Great Basin Mantle Xenoliths Record Active Lithospheric Downwelling Beneath Central Nevada

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 degrees C, in contrast to xenoliths from other localities in the Basin and Range (typically 1,000 degrees 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 (80m) suggest the mylonites deformed at 50MPa. Recrystallized olivines demonstrate isochemical deformation, suggesting the 50-MPa differential stresses were achieved at 1,200 degrees C, implying strain rates of 2x10(-9) to 4x10(-7)/s, and corresponding to effective viscosities of 8.9x10(13) to 1.4x10(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.