A 70-km-wide belt of magnetic highs extends ???1200 km northwest from the southern tip of the Baja California peninsula into southern California. The anomalies are caused by the mafic western belt of the Peninsular Ranges batholith, which is exposed extensively along the northern 800 km length of the magnetic belt. Modeling indicates that the source of the anomalies extends to mid-to lower crustal depths. The linearity and undisrupted nature of the magnetic belt support the hypothesis that the peninsula has behaved as a rigid block and has resisted significant (>50 km) strike-slip deformation during the Tertiary. The absence of a corresponding magnetic belt along the coast of mainland Mexico to the south suggests that the location of rifting in the Gulf of California was influenced by the mafic Western Peninsular Ranges batholithic crust, perhaps because it is thermally and mechanically more resistant to deformation than the surrounding crust.