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Late Oligocene-Early Miocene Volcanism, Faulting, and Sedimentation in West-Central California

Richard G. Stanley

Abundant evidence from the rock record indicates that volcanism, active faulting, and sedimentary-basin development were widespread and nearly synchronous in west-central California during the late Oligocene and early Miocene. Volcanic activity occurred about 24-22 Ma at more than 20 eruptive centers in a northwest-trending belt, extending from Point Arena to the Orocopia Mountains. Volcanism was accompanied by marine transgression, rapid bathymetric deepening, and accumulation of thick piles of predominantly marine clastic sediment in the Point Arena, La Honda, San Joaquin, Pescadero-Ano Nuevo, Salinas, Huasna-San Luis Obispo, and Cuyama basins, as well as in several other basins presently situated offshore. Concurrent movement along basin-margin normal faults resulted i localized deposition of wedges of very coarse nonmarine and deltaic sediment in the La Honda, Cuyama, Lockwood, Soledad, and Diligencia basins. Also, at about the same time, right-lateral strike-slip faulting apparently caused en enchelon folding in both the Cuyama and San Joaquin basins, and formation of a pull-apart feature in the La Honda basin.

Late Oligocene-early Miocene volcanism, faulting, and basin development in west-central California were nearly synchronous over the entire area rather than sequential from south to north, and therefore cannot be explained simply as the effects of the northwestward-migrating Mendocino triple junction. An alternative explanation is that volcanism, faulting, and basin development were caused by an episode of regional extension or transtension (strike-slip with a component of extension) related to a change in plate motions in the circum-Pacific area.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.