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

Abstract

We 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.

Great Basin Mantle Xenoliths Record Active Lithospheric Downwelling Beneath Central Nevada

“…Grain diameters—averaged for at least 100 grains per sample—were measured manually using this software. Results from this approach have been shown to agree with those of the linear intercept method, where grain size is estimated automatically from electron backscattered diffraction (EBSD) maps (Bernard & Behr, ). The experimentally derived empirical olivine paleopiezometer of Van der Wal et al () was used to estimate differential stress magnitudes from the average recrystallized grain sizes, after applying a correction factor of 1.2 to the average grain size to account for 2‐D sectioning (Drury, ; Van der Wal et al, ).…”

“…Such efficient strain partitioning is supported by theory (e.g., Handy, ; Huet et al, ; Reuss, ) and consistent with observations from ophiolitic mantle lithosphere, where high degrees of strain are concentrated within narrow mylonitic shear zones (e.g., Homburg et al, ; Linckens et al, ; Skemer et al, ). Populations of xenoliths recovered from western U.S. localities suggest similar strain localization in subcontinental mantle (e.g., Behr & Hirth, ; Behr & Smith, ; Bernard & Behr, ; Wernicke et al, ).…”

“…Investigation of lithospheric mantle xenoliths sourced from recent volcanoes in actively deforming regions can lend particularly important insight into features visible in high‐resolution geophysical images such as midlithospheric discontinuities (e.g., Abt et al, ), anisotropy variations (e.g., Schulte‐Pelkum & Mahan, ), Moho offsets (e.g., Miller et al, ; Zhu, ), and lithospheric drips or downwellings (e.g., West et al, ). The compositions, pressure‐temperature conditions, paleostresses, and lattice‐preferred orientations recorded by these xenoliths have been used to place constraints on deformation processes ranging from basal shear at the lithosphere‐asthenosphere boundary (e.g., Boyd & Nixon, ; Kennedy et al, ; Tikoff et al, ), localized deformation beneath major crustal fault zones (e.g., Bernard & Behr, ; Behr & Hirth, ; Wernicke et al, ), subhorizontal shear associated with differential lithospheric displacement (e.g., Behr & Smith, ; Tikoff et al, ), to vertical advance of buoyant diapirs sourced from the asthenosphere (e.g., Green & Guegen, ; Mercier, ).…”

Relationships Between Olivine CPO and Deformation Parameters in Naturally Deformed Rocks and Implications for Mantle Seismic Anisotropy

“…For these samples, T used in plots is based on median value within this range. Exact sample T values for Elephant Butte and Cerro Chato samples come from Byerly and Lassiter (2012); Dish Hill and Cima samples from Bernard and Behr (2017). Regional T ranges from the following studies: San Carlos, Galer and O'Nions (1989);…”

“…Measurements were averaged for at least 100 dynamically recrystallized grains or subgrains per sample. Prior work has demonstrated that this method yields virtually identical results compared to the linear intercept method conducted optically or using Electron Backscatter Diffraction (EBSD) data (Bernard & Behr, 2017). Average recrystallized grain sizes, after applying a correction factor of 1.2 to account for 2-D sectioning, were used to estimate deviatoric stress magnitudes during deformation based on the paleopiezometer of Van der Wal et al (1993).…”