MURCHEY, BENITA L., and M. C. BLAKE, JR., U.S. Geological Survey, Menlo Park, CA

ABSTRACT: Evidence for Subduction of a Major Ocean Plate Along the California Margin during the Middle to Early Late Jurassic

The ages of ocean crust along the western North American plate boundary since the Early Jurassic suggest the subduction of an oceanic plate, here called the Klamath plate, during the late Middle to early Late Jurassic. We plotted "spreading ridge age" and "ocean residence time" vs. "time of arrival at the continental margin" for 15 oceanic terranes in a transect from the Klamath Mountain province to the San Francisco Bay region. The resulting curves indicate that, along the continental margin, there was old ocean crust (circa 80 m.y.) in the Early Jurassic, young crust (10-30 m.y.) in the late Middle and early Late Jurassic, and old crust (92- 99 m.y.) in the Cenomanian. The late Middle Jurassic arrival of young ocean crust along the margin recorded the arrival of the Klamath-Farallon spreading ridge. Thereafter, until the Valanginian or later, the absolute age of ocean crust along the margin remained virtually constant, between 166 and 173 Ma. Therefore, little subduction apparently occurred beneath North America following the subduction of the Klamath plate. Our data, coupled with geologic evidence, suggest that the California margin was dominated by sinistral transform motion during this interval. Significant Farallon plate subduction began between 140 and 125 Ma, when crust that had formed on the western side of the Klamath-Farallon ridge became relatively cool and dense. The last of the crust formed by the Klamath-Farallon ridge reached the North American margin in the Cenomanian.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.