HR: 0830h
AN: T11D-0423    [PDF]
TI: The Hayward Fault in the East San Francisco Bay Region, California: A Regional Geophysical and Geological Perspective
AU: * Jachens, R C
EM: jachens@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS989, Menlo Park, CA 94025
AU: Ponce, D A
EM: ponce@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS989, Menlo Park, CA 94025
AU: Graymer, R W
EM: rgraymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS975, Menlo Park, CA 94025
AU: Wentworth, C M
EM: cwent@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS975, Menlo Park, CA 94025
AU: Hildenbrand, T G
EM: tom@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS989, Menlo Park, CA 94025
AB: Regional gravity, magnetic, and geologic data, together with sparse seismic reflection and refraction data, and drill hole logs, provide a regional tectonic perspective and context of the Hayward fault. The fault is at least 85 km long and terminates at both ends at pull-apart basins. The southeastern terminus falls near the northeastern tip of the NW-SE Evergreen basin, which likely formed in the wake of a right step between the Silver Creek and Hayward Faults as a result of large right slip. The northwestern terminus lies along the southwestern edge of the deep basin that lies beneath eastern San Pablo Bay and extends, as the East Bay Trough, both southeast and northwest. Here, a structural manifestation of the Hayward Fault (the southwestern edge of the deep basin) steps right to about trace of the Tolay thrust Fault whereas seismicity and recent movement step farther right to the Rodgers Creek Fault. The deep basin under San Pablo Bay is three times wider than the right step from the Hayward fault to the Rodgers Creek fault, suggesting that if this basin is a strike-slip pull-apart basin, then faults farther east of the Rodgers Creek fault must have been involved in its formation. The local basin that formed during the past few million years in the wake of the Hayward-Rodgers Creek right step probably is no more than 500 m deep, according to a seismic reflection profile. A continuous magnetic anomaly probably reflecting a buried volcanic body lies between the Rodgers Creek Fault and the western deep basin margin north of San Pablo Bay, continues southward to truncate against the Hayward fault beneath most of San Pablo Bay. This body is not dismembered, precluding a simple strike-slip connection between the Rodgers Creek and Hayward Faults with more than about 10 km of offset (compared to the estimated roughly 100 km of total offset across the Hayward fault). The Hayward fault separates two distinctly different crustal blocks. The Bay block to the southwest generally consists of a basement of Franciscan terranes mantled by a veneer of Plio-Quaternary deposits generally $<$400 m thick. One exception is the San Leandro synform west of the central reach of the Hayward fault, which contains about 800 m of folded, probably Tertiary, strata overlain by about 300 m of flat-lying younger deposits. Northeast of the Hayward Fault Franciscan terranes lie beneath Mesozoic Coast Range Ophiolite, Cretaceous Great Valley Sequence, and overlying Cenozoic deposits exposed at the surface. The Valley Sequence and younger sedimentary deposits probably reach thickness in excess of 8 km beneath Livermore Valley and San Pablo Bay, suggesting structural relief on the Franciscan basement of more than 9 km (bottom of Livermore basin to top of the Diablo Range to the south). The central reach of the Hayward Fault lies at the western base of a steeply east-dipping slab of Coast Range ophiolite, indicating that the present-day Hayward Fault is here controlled by the older Coast Range Fault, a wedge fault that regionally separates subduction-related Franciscan rocks from overlying forearc Coast Range ophiolite and Great Valley Sequence strata.
DE: 0920 Gravity methods
DE: 0925 Magnetic and electrical methods
DE: 1219 Local gravity anomalies and crustal structure
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting