PREVIOUSLY UNDOCUMENTED CONTINUITY OF A PALEOGENE STRUCTURAL TREND CONSTRAINS CUMULATIVE STRIKE-SLIP OFFSET ON THE EASTERN SAN FRANCISCO BAY FAULT SYSTEM, CALIFORNIA TO <30 KM

The Eastern Coast Ranges in California’s San Francisco Bay Area largely comprise Upper Cretaceous and Cenozoic rocks deposited on or accreted to the Western Margin of North America. The San Andreas transform margin propagated to this latitude at 12-14 Ma. Graymer et al. (2002) interpret 175 km of dextral offset on faults east of the San Andreas fault. However, a newly identified piercing point disallows large magnitude strike-slip offset east of the San Andreas fault.

In the Central Valley, the Stockton Arch is expressed at the base Miocene unconformity south of the Stockton Fault and is documented by subsurface and gravity data. The 60 km wide arch extends 80 km transverse to the valley. Erosional truncation of Eocene through Upper Cretaceous strata beneath the Miocene unconformity and an anomalously thick Cretaceous section indicate that the arch formed by Paleogene inversion of a Cretaceous normal fault associated with the Rio Vista rift system.

Although analogous sub-Miocene relationships have been mapped at the surface by multiple geologists (e.g., T.W. Dibblee, Jr.), it appears to be newly recognized that the arch extended southwestward from the Central Valley to at least as far as the Hayward Fault. The sub-Miocene erosional limit of Eocene strata on the south flank of the arch is a linear feature which strikes ~E-W in the Central Valley and crops out east of the Diablo Range (15 km northwest of Patterson) and west of the Diablo Range (8 km north of Morgan Hill). This piercing line is eroded west of the Calaveras Fault but, a 35 km gap in the preservation of sub-Miocene Eocene strata along the San Andreas Fault between Palo Alto and a point 4 km west of Loma Prieta suggests that uplift related to the Stockton Arch extended to the San Andras Fault and that cumulative dextral offset across the East Bay is < 30 km.

Conclusions from this analysis are that, despite the dominance of strike-slip motion in both earthquake focal mechanisms and geodetic data, (1) the geologic fabric and seismic velocity structure of the Bay Area is dominated by Late Cretaceous and Tertiary dip-slip structures, and (2) although many of these older faults have been reactivated by the modern oblique stress field, it is the Paleogene vertical motions, rather than Neogene lateral motions, that generated most of the observed stratigraphic variation across East Bay faults.