QUATERNARY SLIP HISTORY OF THE SANTA SUSANA FAULT, WESTERN TRANSVERSE RANGES, CALIFORNIA

The Santa Susana fault (SSF) is the western extension of the Sierra Madre fault system, an east-west trending reverse fault system in the western Transverse Ranges. Slip on the reactivated SSF is associated with growth of the Santa Susana Mountains, south of the Ventura basin. The SSF has a high potential slip rate, with wide uncertainty in the current earthquake hazard model (0.5-10 mm/yr). The faster rates are based on structural reconstructions while the slow rate is based on a subdued geomorphic appearance. Our work better resolves rates over the full reverse slip history of the SSF through the late Quaternary.

We used detrital zircon U-Pb geochronology to refine the provenance history of sediment deposited on the SSF footwall, building on earlier work recognizing growth of the Santa Susana Mountains using clast lithologies in the Saugus Formation. Across the transition from the lower transitional marine to the middle non-marine member of the Saugus Formation there is a disappearance of a 1.2 Ga anorthosite complex signature from the north and a shift to similarity with the upper Neogene units in the SSF hanging wall. This indicates that the SSF hanging wall was a local source of sediment by ~1.4-1.1 Ma. Upper Saugus sediment deposited post-0.6-0.5 Ma is similar to older Neogene strata, providing a second timed increment of deformation. When combined with balanced cross-sections, our revised chronology yields a ~6 mm/yr average slip rate since the activation of the SSF, similar to prior estimates, with a lower upper range. There is a suggestion of slowing rate, although not required considering uncertainties in the data.

Along the western SSF, we have identified a late Quaternary alluvial fan surface that is separated vertically by ~25 m across the fault zone. Based on soil development and other characteristics, we regard its minimum credible age to be latest Pleistocene, which would yield a maximum latest Quaternary slip rate of ~3-5 mm/yr. If this fan surface instead correlates to other prominent aggradational surfaces dated along strike ~35 ka or older, the recent slip rate is much lower. Samples being analyzed will provide direct age estimates for this surface, but our preliminary work suggests the rate has slowed considerably in the late Quaternary, with deformation being transferred to other structures.