CONSTRAINING THE UPLIFT HISTORY OF THE SANTA SUSANA MOUNTAINS, WESTERN TRANSVERSE RANGES, SOUTHERN CALIFORNIA THROUGH U-PB DETRITAL ZIRCON GEOCHRONOLOGY

The Santa Susana fault (SSF) is the western extension of the Sierra Madre fault system, an east-west striking reverse fault system in the Transverse Ranges. A poorly characterized geologic slip rate of 5.9 +3.9/-3.8 mm/yr for the SSF was calculated over the Quaternary using balanced cross sections. However, a lower bound of 0.5 mm/yr was assigned by the UCERF-3 hazard forecast based on geomorphic appearance. Slip on the SSF and related faults was associated with growth of the Santa Susana Mountains forming the southern boundary of the Ventura basin. The Saugus Formation in the footwall of the SSF is poorly lithified Pleistocene sediment composed of conglomerates, sandstones, and siltstones. This southernmost portion of the Saugus Formation was cut off from sediment sources to the north by slip on the SSF and uplift of the mountains. Previous studies estimated uplift to have initiated between 2.3-0.7 Ma through paleomagnetic dating and upsection changes in clast provenance. This study utilizes detrital zircon geochronology to build upon previous clast-lithology studies to examine the provenance of the Saugus Formation and refine the uplift timing of the Santa Susana Mountains.

U-Pb detrital zircon geochronology will be used to identify spatial and temporal trends in sediment contribution to the Saugus Formation. Samples were gathered along a ~21 km long transect from the western San Fernando Valley to the north of Simi Valley. Samples from the Miocene units uplifted in the hanging wall of the SSF were gathered to assess the signature of zircons recycled out of the Cenozoic sedimentary units. Collected samples are undergoing zircon separation and U-Pb dating will occur in the fall. It is expected that the detrital zircon spectra will change upsection in the Saugus Formation due to changes in the sediment sources as structures and topography evolved. Models based on hypotheses of structural growth and sedimentation will be developed based on the review of potential zircon source ages and locations. Further examination of the timing of slip on the SSF will allow estimation of a more constrained slip rate of the fault over the early Quaternary, in addition to providing more information on sediment dispersal trends during Cenozoic deformation in the western Transverse Ranges.