VARIED EVIDENCE FOR A BLIND THRUST BENEATH THE SOUTHWESTERN SANTA CLARA (SILICON) VALLEY, CALIFORNIA: A PRODUCT OF 3-DIMENSIONAL MAP ANALYSIS

The Quaternary range-front thrust system at the SW margin of the Santa Clara Valley, CA, seems to be growing basinward (NE) beyond the outermost faults of the system mapped at the surface (Monte Vista and Shannon). The deepest part of the NE-shallowing Cupertino basin lies beneath these mapped faults according to a gravity inversion of the top of Franciscan basement; this basin is filled with oil-bearing Miocene and older? sedimentary rock. 100s of small earthquakes triggered by the Loma Prieta earthquake form a diffuse zone that rises upward and NE away from the San Andreas fault, subparallel to the mapped faults, from depths of about 7 km to the bottom of the Cupertino basin at 4 km. These earthquakes reach the bottom of the basin at a 7-km-long, northwest-trending intra-basin ridge about 900 m high defined by the gravity inversion. This ridge could mark thrust offset of the basin floor.

The upward projection of a fault through the earthquakes and the NE side of the intra-basin ridge reaches the ground surface 2-3.5 km NE of the mapped faults. At about 2 km, the abrupt NE margin of exposed Santa Clara Formation suggests fault offset. Farther out, the NE limit of a zone of short surface lineaments mapped from aerial photos (Hitchcock and others, 1994) also trends NW. These lineaments could mark the surface effects of underlying deformation; surface deformation associated with the Loma Prieta earthquake was concentrated in parts of this lineament zone (Schmidt and others, 1995), and its spatial coincidence with the lineaments is statistically significant.

The presence of such an outboard fault that cuts most of the water-bearing alluvial sediments can resolve a serious discrepancy between observed and modeled ground-water elevations in the SW Santa Clara Valley and explain a 45-m water-level difference between two wells 1 km apart; these wells locate the barrier near the NE edge of the surface lineaments.

Several lines of evidence thus indicate the likely presence of an active outboard fault in the range-front thrust system that is not fully expressed at the ground surface, although the ground-water evidence suggests that the fault has cut through the Tertiary basin fill and most of the overlying Quaternary alluvium. The presence of such a fault is being tested by shallow reflection profiling across the central part of the Cupertino basin.