Abstract

The continental margin north of the Cape Mendocino triple junction is a region of tectonic transition between structures related to underthrusting on the continental slope and those related to right-lateral strike-slip along the coast. Underthrusting of late Cenozoic rocks of the continental slope by the Gorda lithospheric plate is seen by the presence of magnetic anomaly 3, age 5 m.y., beneath the slope. Folds and faults mapped by reflection profiling on the continental slope are parallel to it, and the faults have dip separations predominantly with west side down. These structures are best explained as resulting from oblique underthrusting, with structural trends controlled by the slope direction. Northwest-trending faults occur along the coast and reported earthquake mechanism solutions show right-lateral first motions. These faults and related folds affect Mesozoic rocks and parallel the San Andreas fault system which lies to the south, most probably sharing its origin—that of shear interaction between the Pacific and North American lithospheric plates.

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