Slip Rates and Kinematics of Active Crustal Faults in the Central Oregon Cascades
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Western Washington University Western CEDAR WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship Spring 2020 Slip rates and kinematics of active crustal faults in the central Oregon Cascades Katherine Alexander Western Washington University, [email protected] Follow this and additional works at: https://cedar.wwu.edu/wwuet Part of the Geology Commons Recommended Citation Alexander, Katherine, "Slip rates and kinematics of active crustal faults in the central Oregon Cascades" (2020). WWU Graduate School Collection. 947. https://cedar.wwu.edu/wwuet/947 This Masters Thesis is brought to you for free and open access by the WWU Graduate and Undergraduate Scholarship at Western CEDAR. It has been accepted for inclusion in WWU Graduate School Collection by an authorized administrator of Western CEDAR. For more information, please contact [email protected]. Slip rates and kinematics of active crustal faults in the central Oregon Cascades By Katherine A. Alexander Accepted in Partial Completion of the Requirements for the Degree Master of Science ADVISORY COMMITTEE Dr. Colin B. Amos, Chair Dr. Douglas H. Clark Dr. Andrew J. Meigs GRADUATE SCHOOL David L. Patrick, Interim Dean Master’s Thesis In presenting this thesis in partial fulfillment of the requirements for a master’s degree at Western Washington University, I grant to Western Washington University the non-exclusive royalty-free right to archive, reproduce, distribute, and display the thesis in any and all forms, including electronic format, via any digital library mechanisms maintained by WWU. I represent and warrant this is my original work, and does not infringe or violate any rights of others. I warrant that I have obtained written permissions from the owner of any third party copyrighted material included in these files. I acknowledge that I retain ownership rights to the copyright of this work, including but not limited to the right to use all or part of this work in future works, such as articles or books. Library users are granted permission for individual, research and non-commercial reproduction of this work for educational purposes only. Any further digital posting of this document requires specific permission from the author. Any copying or publication of this thesis for commercial purposes, or for financial gain, is not allowed without my written permission. Katherine Alexander 4/10/2020 Slip rates and kinematics of active crustal faults in the central Oregon Cascades A Thesis Presented to The Faculty of Western Washington University In Partial Fulfillment Of the Requirements for the Degree Master of Science by Katherine A. Alexander April 2020 Abstract New cosmogenic 3He chronologies and surficial geologic mapping constrain the age of glacial deposits and slip rates of predominantly normal faults in the White Branch and Dilman Meadows fault zones in central Oregon, USA, over the last ca. ~80 kyr. Our mapping of glacial landforms and deposits distinguishes three primary episodes of glacial deposition in the White Branch fault zone. Twenty-two new cosmogenic 3He surface exposure dates indicate that the youngest glacial unit represents a last glacial maximum deposit (ca. 19.4 +10.1/-6.2 kyr). Mapping of outwash terrace surfaces and deposits in the Dilman Meadows fault zone, constrained by two new cosmogenic 3He depth profiles, identifies an older outwash deposit (75.0 +11.3/-9.2 kyr) associated with MIS 5b (~75 ka), as well as a younger last glacial maximum outwash surface (20.8 +0.04/-0.03 kyr). For the first time, glacial chronologies are linked across the Cascades, where west of the crest, where erosion and precipitation rates are high and vegetation cover is dense, and east of the crest, where precipitation and erosion rates are significantly lower and tree cover is sparse. We use offset measurements from fault scarps that show primarily dip-slip motion in combination with the depth profile ages to determine a summed slip rate across the Dilman Meadows fault zone of 0.1 – 0.4 mm/yr since ~75 ka. Surface faulting evident in the White Branch fault zone represents deformation strictly since the last glacial maximum (< 20.3 +6.2/-5.3 ka). Combining offset values from scarp profiles that show purely dip-slip displacement with the cosmogenic 3He surface ages yields a summed dip slip rate across the White Branch fault zone of 0.6 ± 0.5 mm/yr since the last glacial maximum. The White Branch and the Dilman Meadows fault zone both accommodate mostly extension in the Cascade arc and could be mechanisms of Siletzia rotation or volcanic upwelling. iv Acknowledgements This research was supported by funding from the Western Washington University Graduate School, and the Western Washington University Geology department. All lidar DEMs used for mapping and topographic analysis were obtained from the Oregon Department of Geology and Mineral Industries. To all those that helped me with this work, Willy Amidon and Reyne Lesnau at Middlebury College, Greg Balco at the Berkeley Geo Chronology Center, I learned so much from all of you. I want to thank everyone in the geology department for their support during my time at western, –Ben Paulsen for everything you do for the students and our projects, Kate Blizzard for answering my endless questions, and Paul for always checking in. To my cohort, we lucked out. Lexi, Liza, and Paige, I will always be so happy that we got to go through this together ! Cody, thank God you were here and willing to dive into every Thursday afternoon caffeine driven rabbit hole that would lead to nowhere. I miss your fried chicken. To my new fiends in Bellingham, boating, biking, foraging, and exploring would not have been the same or even possible without you. Thanks to people of the Bike House for welcoming me in with open arms, Avery and Nyle you are the first real roommates I have ever had and I couldn’t have ask for better fiends out of all of this. Nolan thanks for taking me on all the adventures, making me dinner, and sticking with me through all of this. My family, Mom, Dad, Mimi, Jos, Rick, Gary, Johnathon, Chris, Nuts, Lil P, and Chugs (in no particular order). Mom and Dad I can’t believe you actually bush-wacked through central Oregon looking for boulders with me, thanks for being a constant source of amusement. Mimi and Rick, thanks for always answering when I call, love you. Jos and Gary, thank you for teaching me about colors and telling me to stay in school, love you. Finally, I want to thank my committee for sticking with me and making me a better scientist. Doug without your advice my maps would not be as they are today. Andrew, I don’t even know what to say. You helped get me here and I am so grateful for all the opportunities and advice that have helped me get to where I am today. Colin, I have loved this project from start to finish and you have taught me how to work independently while also always being there. I am extremely happy that we were able to work on this project and all our other side projects together. #MyAdvisorsAreTheBestAndIAmGoingToMissThis v Table of Contents Abstract ........................................................................................................................................... iv Acknowledgements ......................................................................................................................... v List of Tables and Figures ............................................................................................................. vii Introduction ..................................................................................................................................... 1 Background ...................................................................................................................................... 3 Surficial Geologic Mapping ............................................................................................................ 8 Scarp Profiles ................................................................................................................................. 19 3He Cosmogenic Surface Exposure Dating ................................................................................... 24 Discussion ...................................................................................................................................... 36 Regional Implications .................................................................................................................... 46 Conclusions ................................................................................................................................... 50 Works Cited ................................................................................................................................... 52 vi List of Tables and Figures Figure 1: Tectonic overview of western North America ................................................................. 1 Figure 2: Overview of the Central Oregon High Cascades ............................................................. 3 Figure 3: Surficial geologic map of the White Branch fault zone ................................................. 10 Figure 4: Scarp Profiles from the White Branch fault zone .......................................................... 13 Figure 5: Quick Terrain Modeler images of the White Branch fault zone .................................... 14 Figure 6: Surficial geologic map of the Dilman Meadows fault zone ........................................... 16 Figure 7: Dilman