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Little Lake Valley Basin Is Situated in the Northern California Coast EVOLUTION OF AN INTERMONTANE BASIN ALONG THE MAACAMA FAULT, LITTLE LAKE VALLEY, NORTHERN CALIFORNIA by Gwendolyn Erickson A Thesis Presented to The Faculty of Humboldt State University In Partial Fulfillment Of the Requirements for the Degree Masters of Science In Environmental Systems: Geology May, 2008 EVOLUTION OF AN INTERMONTANE BASIN ALONG THE MAACAMA FAULT, LITTLE LAKE VALLEY, NORTHERN CALIFORNIA by Gwendolyn Erickson Approved by the Master’s Thesis Committee: Harvey M. Kelsey, Major Professor Date Victoria E. Langenheim, Committee Member Date Kevin P. Furlong, Committee Member Date Lori A. Dengler, Committee Member Date Sharon Brown, Graduate Coordinator Date Chris A. Hopper, Interim Dean for Research and Graduate Studies Date ABSTRACT EVOLUTION OF AN INTERMONTANE BASIN ALONG THE MAACAMA FAULT, LITTLE LAKE VALLEY, NORTHERN CALIFORNIA Gwendolyn Erickson Associated with the northern strands of the San Andreas fault system in California is a series of small intermontane basins. While it is tempting to ascribe their formation to simple ‘pull-apart’ tectonics along the dominantly strike-slip fault strands, direct evidence for basin genesis is lacking. In this study, a detailed gravity survey throughout the Little Lake Valley region (Willits, California) provides constraints on mechanisms of basin formation along this young segment of the San Andreas fault system. Interpretation of isostatic gravity anomaly data provides insight into fault geometry, basin structure, and thickness of Quaternary fill in Little Lake Valley, California. Although the active strike- slip Maacama fault zone diagonally trends through the southwest part of the valley, gravity and geologic interpretations indicate the valley conceals an earlier basin and faulting history. Gravity models, mapped geology, and double-difference relocated seismicity data indicate that Little Lake Valley basin geometry records a tectonic history which includes subduction, extension, and presently, dextral strike-slip motion along the Maacama fault. The isostatic gravity anomaly of Little Lake Valley basin is negative (up to 13 mGals) and rhombic in shape. Modeling indicates two splays, less than a km apart, iii of a reverse up-to-the-east East Valley fault. Cumulative vertical fault displacement along the East Valley fault increases in the southern portion of the valley. Gravity modeling and geology indicate two normal, up-to-the-northwest faults; one in the northwest portion of Little Lake Valley and the other to the south in Redwood Valley. Normal faults are presumed to be temporally related to Clear Lake volcanics (2 Ma to 10ka), located 45 km southwest of Little Lake Valley, and migration of the Mendocino triple junction to the north. While focal mechanisms of double-difference relocated seismicity indicate reverse and normal faults no longer accommodate significant vertical motion, portions of these structures are accommodating strike-slip motion. Consequently, the Maacama fault is occupying an upward-branching flower structure which takes advantage of pre-existing fault structures and may sole into a regional detachment at approximately 10 km depth. Although Little Lake Valley was not initiated as a strike-slip basin, development of the Maacama fault may evolve Little Lake Valley into a strike-slip basin. iv ACKNOWLEDGEMENTS I must first express my gratitude towards my major advisor Harvey Kelsey. His enthusiasm, support, and care floated me through this process. Vicki Langenheim, Kevin Furlong, and Gavin Hayes were always there when I needed an explanation, a computer program, a GMT script, or a glass of wine. They made this thesis possible. I would like to thank Lori Dengler for her guidance, and the opportunities she has given me through the years. Thank you Penn State and the U. S. Geological Survey for use of the gravimeters, and David Ponce of U. S. Geological Survey for processing the data. Janet Key, Gary Hensley, Paul Sunderberg, Vicki, and my Man, Todd Williams, generously donated time to help me in the field. Big love to Todd for the GMT lessons. Debt also owed to Cynthia Werner, Julie Bawcom, Bob McLaughlin, Darcy McPhee, Frey Vineyards, John Ford, Golden Vineyards, Mark Edwards, and Mendocino Redwood Company. Completing this thesis was comparable to a marathon. I’m sad it’s over. This is for my Mama. v TABLE OF CONTENTS ABSTRACT.......................................................................................................................iii ACKNOWLEDGEMENTS................................................................................................ v TABLE OF CONTENTS................................................................................................... vi LIST OF TABLES...........................................................................................................viii LIST OF FIGURES ........................................................................................................... ix LIST OF APPENDICES.................................................................................................... xi INTRODUCTION .............................................................................................................. 1 Field Area................................................................................................................ 1 Objectives ............................................................................................................... 2 Regional Tectonics.................................................................................................. 2 Geology of the Little Lake Valley Area ................................................................. 7 RESEARCH APPROACH ............................................................................................... 11 METHODS ....................................................................................................................... 12 Gravity .................................................................................................................. 12 1-D Velocity Models............................................................................................. 16 Rock Density Measurements ................................................................................ 17 Well Log Compilation .......................................................................................... 17 Relocated Seismicity............................................................................................. 18 RESULTS ......................................................................................................................... 19 Densities Derived From 1-D Velocity Models ..................................................... 19 Rock Density Measurements ................................................................................ 20 vi Gravity .................................................................................................................. 20 DISCUSSION................................................................................................................... 25 Little Lake Valley: Thickness of Valley Fill and Slope of Basement Floor......... 25 Structure of the East Valley fault based on Gravity, Seismicity, and Surface Morphology..................................................................................................... 25 Structure of the North Valley fault based on Gravity, Seismicity and Surface Morphology..................................................................................................... 28 Character of the Maacama fault in Little Lake Valley from Gravity and Seismicity........................................................................................................ 29 Redwood Valley and northern Ukiah Valley: Thickness of Valley Fill and Slope of Basement Floor................................................................................. 31 The Laughlin Range, Redwood Valley, and northern Ukiah Valley: Evidence for the Laughlin Range fault........................................................................... 32 Speculations on the Tectonic Evolution of Little Lake Valley............................. 37 CONCLUSIONS............................................................................................................... 39 REFERENCES ................................................................................................................. 42 TABLES ........................................................................................................................... 49 FIGURES.......................................................................................................................... 54 APPENDIX....................................................................................................................... 93 vii LIST OF TABLES Table Page 1 1-D Velocity Model to Density Conversions ......................................50 2 Rock Density Measurements ...............................................................51 3 Well Log Compilation .........................................................................52 4 Approximate Vertical Offsets Along Splays of the East Valley Fault .....................................................................................................53 viii LIST OF FIGURES Figure Page 1 Index map showing geographic locations, faults, major tectonic boundaries, and study area...................................................................55 2 Regional shaded-relief
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