LINKS BETWEEN SURFACE TECTONICS AND MANTLE LITHOSPHERE DEFORMATION IN THE WESTERN US CORDILLERA: CONSTRAINTS FROM MANTLE XENOLITHS (Invited Presentation)

Xenoliths sourced from Western US volcanoes provide insight into how Cenozoic surface deformation connects to the deeper lithosphere. Here I use petrological and microstructural data from deformed xenoliths to demonstrate a range of processes by which the mantle lithosphere contributes to surface deformation, including in accommodating a) basal shear tractions, b) mantle lithospheric drips, and c) localization of faults at the surface to depth. The xenolith suites I focus on include the Oligocene Navajo Volcanic Field in the Colorado Plateau, the Lunar Crater Volcanic Field in Central Nevada and the Cima & Dish Hill volcanic fields in the Mojave desert of Eastern California. All three suites are sourced from different pressure-temperature conditions and different tectonic environments— the one thing they all have in common is deformation to relatively high strains.

• The Navajo Volcanic field includes mylonites and ultramylonites that have been deformed under hydrous, cold conditions. These are interpreted to represent shear within and above the contact between the subducting Farallon slab and the upper plate North American mantle lithosphere during Laramide flat-slab subduction. The stress magnitudes recorded and the inferred effective viscosities in these peridotites are high enough to suggest that the Laramide flat-slab imparted enough shear stress to the upper plate to influence upper plate topography.
• Xenoliths from the Lunar Craters volcanic field record a very different history. This crater is youthful (<1 Ma), and resides within the Basin & Range. The crater location coincides with a well-imaged P-wave anomaly oriented vertically with the upper mantle, interpreted by previous workers to represent lithospheric downwelling beneath the Great Basin. High temperature, moderate stress mylonitic textures within these xenoliths suggest very low viscosities, and are interpreted to reflect deformation in the deep mantle lithosphere along the margin of this lithospheric drip.
• The context of the Mojave mantle xenoliths is also unique, as the Mojave volcanic fields reside within the Eastern California Shear Zone (ECSZ), which is part of the San Andreas Fault System. Our work there suggests that deformation is likely localized in the mantle lithosphere beneath ECSZ upper crustal faults.