Abstract: San Francisco Bay Basin Subsidence: Driven by Offshore Emplacement of Salinia?

Kevin P. Furlong, Jordi Prims

The recent development and subsidence of the San Francisco Bay basin since 4 Ma is exceptional when we consider that this basin is sitting in a locally transpressional fault system. Crustal shortening and mountain building expected in such an environment is observed mainly along the Santa Cruz Mountains which border the San Francisco Bay basin on the west and the south. The synchronaeity of mountain building with basin subsidence, although not the expected style of strike-slip basin development, is a common feature of many other basin types, particularly those formed by flexural downwarping of the lithosphere adjacent to emplaced loads. Here we analyze and evaluate the hypothesis that the San Francisco Bay basin is the consequence of lithospheric flexure of the Pacific pl te as a result of offshore emplacement of the Salinian terrane onto the Pacific plate.

Predicted subsidence of the San Francisco Bay basin agrees with observed sediment thickness. Net sediment thickness should be greater on the east side of the bay than on the west side, and sedimentation should prograde to the north in response to the continuing offshore emplacement of the Salinian terrane. Late Pliocene and Quaternary cover only represents the latest stages of subsidence in the area since the initial subsidence primarily accommodates previous relief. Subsidence in the San Francisco Bay area appears to occur through high angle normal and reverse faulting along the Hayward, Rodgers Creek and other East Bay faults. These dip slip displacements add an additional complication to the dominant strike slip regime.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California