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Structural Evolution of the Himalayan Thrust Belt, West Nepal STRUCTURAL EVOLUTION OF THE HIMALAYAN THRUST BELT, WEST NEPAL ____________________________________________ A Thesis Presented to the Faculty of the Department of Earth and Atmospheric Sciences University of Houston ____________________________________________ In Partial Fulfillment of the Requirements for the Degree Master of Science in Geology ____________________________________________ By An Li August 2013 STRUCTURAL EVOLUTION OF THE HIMALAYAN THRUST BELT, WEST NEPAL _______________________________________________ An Li APPROVED: _______________________________________________ Dr. Michael Murphy, Chairman _______________________________________________ Dr. Alexander Robinson _______________________________________________ Dr. Ran Zhang _______________________________________________ Dean, College of Natural Sciences and Mathematics ii ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my committee chair, Dr. Michael Murphy for his patient guidance and persistent help for my thesis research. Thanks him for taking me to the field trip at Big Bend and New Mexico. The most valuable thing I have learned from him is to be passionate to the work and be optimistic to the life. I would like to thank my committee members, Dr. Alexander Robinson and Dr. Ran Zhang for the support and assistance throughout this research project. Thanks to Sylvia Marshall and Jay Krishnan for answering all my academic and IT questions. Thanks to my classmate Yiduo Liu for giving me generous help in the field trips and constructive discussions about my thesis research. Thanks to my Uncle Faqi and Aunt Fenghua for taking care of my life at Houston. And thanks to my classmates and friends for accompanying and encouraging me in both study and life. Above all, I would like to thank my family, Zhenru, Jinbo, Dongran, Junfang. Thank you for endless love and support that gave me the strength to conquer difficulties and courage to chase my dream. Thank you for helping me to find myself when I feel lost. I dedicate this thesis to my family. iii STRUCTURAL EVOLUTION OF THE HIMALAYAN THRUST BELT, WEST NEPAL ____________________________________________ An Abstract of a Thesis Presented to the Faculty of the Department of Earth and Atmospheric Sciences University of Houston ____________________________________________ In Partial Fulfillment of the Requirements for the Degree Master of Science in Geology ____________________________________________ By An Li August 2013 iv ABSTRACT Himalayan foreland basin strata were recently documented to crop out in a structural window in the central portion of the Himalayan thrust belt. Presently, structural interpretations of the thrust belt do not explain in detail how these strata were incorporated into the thrust belt and why they are not widely exposed throughout the Himalaya. My research provides answers to these two issues. The structural window is located in the Lesser Himalaya of western Nepal and exposes rocks which lie structurally beneath the Main Central thrust (MCT) and Ramgarh thrust (RT) sheets. The thrust sheet consists of Proterozoic metamorphic rocks. Below the thrust sheet, footwall rocks exposed in the window are unmetamorphosed sedimentary rocks which consist of the early foreland basin strata (the Suntar and Swat formations) and the pre-foreland basin strata (the Melpani and Lakharpata formations). These unmetamorphosed rocks are present in the foreland basin beneath Siwalik group, ~100km to the south (towards the foreland). Two geologic maps were created; One covering the structural window and another covering the thrust belt from the High Himalaya to the MFT (Main Frontal Thrust). Several cross-sections were constructed from both maps. Structural reconstruction of these cross-sections reveal the following: 1) ~ 75 km-long hanging wall flat extends northward from its surface trace to the southern margin of the Lesser Himalayan duplex; 2) The geometry of the Jarjarkot klippe is narrower and structurally deeper than klippe to the west. The geometry of the northern flank of the klippe results from stacking of duplex horses, while the geometry of the southern flank results from slip over a ramp in the footwall of the MBT, 3) The early foreland basin strata in the window v are modeled to have originated at the front of the thrust belt, and subsequently buried by the Ramgarh and MCT thrust sheets, and 4) Exposure of these strata results from growth of the duplex which brought them to a higher structural level than most parts of the thrust belt. This structural model explains why the only other exposure of foreland basin strata within the thrust belt is documented within a duplex on the north side of the Dadeldhura klippe. Moreover, this model predicts that foreland basin strata are likely to be exposed wherever duplexes exist. vi CONTENTS 1. INTRODUCTION......................................................................................................... 1 1.1 GEOLOGY OF THE HIMALAYA ........................................................................... 1 1.1.1 Himalayan range and Himalayan fold thrust belt ............................................... 1 1.1.2 Timing of the Indo-Asian Collision .................................................................... 4 1.1.3 Shortening of the Himalayan thrust belt ............................................................. 4 1.1.4 Tectonostratigraphy of the Himalaya .................................................................. 4 1.1.5 Himalayan foreland basin ................................................................................... 7 1.2 STRATIGRAPHY IN THE RESEARCH AREA .................................................... 12 1.3 MOTIVATION AND METHODS ........................................................................... 18 2. STRUCTURAL GEOLOGY OF THE RESEARCH AREA................................... 21 2.1 FAULT ANALYSIS ................................................................................................. 21 2.2 SHEAR ZONE ANALYSIS .................................................................................... 25 2.3 MAJOR STRUCTURAL FEATURES .................................................................... 27 2.4 REGIONAL GEOLOGIC MAPS AND CROSS-SECTIONS ................................ 35 3. DISCUSSION AND MODELS .................................................................................. 39 3.1 TIMING DATA IN RESEARCH AREA ................................................................. 39 3.2 RECONSTRUCTION OF THE REGIONAL CROSS-SECTION ......................... 41 3.3 COMPARISON WITH PUBLISHED CROSS-SECTIONS ................................... 45 3.4 PREDICTION ......................................................................................................... 48 4. CONCLUSIONS ......................................................................................................... 50 5. APPENDIX .................................................................................................................. 51 6. REFERENCES ............................................................................................................ 59 vii FIGURE LIST Figure 1. Topographic map of the Himalaya ................................................................ 2 Figure 2. Tectonostratigraphic Map of the Himalaya ................................................... 3 Figure 3. Foreland basin system ................................................................................... 7 Figure 4. Regional geologic map of the research area ................................................ 14 Figure 5. The concepts of window and klippe ............................................................ 19 Figure 6. The location of field maps ........................................................................... 20 Figure 7. Tectonic map of the research area ................................................................ 22 Figure 8. Structural map of the structural window ..................................................... 23 Figure 9. Stereoplot of the fault surface at station L-4, L-5 and R-7 .......................... 24 Figure 10. Steoreoplot of the S-C fabric at station L-1 and L-6 ................................. 26 Figure 11. Cross-section C-C’ ..................................................................................... 28 Figure 12. Cross-section B-B’ .................................................................................... 30 Figure 13. Cross-section D-D’ .................................................................................... 31 Figure 14. Geologic map of the structural window .................................................... 33 Figure 15. Cross-section A-B south of the structural window .................................... 34 Figure 16. Cross-section B-C through the structural window .................................... 34 Figure 17. Characteristics the Ramgarh thrust sheet imbricate .................................. 35 Figure 18. Regional cross-section A-A’ ...................................................................... 36 Figure 19. Sequential reconstruction of regional cross-section from 20Ma to 10Ma . 42 Figure 20. Sequential reconstruction of regional cross-section from 6Ma to Recent . 43 Figure 21. Talkot cross-section of DeCelles et al. (2001) ........................................... 46 Figure 22. Simikot cross-section of Robinson (2008) ................................................ 46 Figure 23. Kathmandu cross-section of
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