Structural analysis of the San Simeon fault zone, California : implications for transform tectonics

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2008-12

Authors

Coppersmith, Ryan Thomas

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Abstract

The San Gregorio-Hosgri fault zone (SGH), located in the Southern Coast Ranges of California is a 420 kilometer long right-lateral strand of the San Andreas fault system. The San Simeon fault zone is a segment of the SGH that cross-cuts the Nacimiento block which is primarily composed of Franciscan Complex accretionary prism. The Nacimiento block is juxtaposed against the Salinian block, a portion of the Sierra Nevada batholith, by the Nacimiento Fault. The Nacimiento and Salinian blocks have been displaced from the south in a right lateral sense as part of movements within the San Andreas fault system. The San Simeon segment juxtaposes mid-Jurassic Coast Range Ophiolite with Cretaceous Franciscan accretionary prism material. These units are locally overlain by the Oligocene Lospe Formation and Miocene Monterey Formation. To better understand the movement history near the San Simeon fault zone, 33 kilometers of outcrop were examined along the sea-cliff between Ragged Point in the north and Pico Creek to the south. Of this transect, 4 kilometers were buried under marine terrace and sand dunes. No data was collected along 1 kilometer of transect due to the presence of elephant seals. The 28 kilometers of bedrock examined include: 7 kilometers of ophiolitic material, 16 kilometers of Franciscan Complex, 2 kilometers of Lospe Formation, and 3 kilometers of Monterey Formation. In all, 466 minor faults and 254 major (≥0.5 meters exposure length) faults were mapped, and 22 of these major faults juxtapose different formations (n=8) or different units within the ophiolite (n=14). Slickenlines were measured on 517 faults, of which 237 record sense of slip. Of the faults measured, 199 are strike-slip (0-30° rake), 179 are dip-slip (60-90° rake), and 139 are oblique-slip (31-59° rake). Sense of slip indicators record a wide range of movements: 49 right-lateral, 47 left-lateral, 40 normal, 38 reverse, 18 reverse left-lateral, 17 normal left-lateral, 15 normal right-lateral and 13 reverse right-lateral faults. The study transect was divided into structural domains based on fault kinematic patterns. Movement recorded in these data resulted from transform-related faulting. Fault kinematics that differ from the regional N35W strike of the San Simeon fault zone are explained by local variations in movement patterns near the San Simeon fault zone. This variations include local bends and splays off of the fault zone. The Lospe and Monterey Formations that make up 18% of the mapped transect contain 12% of the faults. These formations only experienced transform-related deformation. Faults in the Monterey Formation are parallel to the regional San Simeon fault zone. Faults in the Lospe Formation to the north primarily strike E-W. Ophiolite material contains 25% of the mapped transect and 37% of the faults. These faults primarily indicate right-lateral movement; however, reverse and normal faulting near perpendicular to the regional NW fault trend is common. The Franciscan Complex along 57% of the mapped transect contains 51% of the faults. Faults in the Franciscan Complex and the ophiolite potentially record subduction-related faulting, but evidence from fault kinematics from this study indicates transform-related faulting. Reverse and right-lateral faulting along the splays is indicated. East of San Simeon Point, a 1 kilometer wide San Simeon fault zone is indicated by a cluster of faults between the San Simeon Pier and Broken Bridge Creek, the eastern boundary of the fault zone. The complexity of fault patterns and kinematics in and near the San Simeon fault zone record a long and complex history of transform faulting.

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