STRUCTURE OF THE MONTE VISTA FAULT ZONE, SOUTHWEST SANTA CLARA VALLEY, CALIFORNIA, USING GEOLOGIC, POTENTIAL-FIELD AND SEISMIC DATA

The Monte Vista fault zone (MVFZ) is part of the larger Foothills thrust fault system that bounds the southwest margin of the urbanized Santa Clara Valley. The fault zone offsets Quaternary deposits and experienced triggered slip during the 1989 Loma Prieta earthquake. Here we describe the geometry of the fault zone and its upper plate, which together constitute important elements of the 3-D geologic model of Santa Clara Valley.

Mesozoic Franciscan Complex basement and overlying Miocene and Plio-Quaternary sedimentary rocks form the upper plate of the MVFZ. Gravity anomalies and interpretations of water and oil well logs indicate that Franciscan rocks in the upper-plate form a thin, 2-3-km-wide thrust flap above the lower-plate, which consists of thick (~ 3 km) Miocene rocks of the Cupertino basin overlain by Quaternary alluvium. At the northwestern end of the MVFZ, Plio-Quaternary deposits of the Santa Clara Formation are folded, with Miocene rocks exposed in the core of a hanging-wall anticline. Miocene strata are not exposed south of this anticlinal structure and pinch out stratigraphically or structurally in the subsurface. The fold axes extend parallel to the southeast strike of the MVFZ for 5-6 km, then diverge southward from the fault zone and disappear near Saratoga, to be replaced by deeply dissected, nearly flat-lying Santa Clara Formation and/or Pleistocene fan deposits. These relations suggest that there is a structural transition within the upper plate of the MVFZ. North of this transition, the Santa Clara Formation may be as thick as 600-700 m, if its surface distribution and bedding attitudes represent uniform thickness across the axis of a prominent syncline. South of the transition, the deposits may be half as thick based on interpretation of water well logs. Damage from the Loma Prieta earthquake was less scattered north of this structural transition than to the south, with most damage located within 1-2 km of the MVFZ. South of the transition, damage was as much as 5 km from the fault zone, both in the upper and lower plates. This evidence suggests that the two structural domains in the upper plate of the MVFZ may have influenced the distribution of triggered slip and/or shaking intensity.