The Southern California Continental Borderland and the associated Western Transverse Ranges constitute one of the most distinctive environments on the west coast of North America. Current thinking indicates that the physiography of the region resulted from change in plate motion during the Miocene when a remnant of the Farallon Plate was captured by the Pacific Plate off Southern California. This capture led to extensional deformation of the major upper plates within the subduction zone, rotation and translation of large crustal blocks in the region, and widespread volcanism (e.g., Nicholson et al., 1994). The continental microplate that encompasses the Western Transverse Ranges province eventually rotated at least 90° clockwise, and intense crustal extension affected parts of the region that became the Borderland. Subsequent deformation, including the development of the San Andreas Fault system, led to north-south compression in parts of the region and possible westward escape of crustal blocks. The resulting complex configuration of basins and ranges continues to be seismically active and contains small-scale examples of such physiographic features as canyons, fans, and continental slopes that bear strong similarity to much larger features found in the major oceanic basins.
