3D shear velocity structure beneath the Gulf of California from Rayleigh wave dispersion

Active extension in the Gulf of California is characterized by the transition from continental rifting to seafloor spreading. Puzzling variations in the patterns of both tectonics and magmatism are observed along the length of the gulf and are likely to be related to mantle heterogeneity. Regional-scale mantle structure, however, has been difficult to constrain due to the lack of broadband seismic stations in the region. In this study we utilized new data from the deployment of the NARS-Baja array and other networks, and computed a three-dimensional shear-speed model of the upper mantle beneath the region. Applying a combination of cross-correlation analysis and multimode waveform inversion, we measured interstation Rayleigh wave dispersion for 450 pairs of stations in a broad period range of 9–250 s. We computed phase velocity maps and then inverted the phase-velocity data for shear-speed structure. Our results suggest that the location of the transition from seafloor spreading (South) to continental rifting (North) in the Gulf of California, as well as differences in volcanism across Baja California and the gulf, can be explained by heterogeneity in the upper mantle, in particular by the presence of a slab remnant beneath the south-central part and an absence of such a slab remnant beneath the northern part of the gulf.