No abstract

No abstract

The northwest-trending Silver Creek Fault is a 40-km-long strike-slip fault in the eastern Santa Clara Valley, California, that has exhibited different behaviors within a changing San Andreas Fault system over the past 10-15 Ma. Quaternary alluvium several hundred meters thick that buries the northern half of the Silver Creek Fault, and that has been sampled by drilling and imaged in a detailed seismic reflection profile, provides a record of the Quaternary history of the fault. We assemble evidence from areal geology, stratigraphy, paleomagnetics, groundwater hydrology, potential-field geophysics, and reflection and earthquake seismology to determine the long history of the fault in order to evaluate its current behavior. depression along the fault. No surface trace is evident on the alluvial plain, however, and convincing evidence of Holocene offset is lacking. Few instrumentally recorded earthquakes are located near the fault, and those that are near its southern end represent cross-fault shortening, not strike slip. The fault might have been responsible, however, for two poorly located moderate earthquakes that occurred in the area in 1903. Its southeastern end does mark an abrupt change in the pattern of abundant instrumentally recorded earthquakes along the Calaveras Fault-in both its strike and in the depth distribution of hypocenters-that could indicate continuing influence by the Silver Creek Fault. In the absence of convincing evidence to the contrary, and as a conservative estimate, we presume that the Silver Creek Fault has continued its strike-slip movement through the Holocene, but at a very slow rate. Such a slow rate would, at most, yield very infrequent damaging earthquakes. If the 1903 earthquakes did, in fact, occur on the Silver Creek Fault, they would have greatly reduced the short-term future potential for large earthquakes on the fault.

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This database, identified as 'Preliminary geologic map of the San Jose 30 X 60-minute quadrangle, California: a digital database', has been approved for release and publication by the Director of the USGS. Although this database has been reviewed and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. This database is released on condition that neither the USGS nor the U.S. Government may be held liable for any damages resulting from its use.

The San Andreas fault has long been viewed as a vertical structure that extends from the ground surface to the base of the lithosphere and separates the Pacific plate from the North American plate along a transform plate boundary. Analysis of Neogene structures in the central Coast Ranges indicates, however, that crustal blocks defined by strands of the San Andreas fault system have undergone pervasive compressive deformation that has shifted these blocks and their boundary faults east, relative to North America. This suggests that the blocks and their boundary structures are bounded below by an active decollement that appears to coincide with the midcrustal brittle to ductile transition at the base of the seismogenic zone. These compressive structures may be related to a currently seismically active fold and thrust belt that characterizes the eastern front of the Coast Ranges. Seismic profiles from the Central Valley, east of the range front, show that tectonic wedges ("blind thrusts") developed within stratified'rocks are present throughout this zone of deformation, and that crystalline basement that underlies these stratified rocks is not involved in the deformation. Crystalline basement may underlie most of the Coast Ranges, as indicated by the presence of metamorphic rocks of possible Sierran affinity preserved in Neogene thrust sheets as far west as Loma Prieta and by seismic velocities in the lower crust appropriate for mafic to intermediate granitoid rocks. On the basis of these relations, we suggest that the San Andreas fault system is confined to the brittle crust, above the decollement, and does not penetrate to the base of the lithosphere. Thus the San Andreas fault should not be considered as a "plate boundary." Instead, the functional plate boundary within the Coast Ranges appears to be the inferred subhorizontal midcrustal decollement. oThis decollement corresponds approximately to the 350 C isotherm, the temperature at which quartz becomes ductile. If our hypothesis is proven to be correct by geologic and geophysical investigations currently underway, then extensive revisions in popular plate tectonic models applied to the Coast Ranges will be required. In addition, assessment of seismic hazards will be rendered more difficult, but more realistic, owing to realization of the greater likelihood for occurrence of thrust faulting on blind or buried faults or on faults currently deemed inactive.