Fabrics and Rheology of the Mojave Lower Crust and Lithospheric Mantle

Tuesday, 16 December 2014
Rachel E Bernard and Whitney M Behr, University of Texas at Austin, Austin, TX, United States
We use xenoliths from young (3 Ma to present) cinder cones in the tectonically active Mojave desert region of southern California to characterize the rheological properties of the lower crust and upper mantle. The xenoliths were collected from two localities ~90 km apart: the Cima and Dish Hill volcanic fields. The xenolith suites represent a depth range of ~25-60 km and include spinel and plagioclase facies peridotites and lower crustal gabbros. We document how stress, temperature, water content, deformation mechanism, lattice preferred orientation, and style of localization vary with increasing depth in both xenolith suites.

Key findings thus far include the following:

(1) Both xenolith suites exhibit a wide range of deformation textures, ranging from granular, to protogranular, to porphyroclastic and mylonitic. The 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. This is consistent with recent models of upper mantle post-seismic relaxation following the Landers and Hector Mine earthquakes, which require a component of power-law creep in order to fit the post-seismic surface response.

(3) A- and E-type olivine LPOs occur in both xenolith suites. Further work will determine whether these fabrics are related to changes in water content as inferred from experimental studies.

(4) Deformation in most lower crustal gabbros is weak, but some show strong fabrics associated with plagioclase-rich zones.

(5) Measurements of olivine subgrain sizes in Dish Hill samples are similar to previously published measurements from Cima, suggesting similar stress magnitudes at depth in both locations. Paleopiezometers for olivine and plagioclase indicate stress magnitudes of 11-20 MPa for the uppermost mantle, and 0.1 MPa for the lowermost crust.