HIGH-RESOLUTION LITHOSPHERE VISCOSITY AND DYNAMICS REVEALED BY MAGNETOTELLURIC IMAGING

An uncertain viscosity structure is a major impediment to correctly modeling the behavior and evolution of the lithosphere. This problem is particularly acute at the sub-100 km scale where active deformation and volcanism occur. Here we report an attempt to infer the effective lithospheric viscosity from a high-resolution magnetotelluric (MT) survey across the western United States. The increased sensitivity of MT fields to the presence of electrically conductive fault-occupying fluids makes it a promising proxy for dictating small-scale variations in the mechanical strength of the lithosphere. We demonstrate how a viscosity structure, approximated from electrical resistivity, results in a geodynamic model that successfully predicts short-wavelength topography, deformation across the Basin & Range province, and localized mantle upwelling beneath a region of late Cenozoic volcanism. Comparison with observations suggests that MT imaging provides a powerful constraint on both the net strength and lateral variation of lithospheric viscosity. We propose that MT imaging represents a novel observational constraint for revealing the dynamic evolution of the crust and upper mantle.