THE UNIVERSITY LIBRARY PROTECTION OF AUTHOR ’S COPYRIGHT This copy has been supplied by the Library of the University of Otago on the understanding that the following conditions will be observed: 1. To comply with s56 of the Copyright Act 1994 [NZ], this thesis copy must only be used for the purposes of research or private study. 2. The author's permission must be obtained before any material in the thesis is reproduced, unless such reproduction falls within the fair dealing guidelines of the Copyright Act 1994. Due acknowledgement must be made to the author in any citation. 3. No further copies may be made without the permission of the Librarian of the University of Otago. August 2010 MECHANICS OF FOLDING IN CENTRAL OTAGO An investigation of the Recent deformation of the Rock and Pillar Range. Gillian Greta Salton A thesis submitted for the degree of Master of Science University of Otago 1993 ABSTRACT Cenozoic deformation of the Central Otago Range and Basin province has been investigated to determine the mechanism of range formation. Mapping in the Rock and Pillar Range revealed macroscopic folds in schistosity which correspond in shape and size to the ranges, suggesting topography is controlled by folding of the schist. This congruence has also been documented in the Raggedy Range, further to the west. The role of the Hyde Fault in uplift of the Rock and Pillar Range has been investigated. Available evidence suggests there is no large throw across the fault and that faulting is subordinate to folding in the formation of the range. Thus the Central Otago ranges are interpreted as macroscopic folds of the upper crust. A mechanical framework for the formation of these folds is presented. An elastic-plastic constitutive law for deformation of the Otago Schist has been derived which incorporates the anisotropy of the schist. Plastic components of the model represent slip on schistosity planes. An elastic-plastic plate may buckle under much lower differential stress than a purely elastic plate, but an initial shear on the schistosity planes is required to initiate folding. This may be imposed externally by a fault or internally by misalignment of schistosity planes. Growth of the folds is controlled by the relative weakness and low friction of the schistosity planes, which allow for easy slip. Folds grow until slip on the schistosity planes is no longer favorable; then faulting across schistosity planes occurs. Scale effects on the initiation and growth of the ranges have been assessed. Finite difference computer modelling was used to simulate the growth of the ranges from an initial perturbation to finite displacement folds. Deformation is permanent and asymmetric. Implications for regional tectonics have been assessed. The principal compression direction may be oblique, rather than perpendicular, to the ranges because pervasive lineation of the schist partly controls the orientation of schistosity fold axes. Therefore these folds need not have formed in response to pure compression, but may result from oblique compression. 11 ACKNOWLEDGEMENTS Thanks first and most to my supervisor, Rick Sibson, for making everything possible. His support in every aspect of this project, from my grant application right through the editing of this thesis, has been irreplaceable. Many other people provided input. Of the staff, R. Norris, P. Koons, and D. Craw provided helpful discussions. Of the students, John Youngson continually amazed me with his knowledge of Central Otago. Sean Waters and Guy Simpson shared their experiences in the area. Phaedra Upton provided help with mechanics, and patiently listened to my ramblings. Thanks to Phaedra Upton and Brian McAdoo for assistance in the field, and especially with the plane table. Numerous run­ holders of the Rock and Pillars, generously allowed me free access. Beautiful low-level aerial photography was provided by Mike Hall of Broken Hill Petroleum. Critical information on the strength and elastic properties of the schist was provided by the Clyde Power Project of the Electricity Corporation of New Zealand. Jim Rice provided valuable advice and interesting reading. Sean Waters, Dave Jillett, Cath Samson, and Vern Manville read chapters and provided many helpful comments and ideas. Funding for my stay· in New Zealand was generously provided by the Institute for International Education, in the form of a Fulbright Scholarship. Further support was provided by the University of Otago for fieldwork, and the Earthquake and War Damage Commission of New Zealand for studies on the Hyde Fault. The experience of life in New Zealand is something that will remain with me far beyond the details of this thesis. Thanks to everyone who made me feel at home here, I'll miss you all. Especially, thanks to my flatmates, Sean, Jo, Cogger, and Graham, and the rest of the Herm part-time residents, who provided a creative and intellectually stimulating environment in which to nurture ideas. iii TABLE OF CONTENTS Page TITLE PAGE ABSTRACT i i ACKNOWLEDGEMENTS iii TABLE of CONTENTS iv LIST of FIGURES viii SYMBOLS USED X Chapter 1 Introduction Features of Central Otago and Mechanics of Folding 1.1 GEOGRAPHY 1 1.2GEOLOGY 1 1.3 REGIONAL STRUCTURE & TECTONIC SETTING 3 1.4 THIS STUDY: STRUCTURE AND MECHANICS 5 1 .4.1 Range and Basin peformation 5 1.4.2 Definition of Terms 6 Chapter 2 Post-Metamorphic Deformation of the Otago Schist Field Evidence 2.1 PREVIOUS WORK 8 2.2 ROCK AND PILLAR RANGE 1 0 2.2.1 Nature of Outcrop 1 0 2.2.2 Schistosity 12 2.2.3 Lineation 1 6 2.2.4 Joints 1 7 2.2.5 Faulting 1 8 2.2.6 Tertiary and Quaternary Units 1 9 2.3 RAGGEDY RANGE 1 9 iv Chapter 3 Hyde Fault : Field Evidence of a Controlling Structure? 3.1 PREVIOUS WORK 22 3.2 EVIDENCE OF UPLIFT 24 3.3 EVIDENCE FOR FAULTING 3.3.1 Hyde Diggings 24 3.3.2 Detailed Investigation of Scarps 27 3.3.3 Gouge Petrography 32 3.3.4 Earthquake Activity 33 3.4 EVIDENCE FOR FOLDING 34 3.4.1 Volume of Sediments 34 3.4.2 Composition of Fans 35 3.4.3 Gold Distribution 36 3.4.4 Trace of Range Front 36 3.4.5 Basalt Syncline 37 3.5 SUMMARY 39 Chapter 4 Initiation of Folding : A Mechanical Framework 4.1 PROBLEM STATEMENT 40 4.2 ELASTIC PLATE BUCKLING 42 4.3 MULTI-LAYER MODELS 44 4.4 BlOT THEORY : BUCKLING OF AN ELASTIC ANISOTROPIC PLATE 45 4.5 ELASTIC-PLASTIC ANISOTROPY 49 4.5.1 Simplifying Assumptions 50 4.5.2 Constitutive Law 51 4.6 APPLICATION TO THE OTAGO SCHIST 53 4.6.1 Fault at Depth 54 4.6.2 Slip on Weak Misoriented Planes 55 v Chapter 5 Growth of Folds : A Mechanical Framework 5.1 STABILITY OF DEFORMATION 58 5.1.1 Velocity Strengthening of Material 58 5.1.2 Strain Hardening of System 59 5.1.3 Slip Transfer 60 5.2 STAGES OF FOLD GROWTH 60 5.2.1 Softening and Hardening of Rotation 62 5.2.2 Lock-up and Faulting 62 5.3 EFFECTS OF SCALE ON FOLD GROWTH 64 5.3.1 Gravity Body Forces 64 5.3.2 Fluid Pressure 65 Chapter 6 Computer Modelling A Finite Difference Simulation of Fold Growth 6.1 FLAC CAPABILITIES 6 6 6.2 MODELS 67 6.2.1 Material Parame~ers 67 6.2.2 Grids 69 6.2.3 Boundary and Initial Conditions 71 6.2.4 Loading 71 6.3 RESULTS 71 6.3.1 Reverse Fault 72 6.3.2 Thrust Fault 74 6.3.3 Kinked Grids 74 6.3.4 Combination Grids 77 6.4 SUMMARY 77 Vl Chapter 7 Discussion and Conclusions Mechanical Concepts and the Rock and Pillars 7.1 GEOMORPHOLOGY AND MECHANICS 79 7.1.1 Range Shape and Fold Types 79 7.1.2 Taieri River 81 7.2 FOLDING, FAULTING, AND RANGE GROWTH 7.2.1 Fault- Fold Relations within a Range 81 7.2.2 Fault-Fold Relations among Ranges 82 7.3 STRUCTURE, TECTONICS, AND MECHANICS 7.3.1 Regional Stresses 83 7.3.2 Lineations and Folding 84 7 .3.3 Lateral Deformations 85 7.4 COMPARATIVE STUDIES 7 .4.1 Other Central Otago Ranges 86 7.4.2 Other Crustal Fold Structures 86 7.4.3 Analog Experiments 87 7.5 CONCLUSIONS 89 7.6 SUGGESTIONS FOR FURTHER STUDY 88 REFERENCES 90 APPENDIX A : FLAG Input Files 97 APPENDIX B : Plate Flexure at the Waihemo Fault 102 Vll LIST of FIGURES Page 1-1 Map of Central Otago 2 1-2 Central Otago stress field 4 1-3 Predicted schistosity dips 5 2-1 Location of NE-SW transects, Rock and Pillar Range 9 2-2 Photo of tors on the Rock and Pillar Range crest 1 1 2-3 Photo of schistosity dip on hillside 1 1 2-4 Stereographic projections of schistosity for NW-SE transects 1 2 2-5 Photo of fold propagating out of range. 1 3 2-6 a)Stereographic projections of schistosity along range crest b) Photo of range crest 1 4 2-7 Profiles of topography and schistosity folds 1 5 2-8 Stereographic projections of lineation 1 7 2-9 Stereographic projections of joints 1 7 2-10 Photo of shear zone 1 8 2-11 Location of Raggedy Range transects 1 9 2-12 Stereographic projections of schistosity and lineations for Raggedy Range 2 0 2-13 Profiles of schistosity and topography, Raggedy Range 21 3-1 Location of Hyde Fault 23 3-2 Patterns of stream incision into fans 25 3-3 Geometry of faulting at" Hyde Diggings 26 3-4 Photo of slope break 27 3-5 Location of scarps from air photos 28 3-6 Topographic profiles of slope break 30 3-7 Photo of scarp 31 3-8 Aerial photo of fan surface 31 3-9 Photomicrograph of gouge thin section 32 3-10 Seismogram of earthquake 33 3-11 Geometry of thrusting and erosion 34 3-12 Photo of range front 37 3-13 Photos of basalt syncline 38 4-1 Plate stress diagram 42 4-2 Diagram of orthotropic material 46 4-3 Graphical solution of buckling stability equation 48 4-4 Elastic-plastic shear stress/strain graph 52 4-5 Superposition of plate buckling and bending 55 4-6 Graph of stress ratio needed for fault reactivation 56 Vlll -----'->S---'1"-1P.fl-et&A9-i-e-F-&§-r-a-13-A-&-e-f--m-i-G-a-G-e-OO-S---S-G-i+ist-1-ay-EM-S