HR: 1340h
AN: T43A-1098    [Abstracts]
TI: Evolution of an Intermontane Basin Along the Northern San Andreas System: Evidence from Basin Structure of Little Lake Valley (Willits), Northern California Inferred from Gravity and Geologic Data
AU: * Erickson, G
EM: gje2@humboldt.edu
AF: Geology Department, Humboldt State University, Arcata, CA 95503, United States
AU: Kelsey, H
EM: hmk1@humboldt.edu
AF: Geology Department, Humboldt State University, Arcata, CA 95503, United States
AU: Langenheim, V
EM: zulanger@usgs.gov
AF: Geophysical Unit of Menlo Park, U. S. Geological Survey, Menlo Park, CA 94025, United States
AU: Furlong, K
EM: kevin@geodyn.psu.edu
AF: Geodynamics Research Group, Penn State Univeristy, University Park, PA 16801, United States
AB: 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. The isostatic gravity anomaly of the basin is negative (up to 13 mGals) and rhombic in shape. Modeling indicates two splays, less than a km apart, of an up-to-the-east East Valley fault; the basinward fault is buried by fill and the more easterly fault defines the eastern margin of the basin. Cumulative up-to-the-east vertical fault displacement along the East Valley fault increases southward up to 610 m in the southern portion of the valley. Gravity gradients also suggest approximately east-west trending faults bound the northern and southern sides of the valley and offset Quaternary fill. From gravity and geologic data combined, the basin floor dips approximately 7 degrees to the south in the north part of the valley and both the Quaternary sediment and basin floor dip approximately 13 degrees to the north in the south part of the valley, implying an approximately east-west axis of dip reversal of the basin floor at the northern stretch of East Hill Road (latitude 39.39 degrees N). Faults and basin fill structure are not consistent with any one simple structural model of basin evolution. Although gross fault geometry is consistent with a pull-apart basin origin, a simple pull-apart basin model does not account for the timing of basin initiation, dip of Quaternary basin deposits, and relation of basin bounding faults to the main strand of the Maacama fault.
DE: 8100 TECTONOPHYSICS
DE: 8106 Continental margins: transform
SC: Tectonophysics [T]
MN: 2007 Fall Meeting