ABSTRACT BAYASGALAN, GANTULGA. Late Cenozoic Landscape Evolution in the Khangay Mountains, Mongolia. (Under the direction of Dr. Karl W. Wegmann). Timing, rates, and systems responsible for uplift of intracontinental mountain ranges provide insight into the creation of high-elevation regions on Earth that are distant from active plate tectonic margins. The Khangay Mountains in central Mongolia is an intriguing research site that is suitable for investigating the timing of geologic processes responsible for topographic uplift and the development of continental scale drainage patterns, as well as the climatic-geomorphic responses to such. This dissertation focuses on defining shallow crustal and surficial processes contributing to the development of the Khangay Mountains at both short and long topographic wavelengths, as well as the topographic evolution of the range. The first chapter of this dissertation focuses on cross-strike drainage development via the formation and capture of small tectonic basins (lakes) in front of an active thrust known as the Bayankhongor fault along the southern flank of the Khangay Mountains. The field research site contains ample geomorphological features that I will use to reconstruct the interplay between surface uplift by faulting, temporary formation of lakes in footwall basins and fluvial incision. Although ultimately these landforms (e.g. water gaps) reflect the defeat of rivers during sustained rock uplift, the role of climate oscillations in their formation is an underexplored topic. I used a coupled tectonics–landscape evolution-climate change model to test the hypothesis that a ~130 km2 late Quaternary lake in the Galuut Valley along the southern flank of the Khangay Mountains drained, perhaps catastrophically. I hypothesize that fault-controlled uplift of the original outlet exceeded the elevation of a low drainage divide between an adjacent range-front basin, possibly during a climatically-forced low stand in lake level. The research of my second chapter focuses upon characterizing the relative importance of chemical and physical weathering to landscape development in the Khangay Mountains through an investigation of geochemical major and minor trace elemental analysis. The samples are collected along major stream valleys in the Khangay from well-developed saprolitic paleosols formed in metasediments (middle Orkhon), granite (upper Orkhon), and Miocene fluvial deposits (upper Chuluut) preserved beneath 40Ar/39Ar dated basaltic lava flows with ages of 11.2, 7.6 and 3.1 Ma, respectively. I used the Chemical Index of alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW) derived from the fine-sediment fraction of these paleosols to reconstruct estimates of integrated paleo-mean annual temperature and precipitation in the millennia preceding lava burial. I used these data to compare estimates of geomorphic processes from the end of the middle Miocene and Pliocene with the cold-region, continental climate and physical weathering-dominant geomorphic processes that exist across the Khangay Mountains during the Quaternary. The third chapter focuses on quantifying valley sedimentary fill (alluvium) thicknesses in the Khangay Mountains. My hypothesis is that a regional change from relatively stable, warmer and more humid Miocene and Pliocene climate to the oscillatory cold and dry-dominated climate of the Quaternary resulted in substantial geomorphic process change. Specifically, regional hillslopes transitioned from transport-limited to weathering-limited, which resulted in substantial valley aggradation. The presence of thick packages of aggraded clastic fluvial sediment in both glaciated and non-glaciated valleys of the Khangay Mountains is an indication that glaciers alone are not responsible for the backfilling of regional valleys with sediment. I employed GIS analyses to estimate the thickness (depth to bedrock) and volume of aggraded Quaternary sediments from both Pleistocene glaciated and non-glaciated drainages. Estimates of these parameters provided constraints for reconstructing landscape-scale erosion rates and the modeling of isostatic uplift of the Khangay Mountain due to erosional unloading. © Copyright 2018 Gantulga Bayasgalan All Rights Reserved Late Cenozoic landscape evolution in the Khangay Mountains, Mongolia by Gantulga Bayasgalan A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Marine, Earth, and Atmospheric Sciences Raleigh, North Carolina 2018 APPROVED BY: ________________________________ ________________________________ Karl W. Wegmann Elana L. Leithold Committee Chair ________________________________ ________________________________ Helena Mitasova Ethan Hyland DEDICATION To my parents, L. Bayasgalan and L. Enkhtuya. ii BIOGRAPHY Gantulga Bayasgalan was born in Undurkhaan, Mongolia, and raised in Ulaanbaatar, Mongolia. He earned a Bachelor of Science degree in environmental science from National University of Mongolia in 2002 and a Master of Science degree in Physical Land Resources from the Free University of Brussels in 2007. He spent 5 years working as a lecturer in the Department of geology, Mongolian University of Science and Technology before enrolling in the PhD program at NC State in 2013. iii ACKNOWLEDGEMENTS This work was completed with the help of many people to whom I owe a great debt. My first and foremost thanks go to Prof. Karl W. Wegmann for his support and valued advice throughout my graduate study and for giving me a great opportunity to work and study at North Carolina State University. His guidance and editing in order to make this thesis readable is especially appreciated. I would like to thank the members of my graduate committee Dr. Lonnie Leithold, Dr. Helena Mitasova and Dr. Ethan Hyland. This dissertation has been significantly improved as a result of their valuable guidance and suggestions. I am also grateful to Dr. A. Bayasgalan for his suggestions and support during my Ph.D. program at NCSU. A huge debt of gratitude is owed to Dr. Emanuele Giachetta at the ETH-Zurich for his guidance and help for manipulating SIGNUM landscape evolution model, which intensively used for chapter 1. I would also like to thank Dr. R. Fodor at the NCSU, for his advice for calculating Major Element analysis for chapter 2, Dr. E. Hestir and Dr. D. Bohnenstiehl for their help and encouragement of my research and valuable discussions. Doing research with this effective working group was a great help in finishing my Ph.D. program. I am also indebted to Dr. Kh. Tseedulam, at the Mongolian University of Science and Technology for her help in performing geophysical Vertical Electric Sounding field survey for my research chapter 1. I also would like to thank the professors of the Marine Earth and Atmospheric Science Department at NCSU for their enlightening lectures, especially Jim Hibbard and Gary Lackmann. I immensely thank to Mrs. Meredith Henry, Mrs. Laura Holland and Mrs. Beth Graf for their professional administrative assistance. For the fieldwork in Mongolia, I would like to thank Narangerel Mandakh for his excellent driving and navigation skills throughout the countryside of Mongolia, and Steve Smith, Nathan Lyons, and Matthew Morriss for their great companionship and support both in lab and in the field. Many warm and happy memories accumulated related to my friends. I thank to all of my friends for their longtime friendship. Mongolia research was supported by National Science Foundation Research Grants EAR-1009702 and EAR-1009680. My deepest gratitude goes to my family for their unflagging love and support throughout my life. Without their help and encouragement from them, this thesis would not have been completed. iv TABLE OF CONTENTS LIST OF TABLES………………………………….……………………….……….…..…… viii LIST OF FIGURES……………………….………………………………....…...………….... xii APPENDICES………….………………………………………..…….………….................. xviii Chapter 1: Paleoenvironmental Reconstruction of Late Quaternary Lacustrine Sediments and their Tectonic Implications, Southern Khangay Mountains, Mongolia 1.1. Abstract.................................................................................................................................. 1 1.2. Introduction ........................................................................................................................... 2 1 1.3. Background ............................................................................................................................ 5 1.4. Materials and Methods ..........................................................................................................2 9 1.4.1. SIGNUM modeling .....................................................................................................5 11 1.4.2. Stratigraphy and radiocarbon ages...............................................................................9 12 1.4.3. Geophysical investigation ........................................................................................... 13 11 1.4.3.1. Vertical Electric Sounding (VES) method .......................................................... 13 1.4.3.2. Ground penetrating radar (GPR) survey ............................................................... 14 1.4.4. Geographic Information System Modeling ................................................................. 15 1.4.5. Geomorphological Mapping…………........................................................................