SANTA CLARA VALLEY-BOUNDING FAULTS CHARACTERIZED BY STRUCTURAL SUPERPOSITION

The Santa Clara Valley is bounded to the west by Quaternary-active right-lateral reverse oblique faults (the Foothills Thrust Belt rooting into the San Andreas Fault on the west and the Hayward-Calaveras Fault system on the east) that overprint older right-lateral normal oblique faults. In addition, there are two significant reverse oblique faults (including the west dipping Silver Creek Fault) within the valley. Each of these faults is geologically young (for example, the oldest faults of the Calaveras-Hayward system cut Pliocene gravels). Gravity studies and geologic mapping show that the west-dipping Quaternary Foothills Thrust Belt places relatively high-density rocks in the hanging wall over lower density rocks in the Cupertino Basin, and is superposed on the steeply east-dipping Miocene right-lateral normal oblique fault which bounds the basin. Similar analysis shows that the Calaveras-Hayward system and Silver Creek Fault have overthrust right-lateral normal oblique faults bounding the Evergreen Basin. This transpressional over transtensional superposition has resulted in fault nomenclature problems (for example, both the present reverse-oblique fault and the underlying normal-oblique fault along the west side of the Evergreen Basin have been called Silver Creek Fault), and difficulties in estimating long-term fault offsets (many tens of kilometers of Neogene right-lateral offset were accommodated by the now-buried normal-oblique faults bounding the Evergreen Basin) and basin shape from gravity data (the gravity signal of the Evergreen Basin is partially obscured by dense Mesozoic rocks thrust over the eastern and central parts of the basin). Other examples of structural superposition in the San Francisco Bay region include telescoped Pliocene to late Miocene volcanic fields north of San Pablo Bay, superimposed multiple transtensional basins beneath San Pablo Bay, and thrusting over earlier strike-slip offset along the Moraga Fault east of Oakland. Recognition of structural superposition of this kind can explain apparently contradictory observations and help unravel previously intractable geologic problems.