CONSTRAINTS ON FAULTS AND BASINS WITHIN THE SANTA CLARA VALLEY, CALIFORNIA, FROM SEISMIC-REFLECTION DATA

Thirty kilometers of high-resolution P-wave seismic reflection data provide images of the Santa Clara Valley (SCV) in the 50 to 1400 m depth range. These data help constrain the relationship of Quaternary (Q), Tertiary, and Mesozoic units and clarify some fault boundaries of the three basins beneath the valley: the Q Santa Clara basin (SCB) and the underlying late Miocene to Q(?) Evergreen basin (EB) and Miocene Cupertino basin (CB).

Generally flat-lying and laterally continuous reflections characterize the SCB stratigraphy imaged in the 50 to 500 m depth range. We interpret the base of the SCB at about 500 m as an angular unconformity over the CB that continues eastward across the mid-valley basement high. In the EB we infer the base of the SCB from a high-amplitude reflection associated with a 25 percent increase in seismic velocity, making the Q section here up to 600 m thick. A thickening section of Q sediments on the east side of the SCV and a negative flower structure in Q sediments over the Silver Creek Fault suggest that SCB sediments are deformed by faults that were active during about 140-270 ka and possibly younger, as shallower horizons were not imaged.

The thickening Q sedimentary section on the east side of the EB is underlain by a prominent reflector that forms the top of a wedge of higher-velocity (3150 m/s) rocks that are, from seismic velocity, likely older than Q. Low-angle reflections within the wedge suggest westward-directed thrusting of this wedge into the east side of the EB. The wedge, which forms a ramp-and-flat structure with about 250 m relief, correlates with a flap of Mesozoic rocks modeled by gravity inversion to extend into the basin. The thrust-like features of the wedge may represent the southward transformation of the Hayward fault, which is rolling over to a more east-dipping structure in the Fremont area, accommodating the stepover to the Calaveras fault.

In the CB a prominent 15-20 degree southward dipping reflection package, believed to be the Miocene Monterey Fm. (MF), unconformably underlies the generally flat-lying Q section to at least 1.2 km depth. This relationship confirms previously proposed models for the buried CB filled largely with MF. The only evidence for faulting found over the CB is small (<20m) aligned bends or steps in reflections suggesting two possible small faults within the Q section.