Tectonic Geomorphology of the San Gabriel Mountains, CA By
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Tectonic Geomorphology of the San Gabriel Mountains, CA by Roman Alexander DiBiase A Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Approved September 2011 by the Graduate Supervisory Committee: Kelin X Whipple, Co-chair Arjun M. Heimsath, Co-chair J Ramón Arrowsmith Edward J. Garnero Kip V. Hodges ARIZONA STATE UNIVERSITY December 2011 ABSTRACT The San Gabriel Mountains (SGM) of southern California provide the opportunity to study the topographic controls on erosion rate in a mountain range where climate and lithology are relatively constant. I use a combination of digital elevation model data, detailed channel survey data, decadal climate records, and catchment- averaged erosion rates quantified from 10Be concentrations in stream sands to investigate the style and rates of hillslope and channel processes across the transition from soil- mantled to rocky landscapes in the SGM. Specifically, I investigate (1) the interrelations among different topographic metrics and their variation with erosion rate, (2) how hillslopes respond to tectonic forcing in "threshold" landscapes, (3) the role of discharge variability and erosion thresholds in controlling the relationship between relief and erosion rate, and (4) the style and pace of transient adjustment in the western SGM to a recent increase in uplift rate. Millennial erosion rates in the SGM range from 0.03-1.1 mm/a, generally increasing from west to east. For low erosion rates (< 0.3 mm/a), hillslopes tend to be soil-mantled, and catchment-averaged erosion rates are positively correlated with catchment-averaged slope, channel steepness, and local relief. For erosion rates greater than 0.3 mm/a, hillslopes become increasingly rocky, catchment-mean hillslope angle becomes much less sensitive to erosion rate, and channels continue to steepen. I find that a non-linear relationship observed between channel steepness and erosion rate can be explained by a simple bedrock incision model that combines a threshold for erosion with a probability distribution of discharge events where large floods follow an inverse power- law. I also find that the timing of a two-staged increase in uplift rate in the western SGM based on stream profile analysis agrees with independent estimates. Field observations in the same region suggest that the relict topography that allows for this calculation has i persisted for more than 7 Ma due to the stalling of migrating knickpoints by locally stronger bedrock and a lack of coarse sediment cover. ii ACKNOWLEDGMENTS This dissertation represents the culmination of five years of research in collaboration with a group of professors, post-docs, and graduate students who I have been extraordinarily lucky to work with. There is certainly not enough room to properly acknowledge everyone who has helped me along this path, but I wish to point out a handful of people in particular. First off, I would like to thank my advisors, Kelin Whipple and Arjun Heimsath. I look up to both of you, and I am grateful for the ideal balance of support and independence given to me during my time at ASU. Field work for this dissertation involved 3 more-or-less full summers in the San Gabriel Mountains, a land of steep hillslopes, yucca, poison oak, and few trails. I owe sincerest thanks (and perhaps apologies in some cases!) to all those who have helped me collect samples, survey channels, bushwhack, and explore this rugged, yet strangely metropolitan mountain range: Byron Adams, Erin DiMaggio, Nicole Gasparini, Benny Guralnik, Matt Jungers, Jeni McDermott, Matt Rossi, Melinda Shimizu, Drew Stolar, and Joe Walsh. I have been fortunate to have consistent funding for this project throughout my stay at ASU. The Geomorphology and Land Use Dynamics program of the National Science Foundation (NSF) provided the bulk of funding for this work via two awards (EAR-0724194 to Kelin Whipple and EAR-0518998 to Arjun Heimsath). The NSF- sponsored National Center for Airborne Laser Mapping (NCALM) provided me with 40 km2 of LiDAR data as part of a seed grant awarded in 2007. The remainder of the LiDAR coverage (for a total of nearly 500 km2) was also flown and processed by NCALM as part of a joint effort with Caltech following the devastating Station Fire of 2009. I would also like to thank the ARCS Foundation for their support during my final year here. iii I would like to thank my parents Tony and Deb DiBiase for impressing upon me the value (and providing me the opportunity) of choosing a career based on my passions and not the expectations of others. I thank my sister Olivia, for never letting me fake it and calling me out on every slip. And finally, I would like to especially thank Erin DiMaggio. Not only did you teach me the fundamentals of making pretty figures, but your unfailing support and understanding has made this process infinitely more enjoyable. iv TABLE OF CONTENTS Page LIST OF TABLES ................................................................................................................... x LIST OF FIGURES ................................................................................................................. xi PREFACE ............................................................................................................................. xiii CHAPTER 1 INTRODUCTION ............................................................................................. 1 MOTIVATION ............................................................................................. 1 OUTLINE OF CHAPTERS 2-6 ................................................................... 3 REFERENCES .............................................................................................. 6 2 LANDSCAPE FORM AND MILLENNIAL EROSION RATES IN THE SAN GABRIEL MOUNTAINS, CA ........................................................ 9 ABSTRACT .................................................................................................. 9 INTRODUCTION ...................................................................................... 10 STUDY AREA ........................................................................................... 12 METHODS ................................................................................................. 14 Cosmogenic Erosion Rates ............................................................... 14 Catchment-Mean Hillslope Angle .................................................... 16 Catchment-Mean Channel Steepness Index ..................................... 18 Catchment-Mean Local Relief ......................................................... 20 RESULTS .................................................................................................... 21 Interrelations Among Topographic Metrics ..................................... 21 Spatial Distribution of Topographic Metrics ................................... 22 Erosion Rates and Topography ........................................................ 23 ANALYSIS ................................................................................................. 25 v CHAPTER Page 2 Catchment Mean Slope and Erosion Rate ........................................ 25 Catchment Mean Channel Steepness Index and Erosion Rate ........ 28 DISCUSSION ............................................................................................. 30 Implications for Channel Incision Theory ....................................... 30 Implications for Hillslope Transport Theory ................................... 31 CONCLUSIONS ......................................................................................... 33 ACKNOWLEDGEMENTS ....................................................................... 34 REFERENCES ............................................................................................ 35 FIGURE CAPTIONS ................................................................................. 43 3 THE INFLUENCE OF EROSION THRESHOLDS AND RUNOFF VARIABILITY ON THE RELATIONSHIPS AMONG TOPOGRAPHY, CLIMATE, AND EROSION RATE ......................... 52 ABSTRACT ................................................................................................ 52 INTRODUCTION ...................................................................................... 54 STUDY AREA: SETTING, EROSION RATES, AND ANALYSIS STRATEGY ............................................................................................ 56 ROCK EXPOSURE .................................................................................... 58 SLOPE DISTRIBUTIONS ......................................................................... 60 DRAINAGE DENSITY ............................................................................. 62 DISCUSSION ............................................................................................. 65 CONCLUSION ........................................................................................... 68 ACKNOWLEDGEMENTS ....................................................................... 69 REFERENCES ............................................................................................ 70 FIGURE CAPTIONS ................................................................................. 75 vi CHAPTER Page 4 HILLSLOPE RESPONSE TO TECTONIC FORCING IN THRESHOLD LANDSCAPES ........................................................................................ 88 ABSTRACT ...............................................................................................