LettersMicrostructural and seismic properties of the upper mantle underneath a rifted continental terrane (Baja California): An example of sub-crustal mechanical asthenosphere?

The Gulf of California rift is a young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. The xenolith bearing lavas of the San Quintin volcanic area provide lower crust and upper mantle samples from beneath Baja California peninsula. The microstructures, crystallographic preferred orientations (CPO) and petrology of the San Quintin xenoliths suggest that the continental lithosphere in this region has undergone several stages of deformation, recrystallisation and melt-rock interaction. Melt-rock interactions have led to enrichment in olivine while fine-grained microstructures suggest intense deformation in an active shear zone in the shallow upper mantle. In this study we highlight the effect of the fine-grained mylonitic shear zone development in the upper mantle as an important process of weakening of continental lithosphere. The results of the microstructural study show a reduction in CPO strength with increasing grain size reduction. Most CPOs are consistent with dominant slip on the {0kl}[100] system. As a consequence, corresponding seismic anisotropies decrease for both P- and S-waves with increasing grain size reduction. The shallow crystallographic fabric can be related to active shear zones, which accommodate the relative motion between the Northern Baja terrane and the Pacific plate. Estimates of the strain rate, stress and viscosity indicate that the shallow mantle beneath Northern Baja is thermally and chemically lithospheric but mechanically has similar viscosity as the asthenosphere. The Northern Baja terrane is an interesting case of continental crust lying directly on low viscosity upper mantle.

► We investigate xenoliths from Baja California which sample the mantle below the Gulf. ► Xenolith microstructures show increasing deformation toward the crust-mantle boundary. ► Crystallographic preferred orientations suggest increasing anisotropy with depth. ► Xenolith anisotropy is compared with depth-resolved seismic anisotropy below the Gulf. ► The data are consistent with a very weak mantle immediately below the crust.