Gravity, magnetic and seismic data indicate that the oceanic crust is 9.7 km thick west of the Continental Borderland. The top of the mantle is about 12 km deep under the Borderland, and deepens to 27 km beneath the Peninsular Ranges of Baja California. The mantle is about 20 km below the surface of the Imperial Valley and deepens to 27 km under the area east of the Imperial Valley. The age of the youngest detectable remnant magnetic anomaly over the oceanic crust is about 16.5 million years at 21.3°N Lat. and decreases to the south. A magnetic anomaly expected along the continuation of the San Benito Fault Zone is not detected by this study. A gravity low along the base of the Patton Escarpment is at least partially the result of a buried trench-like depression. In the vicinity of 31.3°N Lat., 119.3°W Long. this depression is filled with 2 km of sediments. The geophysical and geological data are interpreted as indicating a 6 km thick section of Franciscan rocks that extends from the west edge of the Borderland to the Coronado Escarpment. Magnetic data suggest that an ophiolite may be present within or on top of the Franciscan rocks. Several of the ridges in the Borderland have cores of high density rocks which are interpreted as intrusives. The area just south of the San Clemente basin has an anomalously thin upper crust. The gross crustal structure of this region is comparable to the Imperial Valley region and may represent a former site of crustal rifting which occurred when the East Pacific Rise was subducted under this part of the North American plate. North of the Santo Tomas Fault Zone are several basins filled with more than 3 km of sediments, but south of this fault zone the sediment cover is discontinuous and generally less than 2 km thick. / Graduation date: 1978