Neogene contraction between the San Andreas fault and the Santa Clara Valley, San Francisco Bay region, California

In the southern San Francisco Bay region of California, oblique dextral rev erse faults that verge northeastward from the San Andreas fault experienced triggered slip during the 1989 M7.1 Loma Prieta earthquake. The role of th ese range-front thrusts in the evolution of the San Andreas fault system an d the future seismic hazard that they map pose to the urban Santa Clara Val ley are poorly understood. Based on recent geologic mapping and geophysical investigations, we propose that the range-front thrust system evolved in conjunction with development of the San Andreas fault system. In the early Miocene, the region was domi nated by a system of northwestwardly propagating, basin-bounding, transtens ional faults. Beginning as early as middle Miocene time, however, the trans tensional faulting was superseded by transpressional NE-stepping thrust and reverse faults of the range-front thrust system. Age constraints on the thrust faults indicate that the locus of contraction has focused on the Monte Vista, Shannon, and Berrocal faults since about 4 .8 Ma. Fault slip and fold reconstructions suggest that crustal shortening between the San Andreas fault and the Santa Clara Valley within this time f rame is similar to 21%, amounting to as much as 3.2 km at a rate of 0.6 mm/ yr. Rates probably have not remained constant; average rates appear to have been much lower in the past few 100 ka. The distribution of coseismic surf ace contraction during the Loma Prieta earthquake, active seismicity, late Pleistocene to Holocene fluvial terrace warping, and geodetic data further suggest that the active range-front thrust system includes blind thrusts. C ritical unresolved issues include information on the near-surface locations of buried thrusts, the timing of recent thrust earthquake events, and thei r recurrence in relation to earthquakes on the San Andreas fault.