University of Vermont ScholarWorks @ UVM Graduate College Dissertations and Theses Dissertations and Theses 2014 Strain localization and exhumation of the lower crust: A study of the three-dimensional structure and flow kinematics of central Fiordland, New Zealand Alice Newman University of Vermont Follow this and additional works at: https://scholarworks.uvm.edu/graddis Part of the Geology Commons Recommended Citation Newman, Alice, "Strain localization and exhumation of the lower crust: A study of the three-dimensional structure and flow kinematics of central Fiordland, New Zealand" (2014). Graduate College Dissertations and Theses. 312. https://scholarworks.uvm.edu/graddis/312 This Thesis is brought to you for free and open access by the Dissertations and Theses at ScholarWorks @ UVM. It has been accepted for inclusion in Graduate College Dissertations and Theses by an authorized administrator of ScholarWorks @ UVM. For more information, please contact [email protected]. STRAIN LOCALIZATON AND EXHUMATION OF THE LOWER CRUST: A STUDY OF THE THREE-DIMENSIONAL STRUCTURE AND FLOW KINEMATICS OF CENTRAL FIORDLAND, NEW ZEALAND A Thesis Presented by Alice C. Newman to The Faculty of the Graduate College of The University of Vermont In Partial Fulfillment of the Requirements for the Degree of Master of Science Specializing in Geology October, 2014 Accepted by the Faculty of the Graduate College, The University of Vermont, in partial fulfillment of the requirements for the degree of Master of Science, specializing in Geology. Thesis Examination Committee: ______________________________ Advisor Keith A. Klepeis, Ph.D. ______________________________ Laura E. Webb, Ph.D. ______________________________ Chairperson Yves C. Dubief, Ph.D. ______________________________ Dean of the Graduate College Cynthia J. Forehand, Ph.D. August 25, 2014 Abstract In this thesis, I present structural and kinematic data on rock fabrics, shear zones and fault zones from the Cretaceous Malaspina orthogneiss and some of its satellite plutons in central Fiordland, New Zealand. Central Fiordland exposes a large tract of granulite- to eclogite-facies lower crust that was exhumed between late Mesozoic to Cenozoic times. The deformational structures of interest were formed and preserved during the lifecycle of a Cretaceous continental arc that involved thickening to over 60 km followed by collapse and rifting. As such, they provide an excellent opportunity to study strain localization in the deep crust and the process of exhumation. Detailed structural mapping, analysis, and the construction of a 45-kilometer cross section through the Malaspina orthogneiss and adjacent plutons reveal the spatial distribution, sequence, and kinematics of crosscutting deformational structures. The earliest structures record Cretaceous magmatism, high-grade metamorphism at the granulite and eclogite facies, and ductile flow that resulted in widespread (over 1200 km2), disorganized magmatic foliations. These events were followed by regional extension that resulted in the formation of multiple, ≤0.5 km-thick ductile, upper amphibolite facies shear zones that record cooling, hydration, and horizontal flow during the Late Cretaceous. Extension continued but changed obliquity in the early to middle Tertiary and resulted in sets of strike-slip and normal brittle to semi-brittle faults forming a sinistral transtensional system. These faults are distributed across central Fiordland and crosscut and transpose the ductile shear zones and magmatic foliations. Lastly, a change in relative plate motions resulted in the inception of the Alpine fault and the development of a late Tertiary transpressional fault system that crosscuts all previous structures. The dominant factors controlling strain localization in central Fiordland changed from magma, heat, and melting, to fluid activity, plate boundary reorganization, and reactivation of inherited structures. The succession of contrasting strain localization styles in response to changing tectonic and local conditions led to the development of multiple phases of deformation. These multiple phases of deformation allowed the deep crust to be exhumed in a heterogeneous and fragmented, or ‘piecemeal’, way. In particular, the inability of late Cretaceous ductile shear zones to fully exhume the lower crust was compensated by the ability of early Tertiary transtensional faults to simultaneously thin and further exhume the lower crust. Investigations of strain localization patterns in central Fiordland shed light on the causes and mechanisms of crustal exhumation, a phenomenon that is integral to the lifecycle of virtually all orogenic belts. Acknowledgements I am grateful for the support, knowledge, and encouragement I received from numerous people throughout the process of completing this thesis. Above all, I am grateful to Keith Klepeis for his guidance, mentorship, and for providing me with wonderful opportunities to develop my research, teaching, and professional experience. Thank you for your good humor, your time and attention, and for supporting my Fiordland song parodies, to boot. Many thanks go to Laura Webb and Yves Dubief for being part of my thesis committee and for providing valuable feedback that helped to improve this work. I thank all the faculty and staff of the UVM Geology department for being a wonderfully welcoming and supportive group. Thanks go to all the students in the department as well, both graduate and undergraduate. Among the undergraduates, I am especially thankful for the enthusiasm and dedication the “SURGE” students brought to our pilot mentorship seminar, and I owe thanks to Doug MacLeod for many enlightening discussions and for sharing his microstructural expertise with me. Among the graduates, I especially thank Mike Ingram for indispensable field support, and Kathryn Dianiska for her partnership in fieldwork, research, and adventures in and around Vermont. Thank you for sharing so many hours of hard work and laughter with me. Many thanks go to Josh Schwartz, Elena Miranda, and Harold Stowell and their students throughout this collaborative process. Fieldwork for this project would have been impossible without the logistical support so graciously provided by the Institute of Geological and Nuclear Sciences in Dunedin. Thanks in particular to Delia Strong, for ii sacrificing many hours to aid our travels and for introducing us to Southland’s meat pies. Thanks also go to our exceptional crewmembers abroad the Tutoko MV, Sean and Maria. This project was funded by a National Science Foundation grant to Keith Klepeis. I remain grateful to my former professors at Carleton College, for instilling my interests in geology and teaching through their knowledge and enthusiasm. I especially thank Sarah Titus and Clint Cowan for their continued guidance and support. I wish to thank Graham Hagen-Peter for helpful discussions, and for his patience and loving support throughout this ordeal. You are a gem. And finally, I thank my parents, Snow and Charlie, for being my most important supporters, friends, and givers of wisdom. 我 非常 榮幸當妳們的女兒 。 吾心鷹飛。 iii Table of Contents Acknowledgements ........................................................................................................... ii List of Figures ................................................................................................................... vi Chapter I – Introduction and Overview ......................................................................... 1 1. Statement of purpose .................................................................................................. 1 2. Field methods .............................................................................................................. 4 3. Thesis outline .............................................................................................................. 4 Chapter II – Literature Review ....................................................................................... 6 1. Overview of the geologic and tectonic history of Fiordland, New Zealand ............... 6 1.1 The Paleozoic Gondwana margin and Cretaceous arc magmatism ...................... 6 1.2 Cretaceous extension and opening of the Tasman Sea ....................................... 14 1.3 Late Mesozoic to Cenozoic tectonics ................................................................. 15 2. Eclogite-facies rocks in the Breaksea orthogneiss .................................................... 16 3. Deep crustal exhumation .......................................................................................... 18 4. Strain localization ..................................................................................................... 20 Chapter III – Strain localization and exhumation of the lower crust: A study of the three-dimensional structure and flow kinematics of central Fiordland, New Zealand ........................................................................................................................................... 22 1. Introduction ............................................................................................................... 23 2. Geologic background ................................................................................................ 28 3. The relative sequence of structures and their kinematics (D1-D5) ............................ 35 3.1 First (D1) and second (D2) phase structures ........................................................ 43 3.2 Third