Rheology of the Lithospheric Mantle Beneath the Mojave Region from Naturally Deformed Peridotite Xenoliths

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The aim of this project is to characterize the rheological properties of a lithospheric mantle section using a suite of naturally deformed peridotite xenoliths erupted from the dominantly Pleistocene Cima volcanic field in the tectonically active Mojave desert of California. The part of the Mojave from which the Cima xenoliths are derived is a region in which several models of both the transient and steady-state rheology structure of the lithosphere have been made from surface velocity measurements following the Landers (1992) and Hector Mine (1999) earthquakes. It is also a region where geophysical observations (including seismic velocity and attenuation, and receiver function studies) highlight a drop in shear velocity that is rapid in depth, interpreted to be the seismological lithosphere-asthenosphere boundary; the magnitude of the shear velocity drop is too abrupt to be explained by temperature contrasts alone. The compositional and rheological properties of the mantle lithosphere here are therefore critical to understanding what influences both the large-wavelength post-seismic signal, and the abrupt transition from lithospheric lid to low viscosity, low velocity asthenospheric mantle. The study will document how stress, temperature, water content, deformation mechanism, lattice preferred orientation and style of localization vary with increasing depth (down to at least 45 km and as much as 70 km) in the lithospheric mantle from which the Cima xenoliths are derived. These natural measurements of rheological parameters will be used to a) test the applicability of experimentally derived flow laws and predictions of LPO development to mantle rocks, b) develop a naturally constrained stress profile of the upper mantle in the Mojave region and c) examine how the rheological parameters measured might influence a range of larger-scale geophysical models for the region. Some of the critical questions to be addressed include: is the mantle lithosphere beneath this region wet or dry? Does deformation in the mantle occur in narrow zones, or is it widely distributed? Is the mantle lithosphere the strongest ?load-bearing? part of the continental lithosphere, the weakest, or perhaps somewhere in between?Results from this project will shed new light on how Earth?s rigid outer shell, the lithosphere, behaves in response to deformation along an active continental plate boundary. The PI and students will examine samples of the mantle section of the lithosphere entrained and brought to the surface by rapidly ascending magmas beneath young volcanoes. These entrained rocks, known as xenoliths, provide a unique perspective on the structure and physical properties of the lithosphere beneath some of the major earthquake-producing strike-slip faults in eastern California, including the faults that generated the Landers (1992, M=7.3) and Hector Mine (1999, M=7.1) earthquakes.