RHEOLOGY OF THE LOWER CRUST AND UPPER MANTLE IN THE ACTIVE BOUNDARY BETWEEN THE PACIFIC PLATE AND BAJA CALIFORNIA MICROPLATE

How strong is the lower crust compared with the upper mantle in a plate bounding, strike-slip fault zone? Does the strength of the two lithospheric layers change during the seismic cycle? To address these questions, we studied mafic granulite and lherzolite xenoliths from the Holocene San Quintin volcanic field, in western Baja California (Mexico). The xenoliths have been extracted from near the Baja California Shear Zone (BCSZ), which accommodates the relative movement between the Pacific plate and Baja California microplate. Geothermometry and phase equilibria modeling show that crystal-plastic deformation in the granulite xenoliths took place at temperatures of 750–900 °C and pressures of 400–580 MPa, corresponding to lower crustal depths of 15–22 km. Based on the similarity of the equilibration temperatures between the lower crustal and upper mantle xenoliths (800–950 °C), we infer near isothermal conditions in the deep crust (20–30 km). Plagioclase in the mafic granulites is relatively dry (7–317 ppm H₂O) and has been deformed at differential stresses (12–33 MPa) similar to those in the upper mantle (17 MPa). In the granulite xenoliths, microstructures associated with higher stresses are interpreted to form during transient events, while the microstructures in the samples recording lower stresses are interpreted to form during decreasing stress conditions. In both the lower crust and upper mantle, microstructures show deformation by a combination of dislocation creep and diffusion creep, with a significant contribution from grain boundary sliding. Using dry plagioclase and olivine flow laws, we estimate that the viscosity of the lower crust (~10¹⁸ Pa s) at <20 km depth is two orders of magnitude lower compared to the viscosity of the shallow upper mantle (~10²⁰ Pa s). These results indicate the presence of a low-viscosity zone in the lower crust, which may act to transfer the displacement field from the mantle to the upper crust, coupling the two lithospheric layers. Comparing the microstructural and rheological constraints from the xenoliths with the results from post-seismic relaxation studies, we suggest that lower crust is stronger during transient deformation (e.g., post-seismic relaxation period) while the upper mantle becomes stronger during long-term deformation (e.g., interseismic period).