Marine geophysical data from the continental margin of Baja California and the Gulf of California, and geological and geophysical data from the Baja California Peninsula and mainland Mexico, outline the major geologic and tectonic features of the Baja California Peninsula and the surrounding areas from 24.5° N. Lat. to 27.5° N. Lat. A crustal and subcrustal cross section consistent with observed gravity and magnetic anomalies, and constrained by seismic refraction stations and the mapped surface geology shows major variations of density and magnetization in these areas. A geologic interpretation of the cross section indicates the rocks of the Pacific continental margin are composed of unconsolidated and semiconsolidated sediments. Tertiary sedimentary rock, metamorphic rock of Franciscan type, and continental crystalline rock probably corresponding to the Peninsular Range batholith. The depth to mantle under the Baja California Peninsula is postulated to be 20 km. In the Gulf of California a section of low-density mantle beginning at a depth of 11 km is necessary to fit the observed gravity values and accounts for the low seismic velocities associated with the mantle in the Gulf. The correlation between the observed magnetic anomalies on the Pacific continental margin of the Baja California Peninsula and the theoretical magnetic anomalies expected from a spreading center shows that the youngest identifiable remanent anomaly on the Pacific side of south central Baja California is anomaly 3' formed at 6 my B.P. The remanent magnetic anomalies extend 50 km landward from the western edge of the continental slope. / Graduation date: 1978

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