THRUSTING AND MASS WASTING INTERPLAY ON THE BERROCAL FAULT, LOS GATOS, SANTA CLARA COUNTY, CALIFORNIA: IMPLICATIONS FOR GEOLOGIC HAZARD ZONING

The large-scale processes forming the Santa Clara Valley (SCV) are fairly clear: oblique motion partitioned into strike-slip and dip-slip faults; thrusting and uplift of the Santa Cruz Mtns. (SCM). The major fault systems have been recognized.

However, at a fine (site-specific) scale, expression and recognition of these processes is less clear. The complex interplay of geoprocesses can obscure major players (e.g. a nearby fault splay). Formulating fair public policy becomes harder.

Our Los Gatos trench site provides an example of local complexity in a seemingly simple larger picture. In one trench, we found: the eastern fringe of the W-dipping Berrocal fault zone (BFZ), thrusting Franciscan rocks over old fan deposits; opportunistic landsliding of intact upper-plate rocks ejected from the BFZ onto an E-sloping land surface; and minor coseismic extensional structures, related to spreading/extension of the SCM.

Considerations relevant to modelling and geohazard zoning in this setting include: 1) A line joining the apices of adjacent alluvial fans defines a hinge (mass wasting > deposition upslope; mass wasting < deposition downslope). 2) Dissected old fan deposits upslope of the modern hinge indicate the hinge has shifted toward the SCV; how is the range-front fault system evolving? 3) Thrusts upslope of the hinge are relatively easy to recognize. Thrusts downslope of the hinge may be intermittently blind. 4) The activity of thrusts upslope of the hinge is hard to gage because of thin soils. What is the hazard over currently blind thrusts/folds? Which faults are seismogenic, which are sympathetic? 5) The widespread landsliding on the flanks of the SCM indicates inability of other mass wasting processes (e.g. erosion, creep) to keep pace with uplift. Deposits tend to record a punctuated history. 6) Landsliding and creep can result in a fault being masked, or mismapped downslope. Creep can result in reversals of dip direction. 7) If a fault zone at the hinge has multiple traces, fan deposits can be depositional on upper plate rocks between traces.