A Palaeomagnetic Study of Crustal Rotations and Their Relationship to the Tectonics of the Atacama and Domeyko Fault Systems, Northern Chile
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University of Plymouth PEARL https://pearl.plymouth.ac.uk 04 University of Plymouth Research Theses 01 Research Theses Main Collection 1996 A PALAEOMAGNETIC STUDY OF CRUSTAL ROTATIONS AND THEIR RELATIONSHIP TO THE TECTONICS OF THE ATACAMA AND DOMEYKO FAULT SYSTEMS, NORTHERN CHILE RANDALL, DARREN EDWARD http://hdl.handle.net/10026.1/1728 University of Plymouth All content in PEARL is protected by copyright law. Author manuscripts are made available in accordance with publisher policies. Please cite only the published version using the details provided on the item record or document. In the absence of an open licence (e.g. Creative Commons), permissions for further reuse of content should be sought from the publisher or author. A PALAEOMAGNETIC STUDY OF CRUSTAL ROTATIONS AND THEIR RELATIONSHIP TO THE TECTONICS OF THE ATACAMA AND DOMEYKO FAULT SYSTEMS, NORTHERN CHILE DARREN EDWARD RANDALL A thesis submitted to the University of Plymouth in partial fulfilment for the degree of DOCTOR OF PHILOSOPHY Department of Geological' Sciences Faculty of Science In collaboration with Servicio Nacional de Geologi'a y Mineria (SERNAGEOMTN), Chile I June 1996 ! LIBRARY STORE REEERENCE ONLY UNIVERS: SIS Item No. Date 2 5 OCT m 3 R/^N 90 0278975 5 A PALAEOMAGNETIC STUDY OF CRUSTAL ROTATIONS AND THEIR RELATIONSHIP TO THE TECTONICS OF THE ATACAMA AND DOMEYKO FAULT SYSTEMS, NORTHERN CHILE DARREN EDWARD RANDALL ABSTRACT A total of 178 sites have been collected for palaeomagnetic analysis from within two strike-slip fauh systems in northern Chile (25.4°S - 26.4°S); the Atacama Fault System in the Coastal Cordillera, and the Domeyko Fault System in the Andean pre-Cordillera. In the Coastal Cordillera, analysis of Middle Jurassic lavas (La Negra Formation) and Middle Jurassic to Early Cretaceous dyke swarms reveal a consistent clockwise rotation of approximately 42°. The remanence from the La Negra Formation passes both fold and reversal tests and is interpreted as a pre- folding remanence. Four of the five dyke swarms have mixed polarity, suggesting that they too carry a primary or very early remanence. The clockwise rotation of the area is interpreted as occurring due to a domino-type mechanism where the blocks are bounded by sinistral faults operating in a crustal scale strike-slip duplex structure. The rotation is the result of sinistral transpression during the middle Cretaceous as a result of the Peruvian Orogeny leading to abandonment of the Jurassic-Early Cretaceous magmatic arc in the Coastal Cordillera and its subsequent eastward migration. The Domeyko Fault System (DFS) comprises the Domeyko Fault Zone (DFZ), and a series of subsidiary faults to the east which define two distinct but slightly overiapping domains: a fold-and-thrust belt in the north and a domain of sinistral strike-slip faults in the south. Samples were collected from the volcanic rocks of the Sierra Fraga Formation (Middle Jurassic) west of the DFZ, and from the lavas of the Quebrada Vicunita Formation (Late Jurassic) and Cerro Valiente Formation (Palaeocene) from both domains of the DFS. Also sampled in the southern domain, and in a small area between the two domains were sandstones of the Quebrada Monardes Formation (Early Cretaceous). All of the volcanic units have been remagnetised and no tectonic interpretation is made from them. The sandstones in the southern domain record a clockwise rotation of approximately 24°. This is interpreted as being due to compression across the DFS causing the sinistral strike-slip faults, and the blocks between them to rotate clockwise towards the major tectonic structure, the DFS. The sandstones in the central area between the two domains record an anticlockwise rotation of approximately 28°. This is explained by small-scale rotation of thrust sheets or slip on minor dextral strike-slip faults. The deformation and rotation in the pre-Cordillera occurred in response to the Incaic Orogeny during the Miocene-Oligocene. LIST OF CONTENTS ABSTRACT I LIST OF CONTENTS 3 LIST OF FIGURES 10 LIST OF TABLES 16 LIST OF PLATES 18 ACKNOWLEDGEMENTS 19 DECLARATION 20 1. INTRODUCTION 23 1.1 Selection of sampling regions 27 1.1.1 The Coastal Cordillera 29 1.1.2 Thepre-Cordillera 29 1.2 The wider context of this work 31 1.3 Geological background to South America 32 1.3.1 The Andean margin 33 1.3.2 The geology of northern Chile 34 1.3.3 The Arica Deflection 37 2. THE GEOMAGNETIC FIELD, SAMPLING TECHNIQUES AND PALAEOMAGNETIC METHODS 38 2.1 Introduction 38 2.2 The geomagnetic field and acquisition of magnetisation 38 2.2.1 Acquisition of magnetisation 41 2.3 Sample collection and preparation 43 2.4 Specimen measurement 44 2.5 Demagnetisation 44 2.5.1 Thermal demagnetisation 45 2.5.2 Alternating field demagnetisation 48 2.6 Magnetic mineralogy studies 49 2.6.1 Identification of magnetic minerals 49 2.6.2 Grain size determination 51 2.6.3 Measurements of the decay of viscous remanent magnetisation 52 2.6.4 Anisotropy of magnetic susceptibility 53 2.7 Data interpretation and statistics 53 2.7.1 Calculation of rotation, flattening and associated errors 55 3. PALAEOMAGNETIC REFERENCE POLES AND REFERENCE DIRECTIONS . 57 3.1 Introduction 57 3.2 Reliability criteria 59 3.3 Review of previous APWPs and reference directions 66 3.4 Calculating a new reference direction 69 3.4.1 Jurassic reference directions 73 i) Early Jurassic (208-178 Ma) 73 ii) MiddleJurassic(178.157.1 Ma) 76 iii) Ute Jurassic (157.1-145.6 Ma) 76 3.4.2 Cretaceous reference directions 77 i) Early Cretaceous (145.6-131.8 Ma) 77 ii) mid-Cretaceous (131.8-88.5 Ma) 77 iii) Ute Cretaceous (88.5-65 Ma) 78 3.4.3 Cenozoic reference directions 79 i) The early Cenozoic (Palaeogene) 79 ii) The late Cenozoic (Neogene) 80 3.5 Conclusions 80 4. CAUSES OF VERTICAL AXIS ROTATION 83 4.1 Introduction 83 4.2 Rigid bending of the continental margin 84 4.2.1 Oroclinal bending 84 4.2.2 Differential shortening 86 4.3 Mechanisms and models for small-scale in situ vertical axis rotations 88 4.3.1 The role of strike-slip faults in in situ vertical axis rotation 88 4.3.2 Definitions of models 92 4.3.3 Continuum versus discrete deformation 98 4.3.4 Discrete models of in situ vertical axis rotation 99 4.3.5 In situ rotation on thrust sheets 107 4.4 Mechanisms and models for large-scale in situ vertical axis rotation 107 4.5 Conclusions 110 5. THE COASTAL CORDILLERA AND THE ATACAMA FAULT SYSTEM Ill 5.1 Introduction 111 5.2 The geology ofthe Coastal Cordillera and the Atacama Fault System 113 5.2.1 Geology and petrology 113 5.2.2 Structural geology and the Atacama Fauh System 122 5.3 Structural and palaeostress analysis of the dykes 127 5.3.1 Dyke emplacement and orientation 127 5.3.2 Palaeostress analysis 129 5.3.3 Discussion of the palaeostress analysis results 132 5.4 Palaeomagnetic results 137 5.4.1 La Negra Formation 137 i) Palaeomagnetic sampling localities 137 ii) Magnetic mineralogy 137 iii) Anisotropy of magnetic susceptibility 142 iv) Palaeomagnetic remanence measurements 148 v) Discussion of results 158 5.4.2 Magnetic mineralogy of dyke samples 159 5.4.3 Low and intermediate coercivity/unblocking temperature components in the dyke samples 165 5.4.4 Dykes in the Vetado pluton 166 i) Palaeomagnetic sampling localities 166 ii) Palaeomagnetic remanence measurements 166 iii) Discussion of results 171 5.4.5 Dykes from the Flamenco pluton 174 i) Palaeomagnetic sampling localities 174 ii) Palaeomagnetic remanence measurements 174 iii) Discussion of results 177 5.4.6 Dykes from the Las Animas pluton 182 i) Palaeomagnetic sampling localities 182 ii) Palaeomagnetic remanence measurements 182 iii) Discussion of results 185 5.4.7 Dykes from the Las Tazas pluton 190 i) Palaeomagnetic sampling localities 190 ii) Palaeomagnetic remanence measurements 192 iii) Discussion of results 195 5.4.8 Dykes from the Remolino pluton 200 i) Palaeomagnetic sampling localities 200 ii) Palaeomagnetic remanence measurements 201 ii) Discussion of Results 210 5.5 Discussion and conclusions 211 6. THE PRE-CORDILLERA AND THE DOMEYKO FAULT SYSTEM 214 6.1 Introduction 214 6.2 The geology and structure of the Cordillera de Domeyko and the Domeyko Fault System 216 6.2.1 Geology and petrology of lithologies in the study area 217 6.2.2 Structural geology and the Domeyko Fault System 225 6.3 Palaeomagnetic results 229 6.3.1 Sierra Fraga Formation 229 i) Palaeomagnetic sampling localities 229 ii) Magnetic mineralogy 229 iii) Palaeomagnetic remanence measurements 232 iv) Discussion of results 238 6.3.2 Quebrada Vicunita Formation 241 i) Palaeomagnetic sampling localities 241 ii) Magnetic mineralogy 241 iii) Palaeomagnetic remanence measurements 248 iv) Discussion of results 259 6.3.3 Quebrada Monardes Formation 261 i) Palaeomagnetic sampling localities 261 ii) Magnetic mineralogy 263 iii) Palaeomagnetic remanence measurements 263 iv) Discussion of results 274 6.3.4 Cerro Valiente Sequence 276 i) Palaeomagnetic sampling localities 276 ii) Magnetic mineralogy 279 iii) Palaeomagnetic remanence measurements 284 iv) Discussion of results 291 6.4 Discussion and Conclusions 295 7. MODELS FOR ROTATION IN THE ATACAMA AND DOMEYKO FAULT SYSTEM 298 7.1 Introduction 298 7.2 Rigid bending of the Andean margin 303 7.3 Large-scale in situ rotation models 308 7.4 Small-scale in situ rotation models 311 7.5 Proposed models for rotation in the Atacama and Domeyko Fault Systems 315 7.5.1 The Atacama Fault System 317 7.5.2 The Domeyko Fault System 321 7.6 Conclusions 326 8. SUMMARY AND CONCLUSIONS 327 8.1 Introduction 327 8.2 The Coastal Cordillera 327 8.3 The pre-Cordillera 329 8.4 The Andean margin 331 8.5 Recommendations for future work 332 APPENDICES 334 APPENDIX 1 THIN SECTION DESCRIPTIONS 335 APPENDIX 2 PALAEOMAGNETIC DATA 345 REFERENCES 346 10 LIST OF FIGURES 1.