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Latest Pleistocene to Holocene River Terrace Deformation

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Latest Pleistocene to Holocene River Terrace Deformation LATEST PLEISTOCENE TO HOLOCENE RIVER TERRACE DEFORMATION WITHIN THE SOUTHERNMOST EXTENT OF THE LITTLE SALMON FAULT ZONE; GEOMORPHIC INSIGHTS TO FAULT TERMINATION AND RUPTURE HISTORY, VAN DUZEN RIVER, NORTHERN CALIFORNIA By Sylvia R. Nicovich A Thesis Presented to The Faculty of Humboldt State University In Partial Fulfillment of the Requirements for the Degree Master of Science in Environmental Systems: Geology Committee Membership Dr. Mark Hemphill-Haley, Committee Chair Dr. Andre Lehre, Committee Member Thomas Leroy, M.Sc., Committee Member Dr. Melanie Michalak, Committee Member Dr. Christopher Dugaw, Graduate Coordinator July 2015 ABSTRACT LATEST PLEISTOCENE TO HOLOCENE RIVER TERRACE DEFORMATION WITHIN THE SOUTHERNMOST EXTENT OF THE LITTLE SALMON FAULT ZONE; GEOMORPHIC INSIGHTS TO FAULT TERMINATION AND RUPTURE HISTORY, VAN DUZEN RIVER, NORTHERN CALIFORNIA Sylvia R. Nicovich The southern Cascadia subduction zone (CSZ) of northwestern California exhibits northeast-directed contraction, transitioning to north-northwest directed translation within the broad San Andreas fault (SAF) transform margin to the south. The Little Salmon fault (LSF) is one of the southern-most, active thrust faults within the onshore fold and thrust belt of the CSZ, and lies proximal to the transition from compressional to dextral stress across the Mendocino triple junction (MTJ). Thus, it is an ideal location to characterize strain associated with this complex region of transitional stress regimes. High precision topographic data (LiDAR) enabled detailed mapping of geomorphic features otherwise obscured by dense vegetation of the area. The Van Duzen fault (VDF), a northwest trending mole track scarp, sub-parallel and south of the main splay of the LSF is observed on LiDAR imagery. This fault exhibits up-to-the-northeast offset and traverses several Van Duzen River terrace risers and treads that range from Pleistocene to potentially Holocene in age. A shallow, exploratory trench was hand-excavated across the VDF. The shallow, roughly 1.5 m-deep, 16 m-long trench exposed imbricated gravels that dip into the base of the trench in the upper end. Coring within the lower end of the trench mapped ii the southern extent of the unconsolidated, clast-supported gravel deposit revealing vertical separation of 2.5 m, displaying an up-to-the-northeast step. The linear map expression of the VDF across river terraces of varying elevation and age suggests that the fault may be relatively steeply dipping. Exposed offset bedrock display reverse offset along the VDF, but with no stratigraphic constraint to measure offset. River terraces are some of the youngest geomorphic features within the study area. By constraining a sequence of relative ages for terraces associated with the Van Duzen River, in combination with regional uplift assumed to be equivalent to incision rates, rough terrace ages have been determined. Slip rates calculated from estimated net slip along the VDF and inferred terraces ages range from ~ 0.05 to 0.5 mm/yr. The west-northwest orientation of the compressional faults and folds within the study area suggest SAF- parallel compression, possibly near the transition from transform to compressional tectonic regimes. iii ACKNOWLEDGEMENTS First and foremost, I would like to thank all of the faculty and staff at Humboldt State University for making my experience at this fine university transformative, positive, eye opening, and insanely fun. Huge thanks to Dr. Mark Hemphill-Haley for being by geologic sensei through thick and thin, trusting my knowledge, treating me with respect and as a colleague. Infinite thanks to Tom Leroy for formulating the objective of the project and supporting me enthusiastically along the way. Thank you to Humboldt Redwood Company, specifically Shane Beach, Mike Miles and Jason Butcher, for allowing me the use of their LiDAR data and granting me land access to HRC land. Thanks to Dr. Melanie Michalak for consistently mentoring me through my research endeavors, my geological thought process and lending treasured life advise. Thanks to Steve Tillinghast for training me to be the best field geologist I could possibly be in times of steep talus piles and lost bottle openers. Thanks to Laurie Marx for always ridding me of any troubles. Thank you to Dr. Andre Lehre for participating as a member of my graduate committee. Thanks to Dr. Bud Burke for always being available for discussion, impromptu geomorphology lessons, and inspiring me to look at coffee under a new light. Thanks to Dr. Brandon Schwab for kick-starting my experience with research as a senior undergraduate, for beating the bucket drums in our field band, and for gifting me the best piano of all time, enabling me to learn to play ragtime. Thank you to the digging crew; Casey “The Mule” Loofbourrow, Michelle “Dusty Damsel” Robinson, Jason “Montana Sharp Shooter” Padget, Heath “Soils” Sawyer, Maddy “Moo” Schriver, Steven “ Silent iv Blisters” Medina, Vanessa “Momma” Davis and Toby “The Fish” Haskett, for lending your weekend, muscles, and sacrificing your cleanliness to my thesis effort. Thank you to SHN Consulting, specifically Ansen Call, Paul Sundberg, and Jason Buck for alerting me when discovering a bedrock exposure of the VDF and helping around the trench. Thanks to the bore-hole and survey team; Kelsey Conger, Ian Pierce and Lianna Winkler-Prins. Thanks to the Spring 2014 Neotectonics class for imaging terraces, profiling fault scarps, and enjoying the Van Duzen River with me. Thank you to Jim Falls of the California Geological Survey for long discussions of the LSF zone and use of the unpublished Owl Creek Quadrangle. Super thanks to my lovely roommates, Hannah, Jesse, Maia, Maddy, Lukas and Sloane, that have put up with me through the late nights of work, dirty front door threshold, and ambitious laundry loads aiming to eradicate poison oak. Thanks to my oldest brother, Adam, for always answering the phone with the inquiry, “Are you writing your thesis?” Thanks to all my friends and family, and especially wonderful cat, Beaverton. Finally, thanks to my stallion, the Black Mamba, as she sacrificed her life to carry me many long miles to the field and back. v TABLE OF CONTENTS ABSTRACT ........................................................................................................................ ii ACKNOWLEDGEMENTS ............................................................................................... iv TABLE OF CONTENTS ................................................................................................... vi LIST OF TABLES ............................................................................................................. ix LIST OF FIGURES ............................................................................................................ x INTRODUCTION .............................................................................................................. 1 GEOLOGIC SETTING ...................................................................................................... 4 Regional Tectonics ......................................................................................................... 4 Little Salmon Fault ..................................................................................................... 4 San Andreas Fault ....................................................................................................... 6 Stratigraphic, Geomorphic, and Structural Setting ......................................................... 9 Structure and Geomorphology .................................................................................. 11 METHODS ....................................................................................................................... 13 Geospatial Analysis ...................................................................................................... 13 Faults ......................................................................................................................... 13 Bedrock mapping ...................................................................................................... 13 Terrace Mapping ....................................................................................................... 16 Incision and Uplift Rates .......................................................................................... 21 Field Analysis ............................................................................................................... 21 Van Duzen fault ........................................................................................................ 21 Bedrock Mapping ...................................................................................................... 25 vi RESULTS ......................................................................................................................... 26 Faults within the Study Area ........................................................................................ 26 Little Salmon fault .................................................................................................... 26 Van Duzen fault ........................................................................................................ 26 Trench exploration .................................................................................................... 35 Net slip along Van Duzen fault ................................................................................. 37 Terraces
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