CRUSTAL STRUCTURE OF THE PENINSULAR RANGES BATHOLITH FROM AEROMAGNETIC DATA: IMPLICATIONS FOR GULF OF CALIFORNIA RIFTING

A 70-km-wide belt of magnetic highs extends ~1400 km northwest from the tip of Baja California peninsula into southern California, approximately parallel to the Gulf of California/Salton Trough. The anomalies are caused by the more mafic western belt of the Mesozoic Peninsular Ranges batholith (PRB), which is exposed extensively along the northern 1000 km length of the magnetic belt (north of 280N latitude). On the basis of the magnetic data, the PRB extends another 400 km to the southeast. Modeling indicates that the source of the anomalies extends to lower crustal depths (~20 km). The linearity and continuous nature of the Mesozoic magnetic belt support the hypothesis that the peninsula behaves as a rigid block and did not experience significant (>30 km) cumulative offsets by transverse strike-slip faults during the Tertiary. In addition, the eastern edge of the more mafic PRB coincides approximately with late Cretaceous shearing, the westernmost extent of denudation of the PRB during the Oligocene and Eocene, and the western limit of the Gulf Extensional province during the middle and late Miocene. Restoration of Baja California does not show a clear continuation of the PRB magnetic belt southeast along the Mexican mainland. Therefore, we infer that the coherent Baja crustal structure does not extend across the mouth of Gulf of California. We speculate that this narrow 1400-km-long mafic body behaved, relative to its surrounding crust, as a rigid structural element that maintained the integrity of the Baja peninsula during rifting in the Gulf of California and that rifting was focused along its eastern edge.