Great Basin Mantle Xenoliths Record Active Lithospheric Downwelling Beneath Central Nevada

Abstract

Removal of mantle lithosphere by Rayleigh-Taylor (R-T) instabilities is invoked to explain the formation of high plateaus and mountain ranges. Here we report geochemical and microstructural observations from mantle xenoliths from Lunar Crater volcanic field, central Nevada, which we interpret to directly sample a R-T instability beneath the Basin and Range. The xenoliths comprise a suite of mylonitic and granular peridotites with fertile and refractory major and trace element compositions, suggesting a mantle lithospheric origin. Temperatures calculated using several geothermometers are 1,200-1,300°C, in contrast to xenoliths from other localities in the Basin and Range (typically~1,000°C). High Lunar Crater temperatures suggest the xenoliths originate from the base of the mantle lithosphere. The mylonitic peridotites exhibit olivine deformation microstructures characteristic of deformation in the dislocation creep regime; orthopyroxenes experienced brittle deformation. Recrystallized grain sizes (~80 μm) suggest the mylonites deformed at~50 MPa. Recrystallized olivines demonstrate isochemical deformation, suggesting the~50-MPa differential stresses were achieved at~1,200°C, implying strain rates of 2 × 10 −9 to 4 × 10 −7 /s, and corresponding to effective viscosities of 8.9 × 10 13 to 1.4 × 10 16 Pa/s. The most plausible mechanism for producing such conditions in the deep lithosphere beneath central Nevada is extreme strain localization within an actively deforming R-T instability. The most highly strained samples have relatively low effective viscosities, suggesting that strain is preferentially partitioned into weaker rocks within the deforming lithosphere. This study highlights the importance of strain localization and associated weakening as mechanisms for facilitating lithospheric R-T instabilities.

Coseismic fault lubrication by viscous deformation

“…At high magnifications, within the PSZs, grains show low aspect ratio 22 (on an average, nearly equant), display diamond-shapes, quadruple junctions, narrow gaps and numerous grain boundaries aligned with the shear direction (Supplementary Figure S12a-d and Supplementary Section III). These textures are compatible with mechanisms of neighbour-switching that are typical of grain boundary sliding 32,33 (GBS, Figure 3e). Microstructural analysis in calcite experiments suggests diffusion-accommodated GBS 19,20,22 , although we do not exclude dislocation-accommodated GBS for the other materials.…”

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

“…San Quintin, Cabanes and Mercier (1988); Eifel Germany, Witt-Eickschen et al (2003); Ivrea Zone, Jin et al (1998); Kilbourne Hole, Satsukawa et al (2011); Navajo, Behr and Smith (2016); Norway, Pasillas (2015); Newfoundland, Varfalvy et al (1996); Lunar Crater, Dygert et al (2019). e SIMS analyses from this study; FTIR analyses from Bernard and Behr (2017).…”

“…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 (). Regional T ranges from the following studies: San Carlos, Galer and O'Nions (); San Quintin, Cabanes and Mercier (); Eifel Germany, Witt‐Eickschen et al (); Ivrea Zone, Jin et al (); Kilbourne Hole, Satsukawa et al (); Navajo, Behr and Smith (); Norway, Pasillas (); Newfoundland, Varfalvy et al (); Lunar Crater, Dygert et al ().…”

Tectonic Control on Drainage Evolution in Broken Forelands: Examples From NW Argentina

“…In both the Basin and Range Province and the Apennines, widely spaced active normal faults (∼30 km) produce large intermontane basins and significant relief contrasts between basins and ranges (D’Agostino et al., 2001; Densmore et al., 2005; Dickinson, 2002; Zuber et al., 1986). In addition to this short wavelength (∼30 km) basin and range style topography, a longer wavelength (∼200 km), much more subtle topography is often superimposed in extensional regions (e.g., D’Agostino & McKenzie, 1999; Stewart, 1980; Zuber et al., 1986), perhaps due to dynamic processes in the upper mantle (e.g., Dygert et al., 2019). Elevation contrasts between individual basins and ranges are larger compared to regional topography in extensional regions than thin‐ or thick‐skinned wedges.…”