The California continental borderland consists of two geomorphic provinces: the northern borderland which forms the Pacific margin off southern California and the southern borderland which forms the margin off the northern Baja California peninsula. Although these two provinces are longitudinally continuous, bathymetric, geophysical, and bottom sample data suggest that their structure, lithology, and tectonic evolution differ markedly. Relative to the northern borderland, the southern borderland is on the average much deeper (0.5 to 1 km), ubiquitously volcanic (basaltic), and from heat flow and isostatic considerations, underlain by much thinner crust. Seimsic reflection profiles across the southern borderland show a thin veneer (generally < 300 m) of relatively undef rmed strata overlying an irregular nonstratified acoustic basement. Dredge hauls from exposed basement highs along these profiles have yielded chiefly basaltic rocks.

Major northwest-southeast-trending synclinoria bound the northwestern and southeastern limits of the southern borderland (Valero Basin and Vizcaino Bay, respectively), and the synclinal axes of these basins strike directly into the southern borderland. Seismic refraction data across these synclinoria indicate that they contain more than 3 km of sedimentary strata (above 6.6 km/sec basement) or at least 10 times the thickness of sedimentary strata recognized on seismic reflection profiles that cross the southern borderland.

The available data suggest that the southern borderland is a rifted segment of the Pacific margin. The extent of rifting is estimated to be about 260 km on the basis of basalt outcrops and seismic reflection profiles. Rifting had probably begun by 17 m.y. ago, and is inferred to be related to the southward migration of the Rivera triple junction.

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