UPPER MANTLE SEISMIC STRUCTURE AND IMPLICATIONS FOR SUPPORT OF THE WESTERN U.S. CORDILLERAN UPLIFT

The negligible to modest geoid (~0-12m) of the western U.S. cordillera, ~1.5 km mean elevation, indicates near isostatic equilibrium with a maximum depth of compensation in the shallow upper mantle. Receiver function and Pn analyses of EarthScope (ES) data provide a new level of constraint on crust thickness confirming the long-established interpretation that anomalous mantle structure is necessary to support the cordilleran uplift. Thus, knowledge of upper mantle structure is important for identifying the geodynamic processes responsible for uplift. We invert teleseismic travel-time residuals from the ES Transportable Array and more than 1700 additional stations for 3-D P and S velocity perturbations. The inversion uses recent advances in crust models to better isolate the mantle component of residual times, and frequency-dependent 3-D sensitivity kernels to map residuals, measured in multiple frequency bands, into velocity structure. In addition to separate V_P and V_S models, we jointly invert the two datasets for V_P/V_S perturbations, which helps identify regions where non-thermal variations, such as partial melt, contribute strongly to the imaged structure. The magnitude of V_P, V_S, and V_P/V_S variations is greatest in the upper 200 km and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and active small-scale convection in diverse tectonic settings. We find long-wavelength regions of low-velocity mantle beneath the Basin and Range and southern Rocky Mountains bisected by a northeast trending swath of high-velocity mantle underlying the Colorado Plateau and extending across the continental divide to northeast Wyoming. Unreasonably large mantle temperature variations, up to ~900 C at 100 km depth, are required if the peak-to-peak velocity anomalies are attributed to temperature. Furthermore, high velocity mantle underlies some of the highest long-wavelength topography, exacerbating previously recognized problems with invoking thermal expansion for support of high topography in Wyoming. Our P, S, and Vp/Vs tomography results indicate that large lateral temperature variations accompanied by variations in partial melt and bulk composition are necessary to reconcile upper mantle seismic structure with isostatic support for the cordilleran uplift.