Geology of Offshore Central California

David S. McCulloch, Stephen D. Lewis

The offshore central California margin records Mesozoic and Cenozoic events attributable to continental and oceanic plate interactions. Terranes, some carried thousands of kilometers northward on the Kula and/or Farallon plates, were accreted to North America by late Eocene. Following accretion to North America, terranes along the margin were slivered and redistributed by strike-slip motion along the San Andreas and related faults.

During Oligocene global low sea level, much of the coastal margin was subareal and erosion stripped Upper Cretaceous and Eocene strata from elevated basement blocks. These basement blocks subsequently subsided along large-displacement near-coastal faults, and shallow-water marine upper Oligocene and/or lower Miocene volcanics on the present shelf were covered by hemipelagic sediment at bathyal to mid-bathyal depths. This subsidence coincided with tectonism and significant changes in water depth elsewhere around the Pacific margin, and may have been related to a change in Pacific and North American plate relative motion that produced transtensional motion along the California margin. Transtension may have initiated both rupture and volcanism along the San Andreas fault system at about 3 Ma and wrench tectonic deformation on the shelf.

During the late Miocene, growing structural highs on the shelf edge formed elongate shelf basins. A compressional component in Pacific and North America plate relative motion, which began between 5 and 3 Ma, produced strike-parallel folds and compressional faults that now dominate the structural grain of the offshore basins. Continued compressive deformation along coastal California is documented by active thrust faults, rapid coastal uplift, and earthquake focal mechanism solutions indicative of northeast-southwest compression.

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