Abstract

If there has been as much as 1000 km of displacement between the Pacific and North American plates during the past 29 m.y., as is implied from sea-floor magnetic patterns, transform faulting other than that recorded along the San Andreas fault system is required. A model is proposed that accounts for the additional displacement between these two plates.

According to the Atwater and Molnar model, the crust of the Pacific plate that first came into contact with the North American plate, west of the southern California borderland, would have been newly formed and therefore relatively weak. I suggest that as much as 300 km of right-lateral shearing occurred prior to 21 m.y. ago within this newly formed crust. After about 21 m.y. ago cooler and therefore stronger Pacific crust became juxtaposed to the North American plate; this caused an eastward or inboard shift in transform faulting. Eocene rocks from the northern Channel Islands resemble age equivalent rocks from the San Diego; and these data combined with lithologic provenance studies of middle and late Miocene conglomerate and breccia beds on Santa Cruz Island suggest as much as 180 km of right slip on the East Santa Cruz Basin fault in middle and/or late Miocene time. Additional right slip of as much as 100 km is postulated along a borderland fault zone east of the Patton Ridge.

Basalt-rhyolite volcanism commenced in the southern Basin and Range province about 20 m.y. ago. Southward in the area of the proto-Gulf of California, extension was associated with a broad volcano-tectonic rift zone. West northwest-east southeast extension in these areas may be as great as 50 to 200 km. In southern California the San Andreas fault system records 300 km of right slip during the past 12 m.y. The combined movements across all of these faults and extensional areas represent cumulative displacement of as much as 1000 km between the Pacific and North American plates during the past 29 m.y.