We have prepared gravity anomaly maps of the onshore-offshore region of northwest Mexico and southern Alta California. Plates 3 and 4 of this volume show free-air gravity anomalies at sea and simple Bouguer anomalies on land, contoured at a 10 mGal interval. We present eight modeled geophysical sections which cross the peninsula of Baja California, the Gulf of California, the Salton Trough, the Laguna Salada, and mainland Mexico. Three additional geophysical sections cross the Rivera Fracture Zone. The northern sections show a crust about 12 km thick beneath the outer Continental Borderland. The crustal thickness increases to about 17 km beneath the inner Borderland and reaches about 28 km maximum thickness beneath the northern Peninsular Ranges. The maximum crustal thick ess beneath the central and southern Peninsular Ranges is about 22 km. The crust beneath the Salton Trough and the Laguna Salada thins to about 21 km minimum. Beneath the northern Gulf of California the crust thins to 13 km minimum and under the central Gulf it is about 10 km thick. In both the northern and central Gulf the thinnest crust is found near

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the peninsular coast. Beneath the southern Gulf, the crust is about 8 km thick and is no longer asymmetrical. A low density upper mantle (3.10 to 3.15 gm/cm³) is found beneath the entire Gulf. Its volume increases to the south.

Based on onshore geology, section densities, and the mapped anomalies, we trace Franciscan-like rocks continuously beneath the western margin of the peninsula and borderland. Similarly, Mesozoic metamorphic and batholithic rocks that make up the northern Peninsular Ranges continue under the Tertiary volcanics of the central and southern Peninsular Ranges. High density units (2.82 to 2.90 gm/cm³) are emplaced in the upper crust beneath parts of the borderland and Bahia Sabastian Vizcaino.

Mapped anomalies show continuations of transform faults beneath the sediment cover on the flanks of the Gulf, and reveal linear basement highs and lows under the west side of the peninsula. Mapped anomalies also emphasize the continuity of basement highs and lows in the borderland.

The ridges and troughs of the Rivera Fracture Zone are reflected in the crustal structure at depth. The crust thins underneath the ridges and thickens beneath the troughs, probably caused by alteration of the crust and upper mantle by faulting and hydrothermal activity.