--> --> Abstract: Miocene Lavas Constrain Right Slip Movement on the West Huasna Fault in San Luis Obispo County to Less Than 8 Km, by H. McLean; #90992 (1993).

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McLEAN, HUGH, U.S. Geological Survey, Menlo Park, CA

ABSTRACT: Miocene Lavas Constrain Right Slip Movement on the West Huasna Fault in San Luis Obispo County to Less Than 8 Km

The West Huasna fault (WHF) juxtaposes Neogene strata on the southwest limb of the Huasna syncline with melange and associated serpentinite of the Franciscan assemblage exposed along the southwest side of the fault. New detailed 1:24,000-scale geologic mapping along a 30-km segment of the WHF reveals a history of fault-controlled structural and stratigraphic relations that began in the earliest Miocene. The oldest stratigraphically coherent rocks along the WHF are Upper

Jurassic and Lower Cretaceous deposits that comprise the lower part of the Great Valley sequence (GVS). Upper Cretaceous strata gradationally overlie the lower part of the GVS. The Mesozoic rocks are unconformably overlain by a variety of lower Miocene lithofacies including shallow-marine sandstone of the Vaqueros Formation, which locally contains the robust spiral-shaped gastropod Turritella inezana. The distribution of the shallow-marine sandstone appears to parallel the WHF, and the position of the early Miocene shoreline appears to have been controlled by emergence of Franciscan basement. The Vaqueros and correlative rocks grade upward into the Obispo Formation, a sequence of dark-gray marine mudstone that contains interfingering white rhyolitic tuff, basaltic sills and dikes, and lenses of massive, columnar-jointed andesite. The andesitic rocks form an areally restricted swarm, which exists on both sides of the WHF. The medium to dark-gray andesite forms locally columnar-jointed masses and is characterized by a distinctive flinty, cryptocrystalline groundmass with sparse plagioclase phenocrysts. The outcrop distribution of the erosion-resistant andesitic lava appears to limit strike-slip separation on the WHF to a distance of 5-8 km in a right-lateral direction.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.