Abstract: Reinterpretation of the Northern Terminus of the San Andreas Transform System: Implications for Basin Development and Hydrocarbon Exploration

Sara S. Foland, K. J. Enzor

The northern San Andreas transform system was studied to evaluate the tectonic history of offshore Point Arena basin, northern California. The Point Arena basin lies 250 km north of San Francisco and encompasses 8500 sq km² on the outer continental shelf. It is a tertiary basin formed during Eocene subduction and overprinted by Pliocene-Pleistocene strike-slip motion of the San Andreas fault system.

Interpretation of the data yields a new tectonic model for the northern San Andreas fault system and Point Arena basin. Previous models curved the fault system east parallel to the coast, intersecting faults exposed on Point Delgada, and then bending the fault sharply west to join the Mendocino triple junction. The new model projects the San Andreas fault system due northwest, straight into the offshore basin, as a series of parallel faults aligned with the onshore fault trace to directly intersect the triple junction. The new interpretation is supported by aeromagnetic data, which indicates the basin is divided by a major northwest-trending structural boundary and floored by two distinct basement types (Mesozoic Salinian granites and Jurassic Franciscan metasediments). The latest sei mic data contain enough information to determine the genesis and orientation of the offshore fault system and associated folds.

Basin modeling indicates hydrocarbon generation has occurred in the Miocene source beds. The model estimates the Point Arena basin contains multibillion barrel potential trapped in large antiforms associated with the through-going San Andreas system. Integration of all geotechnical data allowed reinterpretation of the tectonic history, and produced and enhanced understanding of Point Arena basin.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994