Constraints from Xenoliths on the Rheology of the Mojave Lower Crust and Lithospheric Mantle

Abstract:

We use xenoliths from young (3 Ma to present) cinder cones in the tectonically active Mojave region of southern California to characterize the rheological properties of the lower crust and upper mantle beneath the Eastern California Shear Zone. The xenoliths, which include spinel and plagioclase facies peridotites and lower crustal rocks (representing a depth range of ~25-60 km), were collected from two localities ~80 km apart: the Cima and Dish Hill volcanic fields. We document how stress, temperature, water content, deformation mechanism, lattice preferred orientation (LPO), and style of localization vary spatially and with depth. Key findings include the following:

(1) Both xenolith suites exhibit a wide range of deformation textures, ranging from granular, to protogranular, to porphyroclastic and mylonitic. Higher strain fabrics show no evidence for static annealing, thus are likely reflecting youthful deformation and strain gradients at depth.

(2) Both xenolith suites show abundant dynamic recrystallization and other evidence for dislocation creep as the dominant deformation mechanism.

(3) A- and E-type olivine LPOs occur in both xenolith suites. In general, E-type LPO is associated with higher strain fabrics than A-type.

(4) Water contents—found using Fourier transform infrared spectroscopy (FTIR) and Secondary Ion Mass Spectrometry (SIMS)—range from 115-254 ppm for clinopyroxene, 35-165 ppm for orthopyroxene, and less than 10 ppm for olivine. We have found no correlation between water content and olivine LPO, despite experimental work associating higher water content with the development of E-type LPO, compared to A-type.

(5) Deformation in most lower crustal gabbros is weak, but some show strong fabrics associated with plagioclase-rich zones. Water content from clinopyroxene in one highly-deformed gabbro is <1 ppm.

(6) Paleopiezometers for olivine and plagioclase indicate stress magnitudes of 16-21 MPa for the uppermost mantle, and 0.1 MPa for the lowermost crust.