Bedrock channel response to tetonic, climatic and eustatic forcings

The response of bedrock channels to external forcings is investigated in this thesis. The approach is to test and constrain a theoretical model for bedrock-channel incision based on shear stress using field data. The primary study area is a series of 21 small, coastal drainage basins in northern California, USA with known, varying rates and history of rock uplift. The initial application of a simple form of the model to the stream profiles suggests that (1) the channels are eroding at rates approximately equal to uplift rates (i.e. steady-state fluvial incision), and (2) erosion processes are proportionally more effective in the high-uplift-rate zone, with factors in addition to channel gradient responding to tectonic forcing. These results lead to the rest of the study, in which some of the assumptions of the simple model are rejected in order to explain the second observation. A more sophisticated model that includes both a stochastic distribution of floods and a threshold shear stress to initiate bedrock erosion (s) predicts that a greater part of the distribution of flood events will exceed the threshold in steeper channels. Therefore, highergradient channels have proportionally higher erosion rates, as is observed in the high-uplift-rate streams of the California field site. The shear-stress model is tested and constrained through a detailed, field-based analysis of topography, lithology, stream morphology and regional hydrology to isolate those factors that respond to tectonics. The stochastic model is able to incorporate the observed variation in stream discharge due to orographic enhancement of precipitation by high topography associated with high uplift rates. This increase in discharge appears to play a second-order role in setting the erosional effectiveness of the high-uplift zone. Other factors, including channel width, lithologic resistance and sediment flux, do not appear to vary in an important way with uplift rate, although this conclusion is based on analyses that have some limitations. The importance of thresholds is underscored by a direct calculation of critical shear stress during a rare bedrock-incision event in a low-erosion-rate creek in New York state (r~100-200 Pa). This event, the only one that caused significant bedrock plucking at the site in an ~40-year period, is consistent with a low erosion rate, with few events that exceed the threshold. In contrast, similar ze values are exceeded during high-frequency flood events in the steep, rapidly eroding California streams. Inclusion of an erosion threshold accounts for the observed relationship between channel gradient and rockuplift rate in the California site. In summary, by using field examples, the shear-stress bedrockincision model with a stochastic distribution of flood events and an erosion threshold is demonstrated to be an effective and powerful tool for exploring relationships amongst climatic, tectonic and surficial processes. In the final section of this thesis, a numerical modeling study couples the shear-stress model for onshore fluvial incision with a simple rule for offshore wave-based erosion of bedrock to explore the response of uplifting streams to eustatic fluctuations. The results highlight the importance of offshore boundary conditions to the onshore response, particularly the position of the edge of the uplifting block and the development of bathymetry. A comparison of model results with the northern California channels suggests that (1) the steady-state hypothesis is consistent with an offshore decrease in rock-uplift rates, and (2) the ubiquitous low-gradient, alluviated mouths of the study-area streams are predicted by the model during uplift of the offshore platform during the late Holocene sea-level stillstand. Thesis Supervisor: Kelin X Whipple, Associate Professor of Geology Acknowledgments First, I wish to thank Kelin Whipple, my advisor. His accessibility, enthusiasm and faith made my graduate-school experience fun and enriching. I will always harbor a certain amount of pride for being the first person to start and finish on the Whipple program. All along I have felt that I am playing for Team Whipple, although my perceived role has changed through the years: early on I thought of myself as a utility infielder always ready to put down the sacrifice bunt to help out the team, and now I'm more of a grumpy veteran trying to pad my batting statistics! I also very much appreciate the help and guidance provided me by Dorothy Merritts and Greg Tucker, who have been collaborators and advisors to this thesis work. I express my gratitude to the other members of my thesis committee, Kip Hodges, John Southard and Rafael Bras for their guidance and suggestions along the way. I also thank professors John Grotzinger, Clark Burchfiel, Wiki Royden and Eric Leonard. Next, I express my love and thanks to Lydia Bergen who has put up with me for pretty much the whole run at MIT. She deserves as much credit as anyone for making this thesis happen. Her devotion and support have been the greatest source of inspiration and renewal over the past 5 years. We've been through a lot together, and I can't wait to see where we go next! I want to mention the series of teachers who put me on this path. Dan Flerlage at the Alternative Community School in Ithaca instilled my initial interest in field science. Professors Dyk Eusden and Mike Retelle at Bates College drew me into the geologist fold and laid my geological foundation, as did Ken Fink, John Creasy and Marita Bryant. I also thank Lee Allison, Gary Christensen and Mike Lowe at the Utah Geological Survey. Jon Stock and Tom Dunne told me get in touch with Kelin, and vouched for his (slightly dicey) character. At MIT, my fellow graduate students made the whole experience fun. I thank the other members of the "gang of four," Eric Kirby, Arthur White, Jose Hurtado, and all of the other students (as well as postdocs) that I overlapped with, including Simon Brocklehurst, Sinan Akciz, Lindsay Schoenbohm, Mark Behn, Julie Baldwin, Mark Schmitz, Karen Viskupic, Jenny Matzel, Becky Flowers, Kirsten Nicolaysen, Anke Friedrich, Jim VanOrman, Steve Parman, Steve Singletary, Astri Kvassnes, Bill Lyons, Amy Draut, John Thurmond, Steve Dibenedetto, Marin Clark, Matt Dawson, Bridget Berquist, Matt Reuer, Frederik Simons, Eliza Richardson, Peter Dodds, Jeremy Boyce, Chris Studnicki-Gizbert, Blair Schoene, Ben Crosby, Cam Wobus, Jeff Parsons, Mike Kroll, Jahan Ramezani, Stephen Lancaster, Jeff Niemann, Daniel Collins, and Nicole Gasparini. These people taught me most of what I know now, especially about earth-science subjects outside of my primary field. I feel fortunate to have them as colleagues. Finally, I thank my family, Laurie Snyder, John Wood, Philip Snyder, Pat Paine, Ben Snyder, Norri Paine and Jonathan Paine for their love and support from all corners of the world. Lastly, I want acknowledge the key role of my friends/housemates/ultimate teammates over the past five years, including Matt Bogyo, Becky Allen, Evan Powers, Dave Owen, Lydia, Jessica Parsons, Tom Evans, Greg Hoke, Chris Heppner and Biz Fitzsimons, for providing necessary diversions from the production of this document.