HR: 13:45h
AN: T12E-01    [PDF]
TI: Pliocene Reorganization of Hayward-Calaveras Fault Junction, San Francisco Bay Region, California, USA
AU: * Graymer, R W
EM: rgraymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025 United States
AU: Ponce, D A
EM: ponce@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025 United States
AU: Jachens, R C
EM: jachens@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025 United States
AU: Simpson, R W
EM: simpson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025 United States
AU: Wentworth, C M
EM: cwent@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS 973, Menlo Park, CA 94025 United States
AB: Paleogeographic reconstruction of the East Bay Fault System has revealed that about 100 km of post 12 Ma offset was passed onto the Hayward Fault from faults to the south (Graymer and others, 2002). This relation is reflected in the present day by transfer of 9 mm/yr of fault slip from the central Calaveras Fault to the Hayward Fault (Working Group on California Earthquake Probabilities, 2003). The surface manifestation of the connection between the two faults is a zone of (oblique?) reverse faults at least 4 km wide and 25 km long. Potential field geophysical studies suggest, however, that the original configuration of the junction involved transfer of Calaveras Fault offset onto a third fault (the Silver Creek Fault of Jachens and others, 2002), and from there onto the Hayward Fault through a right-step resulting in a pull-apart basin (Evergreen Basin). The reorganization of the junction, and the switch from largely extensional to largely compressional deformation, has resulted in the central part of the Evergreen Basin being overthrust by a wedge of Mesozoic rocks along west-vergent faults that root into the central Calaveras Fault. This cap of older rocks is reflected in the local decrease in amplitude of the gravity low associated with the Evergreen Basin. Pliocene non-marine deposits in the footwall of the thrust represent the youngest strata deposited in the pull-apart basin, and suggest that the fault junction reorganization began in Pliocene time. Latest Pliocene to early Pleistocene gravels locally cap thrust faults, suggesting that reorganization was accomplished by that time, although the same gravels are elsewhere themselves faulted, reflecting the ongoing compressional manifestation of the fault junction. However, INSAR studies have detected a ground water barrier in the upper 100 meters of Quaternary alluvium roughly aligned with the Silver Creek Fault that may reflect some amount of continued tectonic activity, though the lack of any geomorphic expression precludes large recent offsets. The westward extent of the overthrusting, the surface manifestation of the Silver Creek Fault in the overthrust region, and the amount of Quaternary offset on the Silver Creek Fault are currently unresolved. Joint detailed geologic and geophysical studies are being applied to produce a 3-D model of the region and address those issues.
DE: 8010 Fractures and faults
DE: 8105 Continental margins and sedimentary basins
DE: 8107 Continental neotectonics
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting