Measurements of upper mantle shear wave anisotropy from stations around the southern Gulf of California

Abstract

Upper mantle seismic anisotropy was quantified at stations around the southern Gulf of California. Measurements on the mainland can be explained by the absolute motion of the North American plate, implying lithospheric drag of the asthenospheric material below. They are consistent with the direction of Basin and Range extension during Miocene time. These observations agree with previous work in the Basin and Range province farther north. The fast E–W anisotropic direction at the southern tip of the peninsula can be explained by asthenospheric flow produced by the subducted Farallón plate, as previously proposed for stations in the northern half of the peninsula. Small delay times and null measurements are dominant in the southern half of the peninsula in contrast with observations in the northern peninsula. The observation of low anisotropy may be caused by the subducted fragments of the Guadalupe and Magdalena microplates. Alternatively, it could be explained by upwelling of hot material from the former Magdalena ridge through the broken Magdalena slab. Station NE75, in the middle of the peninsula, appears to record the transition from the northern to the southern domain.