C. W. Passchier • R. A. J. Trouw

Microtectonics

With 254 Figures

Springer

Contents

1 A Framework of Microtectonic Studies 1 1.1 Introduction 1 1.2 Fstablishing and Interpreting Deformation Phases 3 1.3 Deformation Phases and Metamorphic Fvents 5 2 Flow and Deformation 7 2.1 Introduction 7 2.2 Terminology 7 2.3 Description and Reconstruction of Deformation 7 2.4 Reference Frames 8 2.5 Homogeneous and Inhomogeneous Flow and Deformation 11

2.5.1 Introduction 11 2.5.2 Numerical Description of Homogeneous Flow and Deformation 13

2.6 Deformation and Strain 17 2.7 Progressive and Finite Deformation 18 2.8 Flow and Deformation in Three Dimensions 19 2.9 Fabric Attractor 19 2.10 Application to Rocks 20 2.11 Stress and Deformation 20 2.12 Rheology 21 3 Deformation Mechanisms 25 3.1 Introduction 25 3.2 Cataclastic Flow 25 3.3 Pressure Solution 26 3.4 Intracrystalline Deformation 29 3.5 Twinning 31 3.6 Recovery 32 3.7 Recrystallisation 36

3.7.1 Grain Boundary Migration Recrystallisation 36 3.7.2 Subgrain Rotation Recrystallisation 36 3.7.3 Competing Processes During Deformation 40

3.8 Solid-state Diffusion Creep 40 3.9 Grain Boundary Sliding and Superplasticity 41 3.10 Grain Boundary Area Reduction (GBAR) 43 3.11 Static Recrystallisation 45 3.12 Deformation of Some Rock-Forming Minerals 48

3.12.1 Introduction 48 3.12.2 Quartz 48 3.12.3 Calcite and Dolomite 48

X Contents

3.12.4 Feldspars 49 3.12.5 Micas 50 3.12.6 Olivine 50 3.12.7 Orthopyroxene 50 3.12.8 Clinopyroxene 51 3.12.9 Amphiboles 51

3.13 Deformation of Polymineralic Rocks 51 3.13.1 Introduction 51 3.13.2 Quartz-Feldspar Aggregates 52 3.13.3 Deformed Ignimbrites 53

3.14 Flow Laws and Deformation Mechanism Maps 53 4 Foliations, Lineations and Lattice Preferred Orientation 57 4.1 Introduction 57 4.2 Foliations 58 4.3 Primary Foliation 59 4.4 Diagenetic Foliation 61 4.5 Secondary Foliations 61 4.6 Morphology of Foliations 63 4.7 Continuous Foliation 64 4.8 Spaced Foliation 65 4.9 Mechanisms of Foliation Development 69

4.9.1 Introduction 69 4.9.2 Mechanical Rotation of Tabular or Flongate Grains 69 4.9.3 Solution Transfer 70 4.9.4 Crystalplastic Deformation 74 4.9.5 Dynamic Recrystallisation and Related Processes 75 4.9.6 Mimetic Growth 76 4.9.7 Oriented Growth Defined by a Stress Field 77 4.9.8 Microfolding 77

4.10 Geological Context of Foliation Development 78 4.10.1 Foliations and Folds 78 4.10.2 Foliations and The XY Plane of Tectonic Strain 78 4.10.3 Foliations, Strain and Volume Change 82 4.10.4 Foliations, Lithotype and Metamorphic Conditions 83

4.11 Practical Use of Foliations 84 4.11.1 Introduction 84 4.11.2 Overprinting Relations 86

4.12 Lattice-Preferred Orientation (LPO) 88 4.13 Origin of Lattice-Preferred Orientation 89 4.14 Presentation of LPO Data 90 4.15 LPO Patterns of Quartz 92

4.15.1 Introduction 92 4.15.2 The Fffects of Strain and Recrystallisation 93 4.15.3 Shear Sense Determination Using Quartz Fabrics 95

4.16 LPO Patterns of Other Minerals 95

5 Shear Zones 97 5.1 Introduction 97 5.2 Brittle Fault Rocks 98

5.2.1 Incohesive Fault Rocks 98 5.2.2 Cohesive Fault Rocks 99 5.2.3 Pseudotachylyte 99

Contents XI

5.3 Mylonite 102 5.3.1 Introduction 102 5.3.2 Characteristic Fabric Elements 103 5.3.3 Mylonite Classification 106 5.3.4 Dynamics of Mylonite Development 106 5.3.5 Mylonite and Metamorphic Conditions 106

5.4 Complex Fault Rocks 108 5.5 Sense of Shear 109

5.5.1 Introduction 109 5.5.2 Displacement and Deflection of Markers 109 5.5.3 Foliation Curvature 109

5.6 Microscopic Shear Sense Indicators in Mylonite 110 5.6.1 Introduction 110 5.6.2 Foliation Orientation 110 5.6.3 Oblique Foliations I l l 5.6.4 Shear Band Cleavages I l l 5.6.5 Mantled Porphyroclasts 115 5.6.6 Development of Mantled Porphyroclasts 116 5.6.7 Mica Fish 121 5.6.8 Quarter Structures 123 5.6.9 Lattice-Preferred Orientation 124 5.6.10 Vergence of Asymmetric Fold Sections 124 5.6.11 Potential Shear Sense Markers 124

5.7 Shear Sense Indicators in the Brittle Regime 128 5.7.1 Introduction 128 5.7.2 Incohesive Brittle Fault Rocks 128 5.7.3 Cohesive Brittle Fault Rocks 128 5.7.4 Pseudotachylyte 129

6 Dilatation Sites: Vibrous Veins, Strain Shadwos, Strain Fringes and Boudins 131

6.1 Introduction 131 6.2 Fibrous Veins 133

6.2.1 Introduction 133 6.2.2 Curved Fibres and Fibre Growth Direction 136 6.2.3 Veins in Non-Coaxial Progressive Deformation 139

6.3 Strain Fringes 141 6.3.1 Introduction 141 6.3.2 Fringes on Spherical Core Objects 141 6.3.3 Fringes on Angular Core Objects 143

6.4 Fibres and the Deformation Path 145 6.5 Non-Fibrous Strain Shadows and Strain Caps 148 6.6 Microboudinage 149 7 Porphyroblasts and Reaction Rims 153 7.1 Introduction 153 7.2 Porphyroblast Nucleation and Growth 154 7.3 Inclusions 154 7.4 Classification of Porphyroblast-Matrix Relations 157

7.4.1 Introduction 157 7.4.2 Pretectonic Porphyroblast Growth 159 7.4.3 Intertectonic Porphyroblast Growth 161 7.4.4 Syntectonic Porphyroblast Growth 162 7.4.5 Post-Tectonic Porphyroblast Growth 163

XII Contents

7.4.6 Complex Porphyroblast Growth 163 7.5 Millipede and Deflection-Fold Microstructures 169 7.6 Problematic Porphyroblast Microstructures 170

7.6.1 Inclusion-Free Porphyroblasts 170 7.6.2 Shape and Size of Inclusion and Matrix Grains 170 7.6.3 False Inclusion Patterns 172 7.6.4 Mimetic Growth 172 7.6.5 Deformed Porphyroblasts 173 7.6.6 Uncertain Age Relation of Host and Inclusions 173 7.6.7 Discontinuous Sj and S 176 7.6.8 Rotation of Porphyroblasts 176

7.7 Crystallographically Determined Inclusion Patterns 182 7.8 Reaction Rims 185

7.8.1 Introduction 185 7.8.2 Coronas and Moats 187 7.8.3 Symplectites 190 7.8.4 Establishing the Nature of Reactions 192 7.8.5 Pseudomorphs 194 7.8.6 Relation with Deformation 194

7.9 Reconstruction of Tectono-Metamorphic Evolution 194 8 Natural Microgauges 197 8.1 Introduction 197 8.2 Strain Gauges 197 8.3 Vorticity Gauges 199

8.3.1 Introduction 199 8.3.2 Deformed Sets of Veins 199 8.3.3 Lattice-Preferred Orientation 199 8.3.4 Mantled Porphyroclasts, Fibrous Veins and Fringes 200 8.3.5 Porphyroblasts 200 8.3.6 Tension Gashes and Foliations in Shear Zones 200 8.3.7 Oblique Foliations 200 8.3.8 Al-Cr Zoning in Spinel 200 8.3.9 History 200

8.4 The Concept of Palaeostress Gauges 201 8.5 Gauges for the Orientation of Palaeostress Principal Axes 201

8.5.1 Calcite Twins 201 8.5.2 Deformation Lamellae 201

8.6 Differential Stress Gauges (Palaeopiezometers) 202 8.6.1 Dynamically Recrystallised Grain Size 202 8.6.2 Twins in Calcite and Dolomite 203 8.6.3 Microboudins 203 8.6.4 Deformation Lamellae 203

8.7 Pressure Gauges 203 8.8 Strain Rate Gauges 204 8.9 Temperature Gauges 204 8.10 Rheology Gauges 206

8.10.1 Introduction 206 8.10.2 Folding and Boudinage 207 8.10.3 Mantled Objects 207

9 Special Techniques 209 9.1 Introduction 209 9.2 Cathodoluminescence 209

Contents X'"

9.3 Fluid Inclusion Studies 210 9.4 Electron Microscopy 211 9.3 Scanning Electron Microscope (SEM) 211

9.5.1 Introduction 211 9.5.2 Secondary and Backscatter Electrons 212 9.5.3 Problems with Orientation 213 9.5.4 Sample Preparation 213

9.6 Transmission Electron Microscope (TEM) 216 9.6.1 Introduction 216 9.6.2 Sample Preparation 217

9.7 Image Analysis 217 9.7.1 Introduction 217 9.7.2 Heilbronner Method 218

9.8 Lattice-Preferred Orientation 218 9.8.1 Introduction 218 9.8.2 U-Stage Measurements 218 9.8.3 Sample Selection and Preparation for Measurement 219 9.8.4 Optical Bulk Analysis - Price Method 219 9.8.5 CIP Method 219 9.8.6 Texture Goniometers 220 9.8.7 Channelling 220

9.9 Analogue Modelling 220 9.9.1 Introduction 220 9.9.2 Linear Rig 220 9.9.3 Torsion Rig 223 9.9.4 Triaxial Rig 223 9.9.5 Marker Particles 223 9.9.6 Analogue Experiments 224

9.10 Use of the U-Stage 225 9.10.1 Introduction 225 9.10.2 Preparation of the Stage for Measurement 225 9.10.3 Measurement of c-Axes of Quartz 228 9.10.4 Notation of Measurements 230

10 From Sample to Section 231 10.1 Introduction 231 10.2 Sampling 231 10.3 Orientation of Hand Specimens 231 10.4 Where to Sample in Outcrop 231 10.5 Cutting Samples 233 10.6 Types of Thin Sections 233 10.7 Geometries in Thin Section - a Problem of Dimensions 234 10.8 Choosing the Orientation of Thin Sections 236 11 Exercises 239 11.1 Introduction 239 11.2 Problem Section 239 11.3 Interpretations 250 Glossary 255 References 267 Index 283