SEISMIC CONSTRAINTS ON SMALL-SCALE CONVECTION IN THE WESTERN U.S. UPPER MANTLE

New P- and S-wave upper mantle tomography images provide strong constraints on the form and magnitude of small-scale convection active beneath the western U.S. orogenic plateau. We invert travel-time residuals from the EarthScope Transportable Array and more than 1500 additional temporary and permanent stations for three-dimensional velocity structure to a depth of 800 km. Our tomographic inversion method uses approximate finite-frequency sensitivity kernels to interpret travel-time residuals in multiple frequency bands and recent advances in western U.S. crust thickness and velocity models to better isolate the mantle component of travel-time residuals. Use of all available data results in improved lateral, depth, and magnitude resolution, and affords an opportunity to reconsider the causes and consequences of lithospheric activity beneath the region. Prominent high-velocity features of the tomography include extensively studied and more recently hypothesized convective downwellings, several of which have adjacent low-velocity anomalies that appear to represent partially molten ascending asthenosphere. Continuous, high-resolution P- and S-wave tomograms of the entire western U.S. upper mantle illuminate differences between small-scale heterogeneities, and also permit comparisons of mantle structure between larger-scale geologic regions that have experienced varying degrees of crustal deformation and surface uplift. The geologic location, geometric form, and amplitude of velocity heterogeneity suggests complex subduction history as well as lateral variations in lithospheric structure may give rise to apparently widespread small-scale convection beneath the western U.S.