THREE-DIMENSIONAL GEOLOGIC MAP OF THE SANTA CLARA VALLEY AND ADJACENT UPLANDS, CALIFORNIA

The first 3D geologic map of the Santa Clara Valley and adjacent uplands covers an area 45 km on a side and includes the top 14 km of crust. It extends from SW of the San Andreas Fault to NE of the Calaveras Fault, and from the southern end of San Francisco Bay nearly to Morgan Hill. The map facilitates 3D visualization and analysis, and provides a realistic earth model to support computer simulations of processes such as seismic wave propagation and ground shaking, tectonic strain accumulation and fault interactions, groundwater flow, contaminant transport, and many others. It is assembled in EarthVision (TM Dynamic Graphics, Inc.) software, and can be exported in a variety of formats. The 3D surfaces--the fundamental elements of the map (faults, unconformaties, depositional contacts, land surface)-were constructed based on geologic mapping, potential-field geophysical interpretations, seismic profiling, hypocenter locations, well and well-log analyses, digital elevation models, and regional geologic considerations. The map is broken into blocks by 11 major faults, and the fault-blocks are subdivided into 14 generalized geologic units. Layer thicknesses span 5 orders of magnitude, from a thin water layer to the thick Franciscan block of the Diablo Range. The map distinguishes Bay Mud and Holocene layers, both important for prediction of ground shaking, and a valley-wide layer of Plio-Quaternary alluvium which contains the valley's major groundwater system.

At least 3 distinct basins lie beneath the surface of the Santa Clara Valley; (1) the Cupertino Basin in the SW part of the valley filled with dipping marine Miocene strata, (2) the Evergreen Basin in the NE part of the valley, a strike-slip pull-apart basin in its upper part filled with flat-lying continental deposits, and (3) an overlying alluvial basin ~300-450 m thick that filled mostly during the past 1 Ma. Reverse faulting dominates the present valley margins, with an extreme example being a flap of Mesozoic rocks that locally extends entirely across the late Cenozoic Evergreen Basin. The distribution of seismicity and focal mechanisms of earthquakes between the valley and the San Andreas Fault suggests active thrusting at depth along the SW margin of the valley.