Abstract

Recent studies indicate that the heat flows on the southern California borderland are above average and appear to increase southward. They are similar to fluxes in the California Coast Ranges to the north, but less than those of the Basin and Range Province and Gulf of California. The thermal regime of the borderland represents an integrated record of tectonic events during Cenozoic time, modified to variable but unknown degree by surficial environmental effects such as sedimentation and hydrothermal circulation. The impingement upon the North American continent by the East Pacific Rise ~ 30 m.y.B.P. was responsible for an increased heat flux through the southern California borderland for perhaps 20 million years. This thermal anomaly diminished with time from north to south as the resultant ridge-trench-transform triple junction migrated southward to its present position at the mouth of the Gulf of California. Elevated temperatures in the continental crust and underlying mantle increased their ductility and facilitated crustal thinning, pull-apart and folding in response to extensional and/or lateral shear stresses. Present day heat-flow patterns and the postulated plate boundary thermal interactions are not inconsistent with a diversity of deformational styles on the southern and Baja California borderlands.