Rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure
beneath the Gulf of California. Low-velocity zones (LVZs) are found on axis between 26 and 50 km depth
beneath the Guaymas Basin but mostly off axis under the other rift basins, with the largest feature underlying
the Ballenas Transform Fault. We interpret the broadly distributed LVZs as regions of partial melting in a solid
mantle matrix. The pathway for melt migration and focusing is more complex than an axis-centered source
aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments.
We also consider the existence of solid lower continental crust in the Gulf north of the Guaymas Basin, where
the association of the LVZs with asthenospheric upwelling suggests lateral flow assisted by a heat source.
These results provide key constraints for numerical models of mantle upwelling and melt focusing in this
young oblique rift
