Abstract

Underthrusting of the Franciscan assemblage beneath Great Valley rocks seems to have been completed by the end of the Oligocene in the region west of the Salinian block. Both the Franciscan and Great Valley assemblages are latest Jurassic-Cretaceous in age. Clasts derived from both first appear together in the Vaqueros Formation (Upper Oligocene?-Lower Miocene). The Vaqueros is markedly transgressive and is generally the oldest formation in depositional contact with the Franciscan in the region under discussion. The Vaqueros locally rests on both Franciscan and Great Valley rocks, and extends across a fault separating the two.

If Underthrusting ceased before the end of the Oligocene, the timing is compatible with the termination of northeast-southwest relative motion at the continental margin, and the beginning of northwest-southeast relative motion when North America encountered the Main Pacific plate. Much or all activity on the San Andreas fault followed this event. The fault offset the zone of Underthrusting.

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