Abstract
The active San Andreas fault system today lies at the splintered boundary between the Pacific and North American lithospheric plates, a tectonic arrangement that originated in California in the Oligocene. By late Miocene time sedimentary breccias derived from San Andreas scarps were being shifted laterally from their source areas. During the Pliocene the system extended from the Mendocino triple-junction to the head of the Gulf of California. The Big Bend in the San Andreas developed and the Transverse Ranges rose, in association with the opening of the Gulf of California.
The boundary between the Pacific and North American plates is wide and fragmented and during the late Cenozoic, the crust has yielded laterally by simple shear and deformed through pull-apart stretching and fault-block squeezing and uplift and rotation. Splay faults, such as the San Jacinto, Calaveras, and Hayward, branch from the main fault near constraining bends. The strewn out arrangement of displaced slices may account for displacements of 600 km on the northern San Andreas and only 300 km on the southern across a wide mobile boundary zone that has a total displacement exceeding 1000 km. Strike slip may also have occurred along faults parallel to the system in Cretaceous-Palaeocene times when the Kula Plate was moving laterally northwestward with respect to the North American Plate.
