STRUCTURAL ANALYSIS OF THE SAN SIMEON FAULT ZONE, CALIFORNIA: IMPLICATIONS FOR TRANSFORM TECTONICS

This study provides a detailed structural analysis of the abundant bedrock faulting along the sea-cliff exposure neighboring the San Simeon fault zone (SSFZ). The San Gregorio-Hosgri fault zone (SGH), located in the Southern Coast Ranges of California, is a 420 km long right-lateral strand of the San Andreas fault system. The SSFZ is a segment of the SGH that crosscuts the Nacimiento block, which is primarily composed of Franciscan Complex accretionary prism. The Nacimiento fault juxtaposes the Nacimiento block with the Salinian block, a portion of the Sierra Nevada batholith. Both blocks have been displaced from the south in a right-lateral sense due to movement within the San Andreas fault system. The SSFZ juxtaposes mid-Jurassic Coast Range ophiolite with Cretaceous Franciscan accretionary prism material. These units are locally overlain by the Oligocene Lospe Formation and the Miocene Monterey Formation.

To better understand the movement history near the SSFZ, 28 km of outcrop were examined along the sea cliff between Ragged Point and Pico Creek. The bedrock exposure included 7 km of ophiolitic material, 16 km of Franciscan Complex, 2 km of Lospe Formation, and 3 km of Monterey Formation. In all, 718 faults were mapped, and 22 of these juxtapose formations or different units within the ophiolite. Slickenlines were identified on 517 faults, of which 237 record sense of slip. Of the faults measured, 199 are strike-slip, 179 are dip-slip, and 139 are oblique-slip. Kinematic 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. Fault kinematics that differ from the regional N35W strike of the SSFZ are explained by local variations in movement patterns that include local bends and splays off of the fault zone The fault splays that juxtapose mélange with ophiolite have multi-kilometer offset. This study finds that the kilometer-scale offset occurred during transform movement rather than during earlier subduction-related movement. Further, the study redefines the SSFZ as a 1 km wide zone indicated by a cluster of faults near the San Simeon Pier that define the eastern boundary.