LATE CENOZOIC STRATIGRAPHIC AND TECTONIC HISTORY OF THE SANTA CLARA VALLEY, CALIFORNIA

In early Miocene time, the area that became Santa Clara Valley consisted of eroding Franciscan Complex basement structurally overlain in places across the Coast Range Fault by Coast Range Ophiolite and Mesozoic Great Valley Sequence. In middle Miocene time this landscape subsided beneath a shallow sea, and was deformed locally into deeper depressions such as the Cupertino Basin in the SW valley. Marine deposition during the next ~6 Ma laid down thin deposits in shallow water and thick deposits in the Cupertino Basin. During this sedimentation the San Andreas Fault system encroached into the valley, with most offset partitioned onto the San Andreas Fault SW of the valley, and the southern Calaveras-Silver Creek-Hayward Fault system in the NE part of the valley. A 6 km-wide right step between the Hayward and Silver Creek Faults formed the 40-km-long Evergreen pull-apart basin along the NE margin of the valley, leaving a basement ridge between it and the Cupertino Basin. No other major San Andreas system faults break the central basement block between the present-day range-front faults bounding the valley. The Silver Creek Fault was abandoned ~2.5 Ma in favor of a compressional left step between the Calaveras and Hayward Faults. Sometime between 9 and 4 Ma (9 and 1 Ma for the central block) the area rose above sea level and erosion carved a regional unconformity into the Mesozoic and Tertiary rocks. Alluvium of the Santa Clara gravels (SCg) was deposited on this unconformity along the margins of the valley beginning ~4 Ma, but may not have prograded onto the central block until ~ 1 Ma, because no older equivalents of the SCg have been found there. Thus, either the central block was high enough relative to the surrounding areas that SCg were never deposited on it, or any SCg deposited there was stripped away before ~1 Ma. Sequence-stratigraphic analysis of alluvium on the central block implies a remarkably uniform, piston-like, subsidence of the valley of ~0.4 mm/y since 1 Ma, possibly extending north to Angel Island. Today, the central block continues to subside, the range front reverse faults are active, and the major active faults of the San Andreas system are mostly outside the valley. The Silver Creek Fault, although largely abandoned, seems to have accommodated some slip during the past few 100 Ka.