematically along a fault. At this point, we know what a structure is and we (a) A translated fault block; (b) a rotated fault block in the hanging wall. know what a geologist means by deformation. We also know that there is a group of people who call them- selves structural geologists. But what do structural geologists do? One way to gain insight into the subject of structural geology is to think about the type of work m n m' n' that structural geologists carry out. Not surprisingly, structural geologists do structural analysis, which involves many activities (outlined in Table 1.1). Throughout the book you see that we use tables like p o this to summarize concepts and terms. Many terms not p' o' (a) (b) specifically mentioned in the text can be found in these F I G U R E 1. 8 Deformation represented as a coordinate tables, which serve as convenient reference points transformation. Points m, n, o, and p move to new positions m′, throughout the text. n′, o′, and p′. Looking at Table 1.1 you will note that in many of the definitions we have to refer to the scale of obser- vation. For the results of a structural analysis to be square in a Cartesian coordinate system. If the square interpretable, the scale of our analysis must be taken is sheared by stresses acting on the top and bottom into account. For example, a bed of sandstone in a sin- surfaces, as indicated by the arrows, and moved from gle outcrop in a mountain may appear to be unde- its original location, it changes into a parallelogram formed. But the outcrop may display only a small part that is displaced from the origin (Figure 1.8b). The of a huge fold that cannot be seen unless you map at the deformation can be described by saying that points m, scale of the whole mountain. Structural geologists com- n, o, and p moved to points m′, n′, o′, and p′, respec- monly refer to these relative scales of observation by a tively. In other words, coordinates of all four corners series of subjective prefixes. Micro refers to features of the square have been transformed. If you are math- ematically adept, you will probably realize that this transformation can be described by a mathematical function, but we won’t get into that now... wait until Chapter 3. 1Paleomagnetic and paleontologic methods are primarily used for this in In many real circumstances, we don’t have an exter- the Paleozoic. In the Mesozoic and Tertiary, ocean-floor magnetic anom- nal reference frame, so we can only partially describe alies are available as well, but in the Precambrian only the paleomagnetic a deformation. For example, at an isolated outcrop we approach remains. 8 OVERVIEW 2917-CH01.pdf 11/20/03 5:07 PM Page 9 TABLE 1.1 CATEGORIES OF STRUCTURAL ANALYSIS Descriptive analysis The characterization of the shape and appearance of geologic structures. It includes development of a precise vocabulary (jargon) that permits one geologist to create an image of a structure that any other geologist can understand, and development of methods for uniquely describing the orientation of a structure in three-dimensional space. Kinematic analysis The determination of the movement paths that rocks or parts of rocks have taken during transformation from the undeformed to the deformed state. This subject includes, for example, use of features in rocks to define the direction of movement on a fault. Strain analysis The development of mathematical tools for quantifying the strain in a rock. This activity includes the search for features in rock that can be measured to define strain. Dynamic analysis The development of an understanding of stress and its relation to deformation. This activity includes the use of tools for measuring the present-day state of stress in the Earth, and the application of techniques for interpreting the state of stress responsible for microstructures in rocks. Mechanism analysis The study of processes on the atomic scale to grain scale that allow structures to develop. This activity includes study of both fracture and flow of rock. Tectonic analysis The study of the relationship between structures and global tectonic processes. This activity includes the study and interpretation of regional-scale or megascopic structural features, and the study of relationships among structural geology, stratigraphy, and petrology. that are visible optically at the scale of thin sections, or that may only be evident with the electron microscope; the latter is sometimes referred to as submicroscopic. Meso refers to features that are visible in a rock out- crop, but cannot necessarily be traced from outcrop to outcrop. Macro refers to features that can be traced over a region encompassing several outcrops to whole mountain ranges. In some circumstances, geologists use the prefix mega to refer to continental-scale defor- mational, such as the movements of tectonic plates over time. Of course there are no sharp boundaries between these scales, and their usage will vary with context, but a complete structural analysis tries to integrate results from several scales of observation. Each scale of observation has its own set of tools. For example, optical and electron microscopes are FIGURE 1.9 Field area in Antarctica. used for observations on the microscale, and satellite imaging may be used for observations on the affair unless you are working in the High Himalayas, macroscale