Evidence for Northward Increasing Slip along the San Gregorio-San Simeon-Hosgri Fault

Langenheim, V. E.; Jachens, R. C.; Graymer, R. W.; Colgan, J. P.; Wentworth, C. M.; and Stanley, R. G.
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Estimates of the dip, depth extent and amount of cumulative displacement along the major faults in the central California Coast Ranges are controversial. We use detailed aeromagnetic data to estimate these parameters for the San Gregorio-San Simeon-Hosgri and other faults. The recently acquired aeromagnetic data provide an areally consistent dataset that crosses the onshore-offshore transition without disruption, which is particularly important for the mostly offshore San Gregorio-San Simeon-Hosgri fault. Our modeling, constrained by exposed geology and in some cases, drillhole and seismic-reflection data, indicate that the San Gregorio-San Simeon-Hosgri and Reliz-Rinconada faults dip steeply throughout the seismogenic crust. Deviations from steep dips may result from local fault interactions, transfer of slip between faults, or overprinting by transpression since the late Miocene. Given that such faults are consistent with predominantly strike-slip displacement, we correlate geophysical anomalies offset by these faults to estimate cumulative displacements. We find a northward increase in right-lateral displacement along the San Gregorio-San Simeon-Hosgri fault that is mimicked by Quaternary slip rates. Although overall slip rates have decreased over the lifetime of the fault, the pattern of slip has not changed. Northward increase in right-lateral displacement is balanced in part by slip added by faults, such as the Reliz-Rinconada, Oceanic-West Huasna, and (speculatively) Santa Ynez River faults to the east, although how slip is accommodated through Santa Maria basin is still not understood.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013