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The Tectonic Evolution of the Central Himalaya, Marsyandi Valley, Nepal

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The Tectonic Evolution of the Central Himalaya, Marsyandi Valley, Nepal The Tectonic Evolution of the Central Himalaya, Marsyandi Valley, Nepal by Margaret E. Coleman M.Sc. Geology Carleton University (Ottawa), 1990 B.A., Geology Middlebury College, 1987 Submitted to the Department of Earth, Atmospheric, and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy at the Massachusetts Institute of Technology August, 1996 © Massachusetts Institute of Technology, 1994. All rights reserved. Signature of Author bepartaent of Earth, Atmospheric, and Planetary Sciences August, 1996 z Lj Certified by Kip V. Hodges Thesis Advisor Accepted by Thomas H. Jordan Department Head alASSACHUSETTS INST1G~ U I E. OF TECHNOLOGY AUG 1 31996 -. M LIBRARIES 2 The Tectonic Evolution of the Central Himalaya, Marsyandi Valley, Nepal by Margaret E. Coleman Submitted to the Department of Earth, Atmospheric, and Planetary Sciences at the Massachusetts Institute of Technology on August 5, 1996, in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Geology ABSTRACT The metamorphic core of the central Himalaya, the Greater Himalayan sequence, is exposed in a 15 km thick section of amphibolite facies gneisses in the Marsyandi Valley of central Nepal. The sequence is bound by the Miocene north-dipping Main Central Thrust (MCT) at the base, and roofed by coeval north-dipping normal faults of the South Tibetan detachment system. Structurally above the Greater Himalayan sequence are lower-grade rocks of the Tibetan sedimentary sequence, and below are greenschist facies rocks of the Lesser Himalayan sequence. All of these rocks are Indian crust that was deformed and metamorphosed during intercontinental collision between India and Eurasia. Collision initiated in the Eocene and continues today. This study combines structural analysis with U-Pb and "Ar/ 39 Ar geochronology and metamorphic petrology to determine the structural and thermal history of the central Himalayas. The results are threefold and include 1) significant crustal thickening within the Indian plate, north of the MCT, by the mid- Oligocene; 2) Early Miocene orogen parallel extensional deformation within the upper half of the Greater Himalayan sequence during peak metamorphism; 3) a transition from north- south to east-west extension within the middle and upper crust of the northern Himalaya and southern Tibetan plateau during the Miocene. A minimum Oligocene age for crustal thickening within the Tibetan sedimentary sequence is constrained by -30 Ma *Ar/39Ar cooling ages from phlogopite that is synkinematic with south verging folds. The Oligocene syn-metamorphic deformation is preserved within the hanging wall of the South Tibetan detachment. The South Tibetan detachment in the Marsyandi valley dips to the NW and footwall mylonites indicate top-to- the-west displacement (orogen-parallel) at sillimanite through greenschist-grade conditions. Uranium-lead geochronology of migmatite from the base and the top of the Greater Himalayan sequence and from undeformed leucogranite at the top indicate syn- metamorphic displacement on the South Tibetan detachment and the MCT between 18 and 22 Ma. Thermobarometric results indicate that the metamorphic core was at a minimum of 900K throughout by 22 Ma. A minimum age of 14 Ma for east-west extension of the southern Tibetan plateau was determined by 4Ar/39Ar dating of muscovite-filled extensional fractures. Thesis advisor: Kip V. Hodges, Professor of Geology 4 Acknowledgments The following section provides an opportunity to thank those who have helped with this project. A few simple twists of fate lead me to embark on a Ph.D. in tectonics at MIT, and I owe those largely to Sharon Carr and Allison Macfarlane. Sharon Carr did not have time to accompany Allison to do field work in Nepal, but she was kind enough to inform Allison that I did. In November of 1990, somewhere up the Langtang Valley, Allison convinced me (over lunch and a particularly nice view), that pursuing a Ph.D. in tectonics at MIT was a good idea. I am grateful to Allison as well as my friends and mentors from Carleton University, in particular Randy Parrish, Dick Brown, Sharon Carr and Lisel Currie, for convincing me to follow this path. The essential advice, scientific interaction, editorial prowess, and friendship of my thesis advisor Kip Hodges are thankfully acknowledged. Kip's approach to Himalayan tectonics and tectonics in general will always be an inspiration. I would also like to thank my committee members Clark Burchfiel, Tim Byrne, Maureen Raymo, and Kip Hodges, for their time and complete reviews of this dissertation, and for actually making the defense a pleasant experience. I am particularly indebted to those individuals who helped me carry out the field work. First I must thank Allison Macfarlane for thoroughly teaching me how to manage the logistics of doing field research in Nepal, including introducing me to all the right people in Kathmandu, showing me how to get research permission, how to get rocks out of the country, introducing me to the geology, etc., and also for being a great friend. The other person who I cannot thank enough is Kate Bull, my dear friend and most excellent field partner (she definitely does not qualify as assistant!). Kate spent both entire field seasons with me and contributed a great deal to the success of the field mapping component of this project. Kate is one of the finest geologists/mountaineers/ EMT/Black belt/ Didis I have the pleasure of knowing and I hope we have more opportunities for mountain adventures together. Thanks for everything Kate! Several Nepalese also helped us carry out the field work, in particular Mangale Sherpa and D.R. Thulung; they were fantastic guides and cooks during the field season and their hard work and good cheer will never be forgotten. There is also a long list of Nepalese who helped us carry our gear and rocks around the Marsyandi region and they are sincerely thanked for their hard labor. I am grateful to Dr. B.N. Upreti and Surrendra Shresthra for helping me obtain research permission, and to the Nepali Government for granting research permission. U-Pb geochronology was carried out at the Geological Survey of Canada under the supervision of Randy Parrish. I would like to thank Randy for his help with the U-Pb work as well as the rest of the GSC geochronology lab in particular V. McNicoll, P. Hunt, K. Santowski, J. Crowley, and M.Villaneuve. During my visit to Ottawa I enjoyed the hospitality of the Hodges family, who generously offered me a home for an entire month during their sabbatical. Many thanks to Larky, Kip and Rachel for all their hospitality over the past five years. Bill Oszewski has been an excellent source of information and provided essential assistance in the CLAIR facility. Mike Jerconovic and Neel Chatterjee provided patient and thorough technical support in the mirocprobe lab. Pat Walsh took care of all the administrative details (and then some) and always provided a healthy perspective on life here. Nancy Dallaire was also a great source of administrative information (happy retirement!). Many graduate students and former graduate students have been influential during the course of this study and these include, Allison Macfarlane, Martha House, David Applegate, Audrey Huerta, Anke Friedrich, C.J. Northrup, and David Hawkings. C. J. Northrup, Drew Coleman, and Anke Friedrich are thanked for providing helpful comments on earlier drafts of various chapters. Beverly Saylor, Mousumi Roy, Jim Van Orman, Eric Kirby, Shane Pelechaty, Dave Dinter, Dawn Sumner, Jen Carlson, Nancy Harris, Ellen Dotson, Yu Jin, Peter Kaufman, Mark Simons, Kirstin Nicolaysen, and Mark Schmitz make up most of the other graduate students that I have enjoyed interacting with and learning from over the course of the past five years. Special thanks to Audrey Huerta for providing me with a safe haven over the past couple of months. The past year would not have been nearly as fun nor sane were it not for the almost daily visits to the pool with Beverly Saylor. Thank you Beverly, finishing at the same time made it much more enjoyable! Outside of MIT there are several people who provided me with a tremendous amount of encouragement and support. They include my immediate family, and good friends Hanna Steffian, Heather Sivert, and Buck deWolf. Special thanks to Hanna for sharing her house over the past two years and making it a real home for me, and Heather for many memorable meals! Weekend and holiday retreats to my parents home in New Hampshire and Maine sustained me at many different stages of graduate school and is greatly appreciated along with their unconditional love and support. A good portion of this thesis was written at Yale and I thank Philip Bogden and the GFD group for their hospitality. I credit much of my happiness over the past three year to Philip Bogden, who has been a wonderful distraction as well as a source of support and encouragement; and in addition an excellent incentive to finish (yes Kip, you provided me with plenty of reasons too!). 8 To William and Mildred Coleman 10 TABLE OF CONTENTS Title P ag e ..................................................................................................................... 1 A b stract ........................................................................................................................ 3 Acknowledgements .................................................................................................... 5 D ed ic atio n ....................................................................................................................
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