Rheological properties of the mantle lid beneath the Mojave region in southern California

- Xenoliths constrain the modern-day rheology of the Mojave mantle lithosphere.
- The Mojave mantle is weak compared to the lower crust.
- Deformation in the Mojave mantle is localized into narrow shear zones.

Recently deformed mantle peridotite xenoliths derived from Moho depths constrain the geothermal gradient, stress magnitude, effective viscosity, and degree of localization in the uppermost mantle within the tectonically active Mojave region of California. Microstructural observations and water content measurements in the xenoliths indicate that upper mantle deformation is accommodated by dislocation creep of modestly hydrated olivine. Differential stress measured in the xenoliths using olivine paleopiezometry is 13-17 MPa, which is at least one order of magnitude less than peak stresses estimated for the gabbroic lower crust and the brittle upper crust. Similarly, the mean effective viscosity of ∼3×1019 Pas for the uppermost mantle is one to two orders of magnitude less than the mean viscosity estimated for the lower crust, consistent with recent models of postseismic relaxation following the Landers (1992) and Hector Mine (1999) earthquakes. These results support a rheological model for the Mojave region in which the peak stress resides in the crust, rather than within the lithospheric mantle (consistent with the 'crème brûlée' model of lithospheric strength). Temperatures and pressures recorded in the xenoliths indicate a high geothermal gradient of at least ∼30 °C/km, which may explain the regional weakness of the mantle lid. Strain rates calculated for the uppermost mantle using the xenolith data and olivine flow laws for wet dislocation creep are ∼7-70 times faster than bulk strain rates estimated across the central Mojave region from GPS-constrained surface velocities. This suggests that faults at the eastern border of the Eastern California Shear Zone persist through the seismogenic zone and retain their identities as narrow ductile shear zones into the mantle beneath the Moho. The observation that recent deformation is localized into only 10-25% of the rock body may explain why upper mantle seismic anisotropy in the Mojave region is highly oblique to present-day plate boundary motion.