A 40-KM-LONG CONCEALED BASIN SUGGESTS LARGE OFFSET ON THE SILVER CREEK FAULT, SANTA CLARA VALLEY, CALIFORNIA

A 40x10-km gravity low at the NE margin of Santa Clara Valley marks an extensional strike-slip basin (the Evergreen basin) between the Silver Creek fault (SCF) and Hayward fault (HF) that resulted from large right slip across this previously unrecognized right step in the San Andreas fault system. Quaternary alluvium lies at the NW end where the low is widest and deepest (~-35 mGal), whereas Tertiary gravels and older bedrock crop out in places to the SE, where the low is shallower. Detailed seismic tomography (Michael, 1988) shows low velocities extending down to > 4 km within the low, indicating that it results from a concealed basin of slow, and thus relatively young, sediments. Inverting the gravity with an iterative 3D basin-modeling method and a density/depth function from nearby Livermore Valley yields a 40-km-long model basin that narrows from 8 to 5 km and shallows from 6 to 4 km toward the SE. The basin is bounded on the NE by the Calaveras fault and the southward extension of the HF system (the Evergreen and related faults), and on the SW by the trace of the SCF and its probable concealed extension to the NW. The faults on the NE dip NE and show reverse separation. Along the SE half of the basin, gravity and magnetic anomalies interpreted in conjunction with exposed Franciscan rocks and Coast Range Ophiolite SW of the SCF strongly suggest that the SCF dips SW, although other interpretations are possible.

We propose that the Evergreen basin formed in the wake of a right stepover from the SCF to the southernmost HF. Its length and regular, non-segmented morphology imply at least 40 km of right slip along the SCF. Outward dips of the bounding NW-trending faults and a thin, flap of Mesozoic Great Valley sequence and Coast Range Ophiolite rooted to the NE that conceals much of the SE part of the basin indicate late cross-basin shortening, perhaps caused by basin development with bounding faults that diverge slightly northwestward. In our interpretation, palinspastic reconstruction of the 175 km of San Andreas fault system offset partitioned to the East Bay faults juxtaposes two dispersed 40-km-long, strongly magnetic, tabular bodies against each other across the SCF: serpentinite of Yerba Buena ridge/Oak Hill in Santa Clara Valley against a concealed, flat-lying body (inferred to be serpentinite) now NW of Parkfield, California.