Neogene Contraction between the San Andreas Fault and the Santa Clara Valley, San Francisco Bay Region, California

Abstract

In the southern San Francisco Bay region of California, oblique dextral reverse faults that verge northeastward from the San Andreas fault experienced triggered slip during the 1989 M7.1 Loma Prieta earthquake. The role of these range-front thrusts in the evolution of the San Andreas fault system and the future seismic hazard that they may pose to the urban Santa Clara Valley 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 dominated by a system of northwestwardly propagating, basin-bounding, transtensional faults. Beginning as early as middle Miocene time, however, the transtensional fault ing was superseded by transpressional NE-stepping thrust and reverse faults of the range-front thrust system.Age constraints on the thrust faults indicate that the locus of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 frame is ~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 dis tribution of coseismic surface 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. Critical unresolved issues include information on the near-surface locations of buried thrusts, the timing of recent thrust earthquake events, and their recurrence in relation to earthquakes on the San Andreas fault.

Modeling and Validation of a 3D Velocity Structure for the Santa Clara Valley, California, for Seismic-Wave Simulations

“…These rocks are overlain by Pleistocene and Holocene alluvial fan and stream deposits and estuarine deposits around the margin of the San Francisco Bay. Pliocene and Pleistocene fanglomerates of the Santa Clara and related formations overlie the older rocks along the margins of the valley (McLaughlin et al, 1999;Wentworth et al, 1999). have proposed a tectonic evolution for the Santa Clara Valley during the late Cenozoic, starting with a Franciscan Complex basement that subsides beneath a shallow sea in the middle Miocene.…”

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

“…Average shortening rates of these faults during the past 5 m.y. are similar to those accommodated by folding, 0.5-0.6 mm/yr (McLaughlin et al, 1999). During this time interval, the San Andreas fault reorganized, abandoning the Pilarcitos strand and straightening its trace (McLaughlin et al, 2007).…”

“…1) Oil and Gas, 1982;Robbins, 1982;Rogers, 1993 (Figs. 2 and 4B), which has an unknown amount of strike slip during this time frame but is likely to be on the order of several kilometers (which restores ophiolitic and Franciscan complex rocks into reasonable proximity; McLaughlin et al, 1999). Robbins (1982) modeled the gravity and magnetic anomalies in this area as refl ecting a substantial section of late Miocene (?)…”

Mountain Rivers Reveal the Earthquake Hazard of Geologic Faults in Silicon Valley

“…However, knowledge of the frequency of these and larger earthquakes is limited by the unfavorable geometry of the faults for performing paleoseismology studies, urban development in the area, and a lack of earthquake‐related deposits, all of which hamper efforts to understand rates at which seismic moment may be accruing. Fault‐offset terraces, which serve as markers of past river levels, and piedmont deposits provide some information about their Quaternary slip rates (∼0.45 mm/a) (Hitchcock & Kelson, 1999; Mclaughlin et al., 1999), but the location of available sites and a lack of age control obscures the potential hazard that they pose.…”

“…Previous geologic mapping and geophysical imaging constrain the surface outcroppings of rocks and subsurface geometries of faults (see Supporting Information ). Slip along the range‐bounding thrusts (Mclaughlin et al., 1999) has caused average rock uplift‐rates of ∼0.8–1.2 mm/a over the last 4 Ma in the southern area (Bürgmann et al., 1994) (Figure 1a). This uplift stands in contrast to adjacent areas located northeast of the range, where thick Pliocene‐Holocene deposits of the Santa Clara Valley indicate stability, and possibly subsidence during this time (Langenheim et al., 2015).…”