NEOTECTONIC INVESTIGATION OF THE SOUTHERN RODGERS CREEK FAULT, SONOMA COUNTY, CA

The 60-km-long Rodgers Creek fault (RCF), located between San Pablo Bay and Santa Rosa, California, strikes approximately N35°W, and is characterized by a late Holocene right-lateral slip rate of 6.4 to 10.4 mm/yr. Field studies along the southern RCF have resulted in: 1) new insight concerning the structural relations across the RCF and the long-term slip budget on the East Bay fault system; 2) a new annotated strip map documenting details of the tectonic geomorphology of the fault zone; and 3) new paleoseismic data. Structural relations found west of the RCF indicate that thrust klippen of the Donnell Ranch Volcanics (DRV; ⁴⁰Ar/³⁹Ar 10.64-9.28 Ma) were emplaced over the Petaluma Formation (⁴⁰Ar/³⁹Ar 8.52 Ma) along east-vergent thrust faults, rather than along west-vergent thrusts that splay from the RCF, as previously proposed. This implies that: 1) the allochthonous DRV, which have been correlated to volcanic rocks in the Berkeley Hills (⁴⁰Ar/³⁹Ar 9.99-9.2 Ma), must have originated from west of the Tolay fault; and 2) much of the 45 km of northward translation of the DRV from the Berkeley Hills may have occurred along the Hayward -Petaluma Valley system of faults, rather than along the Hayward-RCF system. Long-term post middle to late Miocene offset along the RCF may be estimated by matching similar aged Sonoma volcanic tuffs (7.12 and 7.05 Ma) across the fault, which suggests about ~20 km of net right-lateral translation. This estimate is more consistent with independently derived paleogeographic reconstructions as well as Pliocene sedimentary units of the Roblar Tuff across the southern RCF (Sarna-Wojcicki, 1992; McLaughlin et al., 1996). An annotated strip map compiled for the southern 12 km of the fault has resulted in new details on the geologic deposits and tectonic geomorphology along the fault. The maps provided the basis for a site-specific paleoseismic investigation. We opened a 60-meter-long trench located 2 km northwest of Wildcat Mountain, in southern Sonoma County. The trench exposed two main traces of the fault bounding a 7-meter-wide tectonic depression. Stratigraphic and structural relations in the trench have provided evidence for six mid to late Holocene faulting events, the youngest of which may have ruptured to the modern ground surface.