Deformation, Sandstone Detrital Zircon Ages, and Provenance in and near the Eocene Leavenworth Fault Zone, Washington

Abstract

The North Cascades is an excellent area to study basins and strike-slip faults that formed during regional transtension. This structural, petrographic, and U-Pb geochronological study focuses on the Eocene Leavenworth fault (LVF), which separates Eocene non-marine clastic rocks of the Chumstick Formation and older Swauk Formation. The LVF has been interpreted as a dextral strike-slip fault active during Chumstick deposition and as a reverse fault. Poles to beds (n=207) in the Swauk Formation trend WNW, oblique to the ~320˚-striking LVF, and commonly dip SSW, defining a girdle and a fold axis plunging 4˚ toward 297˚. Beds (n=50) in the Chumstick Formation trend NW and generally dip NE, and poles to beds define a fold axis plunging 13˚ towards 306˚. Overall, 70% of folds in the formations trend NW to WNW, <30˚ counterclockwise to the main strand of the LVF, compatible with dextral transpression. Most faults in the Swauk and Chumstick formations strike >45˚ or <30˚ to the main strand of the LVF. Their movement sense is unknown. Source terranes for the Swauk Formation likely include the Cascades crystalline core to the N and NE and a combination of other relatively local sources to the N, NE, and SW. Detrital zircon maximum depositional ages in the Swauk Formation range from 67.2 ± 1.9 to 50.46 ± 0.17 Ma, constraining the timing of deposition and early folding of the Swauk Formation. Distinctive ~91 Ma tonalitic clasts in Chumstick conglomerate were likely transported ~20 km from their source by dextral slip of the Leavenworth fault zone. v ACKNOWLEDGMENTS I would like to take this opportunity to thank the people that made this thesis possible. First and foremost I want to thank my family, who have always encouraged and supported me. Especially my parents, who have devoted their lives to giving me the best possible future, and for this, I would like to dedicate this thesis to them. To Robert Duffala, your motivation and constant optimism has helped me greatly through this process. Thank you for being my support during times when everything seemed impossible. I would like to extend special gratitude and admiration to my thesis advisor, Bob Miller, for giving me the opportunity to work in the North Cascades, for his incredible mentorship and guidance through my graduate studies, and for his insight in the field and in the office. I value greatly all the time and effort you put into making this thesis possible. I thank my thesis committee members, Dave Andersen and Jonathan Miller, your doors were always open and I appreciate your insightful revisions and knowledge. I especially give thanks to my field assistant Natasha Budimirovic for her encouragement and companionship which kept me sane through my field work. I would like to express gratitude to my fellow geology graduate students and friends, especially Kelly Dustin and Katie Bryant. I knew I could go to you with any questions and you were always reliable. Thank you for listening to me practice my thesis defense and giving me creative feedback. Most importantly, I a...