Bathymetric and magnetic ship surveys of the southern continental borderland (the southern portion of a submarine province of ridges and basins west of northern Baja California) reveal the following structural features: (1) a fault zone trending south-southeast from Islas Los Coronados to Bahia Todos Santos, (2) southeast-northwest faults within the borderland, especially a trend toward San Clemente Island, (3) the Santo Tomas fault zone trending west-southwest across the borderland from Bahfa Soledad [(1), (2) (?), and (3) are associated with the continental Agua Blanca fault], (4) several east-west fault zones, (5) faults along the seaward margin of the borderland, (6) very large geomagnetic anomalies associated with the present coast line, (7) north-south geomagnetic anomalies in the deep-sea crust, (8) many minor faults, and (9) broad folding of the borderland. The recorded seismicity is associated only with the first three. Most (if not all) of the faults are observed or interpreted to dip steeply. Many are observed or interpreted to have large strike-slip components of displacement in addition to obvious dip-slip components.
Heat-flow measurements by Von Herzen (1964) reveal that heat flow increases to the east and probably indirectly causes the strength of the crust under the borderland to diminish to half the strength of the oceanic crust to the west where heat flow is normal. This largely explains the severe deformation of the borderland crust relative to the oceanic crust. The high heat flow helps to explain the ubiquitous volcanic rocks. Isostatic computations suggest a test for determining some structure and history of the underlying mantle.
The development of the continental borderland apparently began in the Mesozoic and reached its peak during the Miocene Epoch. It is still proceeding. The borderland results from a combination of many processes including right-lateral faulting between the oceanic basin and the continent plus east-west dilation.