_____________________________________________________________________________________ ANALOGUE MODELLING OF STRIKE-SLIP SURFACE RUPTURES: IMPLICATIONS FOR GREENDALE FAULT MECHANICS AND PALEOSEISMOLOGY BY PERI JORDAN SASNETT A thesis submitted in partial fulfilment of the requirements for the degree of Master in Science of Geology at the University of Canterbury 2013 i ABSTRACT Analogue modelling of strike-slip faulting provides insight into the development and behaviour of surface ruptures with accumulated slip, with relevance for understanding how information recorded in paleoseismic trenches relates to the earthquake behaviour of active faults. Patterns of surface deformation were investigated in analogue experiments using cohesive and non-cohesive granular materials above planar strike-slip basement faults. Surface deformation during the experiments was monitored by 3D PIV (particle image velocimetry) and 2D time lapse photography. Analysis focused on fault zone morphology and development, as well as the relationship of the models to surface deformation observed at the Greendale Fault that resulted from the 2010 Darfield earthquake. Complex rupture patterns with similar characteristics to the Greendale Fault (e.g. en echelon fractures, Riedel shears, pop-up structures, etc.) can be generated by a simple fault plane of uniform dip, slip, and frictional properties. The specific structures and the style of their development are determined by the properties of the overburden and the nature of the material surface. The width of the zone of distributed deformation correlates closely with sediment thickness, while the width of discrete fracturing is controlled by the material properties as well as the thickness of the overburden. The overall deformation zone width increases with the growth of initial, oblique fractures and subsequently narrows with time as strain localizes onto discrete fractures parallel to the underlying basement fault. Mapping the evolution of fracture patterns with progressive strain reveals that Riedel shears, striking at 90-120° (underlying fault strike = 90°) are more frequently reactivated during multiple earthquake cycles, and are thus most likely to provide reliable paleoseismic records. ii This will help identify suitable locations for paleoseismic trenches and interpret trench records on the Greendale Fault and other active, strike-slip faults in analogous geologic settings. These results also highlight the tendency of trenching studies on faults of this type to underestimate the number and displacement of previous ruptures, which potentially leads to an underestimate of the magnitude potential and recurrence interval of paleoearthquakes. iii ACKNOWLEDGEMENTS So many people have combined to help this thesis come together, and first and foremost among them are my supervisors. Mark, I’m so glad you agreed to take me on as a student and make this opportunity possible. You’ve made a lot of time for me at a point when you didn’t have much to spare, and I appreciate it. Sandy and David, you both looked after me so well in Australia. Having minimal experience with analogue modelling, I was reliant on you to figure out how to make my grand ideas into an actual model, and you totally came through. Pilar, you are the best brainstorming partner ever. The days you spent with me talking through my thesis and outlining the final version have made these few months of writing infinitely easier, and I am very grateful. Also, thanks for teaching me to make mole! The Fulbright US Student Program made it possible for me to come back to New Zealand for postgraduate study. Fulbright New Zealand, specifically, did a wonderful job welcoming me and the other grantees to the country and supporting us during our time here. I also feel very lucky to have such awesome fellow Fulbrighters—you guys are the best, and I can’t wait to see you all back in the States. I’m also grateful to the University of Canterbury geology community that adopted me so effortlessly. Darren, Sam, and Anekant: Frontiers was the perfect re-introduction to New Zealand, both geologically and otherwise. Thanks to the staff as well—you have all been inexplicably eager to help me solve any problem, from the initial last minute decision to do a Masters, to constant requests for plane tickets a week in advance (sorry!). To the other UC postgrads, you have all been a great source of support and fun, from field work to conferences to iv evenings (or afternoons…) at the Shilling Club. Also, a big shout out goes to the UC wiffleball team—let me know when it’s time to organize an international tour. Last but certainly not least, thanks to my friends, flatmates, and family. Past and present Peer Streeters—and honorary ones, you know who you are—there’s no way I would have gotten this far without your constant, loving harassment. Kat, Kate, Kate, Courtney, and Louise, thanks for all the fun times, great chats, and many (many) wines. Mom and Dad—you guys get all the credit for gently suggesting I come back to New Zealand, and even more credit for actually letting me go abroad for so long. I’m so grateful for your support and encouragement, and your many postcards and excessively hyperlinked emails have made me feel not so far from home. v TABLE OF CONTENTS ABSTRACT .................................................................................................................................... ii ACKNOWLEDGMENTS .............................................................................................................. iv TABLE OF CONTENTS ............................................................................................................... vi LIST OF FIGURES ....................................................................................................................... ix LIST OF TABLES ...................................................................................................................... xiv 1. INTRODUCTION AND REVIEW OF CURRENT KNOWLEDGE ........................................1 1.1 Introduction .................................................................................................................1 1.2 Regional Geology ........................................................................................................3 1.2.1 Regional Tectonic Context ...........................................................................3 1.2.2 Greendale Fault and Canterbury Plains Stratigraphy ................................5 1.3 Previous Studies of Strike-Slip Deformation ...........................................................9 1.3.1 Field Observations .......................................................................................9 1.3.2 Analogue Modelling Studies ......................................................................14 1.4 Research Questions and Objectives .......................................................................16 2. METHODS ..............................................................................................................................18 2.1 Experimental Setup .................................................................................................18 2.2 PIV Setup ..................................................................................................................19 2.3 Materials Tested .......................................................................................................19 2.4 Material Properties ..................................................................................................25 2.5 Scaling .......................................................................................................................30 2.6 Experiments ..............................................................................................................32 2.7 Greendale Fault Datasets ........................................................................................34 2.8 Analysis of Analogue Models and Greendale Fault Data.....................................36 3. FAULT ZONE DEVELOPMENT AND CONTROLS ON SURFACE MORPHOLOGY FROM ANALOGUE EXPERIMENTS ...................................................................................38 3.1 INTRODUCTION....................................................................................................38 vi 3.2 DATA AND RESULTS ...........................................................................................39 3.2.1 Experimental Results ...............................................................................39 3.2.1.1 TALC and TALC_PIV ....................................................................39 3.2.1.2 TALC_SED .....................................................................................46 3.2.1.3 TALC_ER .......................................................................................51 3.2.1.4 TALC_SAND and TALC_SAND_PIV ............................................56 3.2.1.5 SAND_PIV .....................................................................................64 3.2.1.6 General Summary ..........................................................................68 3.2.2 Greendale Fault Trace .............................................................................71 3.2.2.1 Stepovers ........................................................................................76 3.3 DISCUSSION ...........................................................................................................78 3.3.1 Comparisons with the Greendale