RHEOLOGICAL PROPERTIES OF THE MANTLE LID BENEATH THE MOJAVE REGION

Recently deformed mantle peridotite xenoliths derived from Moho depths constrain the geothermal gradient, stress magnitude, viscosity, and degree of localization in the upper 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 olivine with ~250 ppm H/Si. Differential stresses measured using olivine paleopiezometry are 12-15 MPa, several tens to hundreds of MPa less than stresses estimated for the gabbroic lower crust and the brittle upper crust. Similarly, a calculated mean viscosity of 5 x 10¹⁹ Pa-s for the uppermost mantle is approximately two orders of magnitude less than estimated for the lower crust, consistent with recent models of the Mojave region based on postseismic relaxation following the Landers (1992) and Hector Mine (1999) earthquakes. These results confirm that the Mojave region fits a `creme brulee' model of lithospheric rheology structure, in which the peak stress resides in the crust, rather than within the lithospheric mantle. 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 at least four times higher than bulk strain rates estimated across the central Mojave region from GPS-constrained surface velocities. This implies either 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; or alternatively, that a zone of high strain just below the Moho accommodates differential motion of upper and lower lithospheric layers.