"Crustal deformation, and associated seismic hazards, along active strike-slip faults are likely to be highly dependent on broad shear zones that occur within the uppermost mantle. The zones of localized mantle flow are physically continuous with crustal faults, and may control the bulk rheology of these systems. The main goal of this proposal is to characterize the rheological properties of naturally deformed mantle rocks and to provide insights into the control of mantle processes on deformation in lithospheric fault zones. We will utilize the mantle rocks exhumed in three major strike-slip fault shear zones (San Andreas Fault, US; Bogota Peninsula Shear Zone, New Caledonia; and Southern Pindos-Vourinos Shear Zone, Greece) as ""natural laboratories"". Through a powerful combination of conventional and emerging micro-analytical techniques, we will determine 1) microstructural relationships among constituent minerals, 2) shape preferred orientation of phases, 3) crystallographic preferred orientation using Electron Backscatter Diffraction (EBSD), 4) water content of major phases by means of Fourier Transform Infrared (FTIR) spectroscopy and nano Secondary Ion Mass Spectrometry (nanoSIMS) and 5) deviatoric stress through paleopiezometry. These methods will be used, in aggregate, to constrain the deformation mechanisms that promote weakening and strain localization in the upper mantle, with emphasis to the effect of water content and grain size reduction. Moreover, we will quantify viscosity and seismic anisotropy changes induced by strain localization in the mantle that can be used to refine interpretations of geophysical datasets. Finally, we will apply mathematic techniques (Lie groups and Lie algebras) to statistically characterize fabrics within deformed rocks, in order to recognize the presence of strain/fabric gradients, strain history, and strain-dependent deformation processes. Research will be carried out at the Universities of Wisconsin-Madison and Utrecht."