The Late Cenozoic tectonic evolution of Gurla Mandhata, Southwest Tibet By Andrew T. McCallister B.S., University of Arizona, 2010 Submitted to the graduate degree program in Geology and the Graduate Faculty of the University of Kansas in partial fulfillment of the requirements for the degree of Master of Science. ________________________________ Chairperson: Michael H. Taylor ________________________________ Andreas Mӧller ________________________________ Leigh A. Stearns ________________________________ Daniel F. Stockli Date Defended: October 10/10/2012 The Thesis Committee for Andrew T. McCallister certifies that this is the approved version of the following thesis: The Late Cenozoic tectonic evolution of Gurla Mandhata, Southwest Tibet ________________________________ Chairperson: Michael H. Taylor Date approved: 12/3/1012 ii Abstract How strain within the Tibetan plateau is geodynamically linked to that within the Himalayan thrust belt is a topic receiving considerable attention. The right-lateral Karakoram fault plays key roles in models describing the structural relationship between southern Tibet and the Himalaya. Considerable debate exists at the southeastern end of the Karakoram fault, where the role of the Karakoram fault is interpreted in two very different ways. One interpretation states that slip along the Karakoram fault extends eastward along the Indus-Yalu suture zone, thereby bypassing the Himalayan thrust belt to its north. The other, interprets that a significant component of the slip is fed southward into the Himalayan thrust belt along the Gurla Mandhata detachment. To evaluate this debate, the late Miocene fault slip rate history of the Gurla Mandhata detachment system is reconstructed from thermokinematic modeling with Pecube of zircon (U-Th)/He and biotite and muscovite 40Ar/39Ar thermochronometric ages. This slip rate history is then compared to that of the Karakoram fault. Zircon (U-Th)/He thermochronometric data from 3 east-west footwall transects reveal cooling of the Gurla Mandhata footwall through the zircon partial retention zone, from 8.01±1.31 Ma to 2.56±0.7 Ma. Results from ~21,100 Pecube models show a southward progression of decreasing fault slip magnitude and rate along the Gurla Mandhata detachment system. The northern transect modeling results show an initiation age from 14–11 Ma with a mean fault slip rate of 5.0±0.9 mm/yr. The central transect modeling results show an initiation age from 14–11 Ma with a mean fault slip rate of 3.3±0.6 mm/yr. The southern transect modeling results show an initiation age from 15-8 Ma with a mean fault slip rate of 3.2±1.6 Ma. These fault initiation ages and fault slip rate results match estimates obtained for the Karakoram fault across several timescales, supporting the idea that the two are kinematically linked. Specifically, the data are consistent with the Gurla Mandhata detachment iii acting as a right-step extensional stepover along which the Karakoram fault slip is transferred into the Himalayan thrust belt of western Nepal. iv Acknowledgments This work was funded by the Tectonics Division of the National Science Foundation, and by the University of Kansas (Student Summer Scholarship). I would like to thank my advisor Michael Taylor for the guidance and support provided during the course of this project. I would also like to thank Andreas Möller, Danny Stockli, and Leigh Stearns for being on my committee. A large thanks is owed to Michael Murphy at the University of Houston, for without the samples, data, help, and guidance he provided this project would not have been possible. Additionally, thanks to Paul Kapp from the University of Arizona and Allex Pullen from Rochester University who provided samples to this project. I want to thank the Taylor group Richard Styron, Kurt Sundell, Gabriel Velosa, and Maureen Logan for their assistance and discussion that helped make this project what it is. Thank you to Roman Kislitsyn for the help and guidance in the lab. I would like to thank my lab mates Joe Miller, Josh Burrus, Sarah Evans, Josh Feldman, Bad’r Ghorbal, Jeff Oalmann, Willy Rittase, and Sam Coleman for their training, assistance, and discussions in the lab. Thanks to Tandis Bidgoli for all the great discussion and advice throughout the past two years. Also thank Richard Styron for his help with the PeCube modeling, and for his assistance and guidance in the field. Thank you to my family for the love and support during this adventure. Finally to Melissa McMillan, thank you for being my rock during the good and bad. v Table of Contents ABSTRACT ................................................................................................................................. III ACKNOWLEDGMENTS ........................................................................................................... V TABLE OF CONTENTS ........................................................................................................... VI THE LATE CENOZOIC TECTONIC EVOLUTION OF GURLA MANDHATA, SOUTHWEST TIBET .................................................................................................................. 1 1. INTRODUCTION: ....................................................................................................................... 1 1.1 Models for deformation of the Himalayan-Tibetan orogen .............................................. 2 1.1.1 Oroclinal Bending ...................................................................................................... 3 1.1.2 Radial Spreading ........................................................................................................ 4 1.1.3 Oblique Convergence ................................................................................................. 4 1.1.4 Lateral Extrusion ........................................................................................................ 5 1.2 End-Member Models: ....................................................................................................... 6 1.2.1 Specific Model Predictions ........................................................................................ 6 2. GEOLOGY: ................................................................................................................................ 7 2.1 Geologic Setting: .............................................................................................................. 7 2.2 Geology of Gurla Mandhata: .......................................................................................... 10 2.3 LITHOLOGIC UNITS .............................................................................................................. 11 2.3.1 Hanging Wall Units ..................................................................................................... 11 2.3.2 Footwall Units ............................................................................................................. 12 3. METHODS ............................................................................................................................... 13 vi 3.1 ZIRCON (U-TH)/HE THERMOCHRONOLOGY ......................................................................... 13 3.1.1 Sample Processing ....................................................................................................... 14 3.1.2 Samples ........................................................................................................................ 14 3.1.3 Northern Transect ........................................................................................................ 14 3.1.4 Central Transect .......................................................................................................... 15 3.1.5 Southern Transect ........................................................................................................ 15 3.1.6 Results .......................................................................................................................... 16 4. THERMOKINEMATIC MODELING ............................................................................................. 17 4.1 Pecube: ........................................................................................................................... 17 4.2 Northern Transect ........................................................................................................... 19 4.3 Central Transect ............................................................................................................. 21 4.4 Southern Transect ........................................................................................................... 23 5. DISCUSSION ........................................................................................................................... 26 5.1 Development of the Gurla Mandhata Detachment ......................................................... 26 5.2 Comparison with other Himalayan Extensional Structures ........................................... 28 5.4 Comparison of GMD to KF ............................................................................................ 29 5.5.1 Lateral Extrusion ...................................................................................................... 31 5.5.2 Oroclinal Bending .................................................................................................... 31 5.5.3 Radial Spreading .....................................................................................................