Fabric heterogeneity in the Mojave lower crust and lithospheric mantle in Southern California

Abstract

AbstractWe use xenoliths from young cinder cones in the eastern Mojave region of Southern California to investigate deformation fabrics and their implications for strain localization, lithospheric viscosity, and controls on mineral lattice‐preferred orientations (LPOs) and seismic anisotropy at Moho depths. Lower crustal gabbros and upper mantle peridotites were collected from two areas separated by ∼80 km—the Cima and Deadman Lake Volcanic Fields. At both localities, mantle peridotites exhibit solid‐state deformation fabrics that show strong LPOs and other evidence for dislocation creep as the dominant deformation mechanism. The preservation of microstructures ranging from annealed, granular, to mylonitic indicates that there is substantial strain localization in the Mojave mantle near the Moho. Paleopiezometry conducted on subgrains in olivine indicates that stress magnitudes during deformation were 12–26 MPa. Calculations using olivine flow laws yield strain rates on the order of ∼10−12/s and an associated viscosity of ∼1019 Pa s, consistent with estimates of mantle lid viscosity from postseismic relaxation studies. Two types of olivine LPOs were observed in the peridotites: A‐type and E‐type fabrics, both of which predict seismic fast axes that align parallel to the lineation within the foliation plane. The occurrences of the two fabric types appear to correlate with strain magnitude but not with water contents measured using Fourier transform infrared spectroscopy. Lower crustal fabrics are dominated by magmatic foliations and LPOs in plagioclase, which produce seismic fast axes oriented perpendicular to the foliation plane. This suggests that even where mantle and lower crustal fabrics are kinematically linked, their seismic fast axes may appear anticorrelated across the Moho.