Multi-scale seismic heterogeneity and convection in the western U.S. upper mantle (Invited)

New P- and S-wave tomography of the U.S. upper mantle from the Pacific Coast to the Great Plains reveals strong multi-scale heterogeneity closely correlated with tectonic and magmatic activity. We invert teleseismic travel-time residuals from the USArray and more than 1700 additional temporary and permanent stations for 3-D velocity perturbations to a depth of 1200 km. The inversion uses recent advances in western U.S. crust models to better isolate the mantle component of travel-time residuals, and frequency-dependent 3-D sensitivity kernels to map travel-time residuals, measured in multiple frequency bands, into velocity structure. In addition to separate Vp and Vs models, we jointly invert the two datasets for Vp/Vs perturbations by imposing a smoothness constraint on the dlnVs/dlnVp field. The joint inversion helps us identify regions where partial melt is probable. The amplitude of Vp, Vs, and Vp/Vs variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small-scale convection. Unreasonably large mantle temperature variations, up to ~900 C at 100 km depth, are required if the entire magnitude of velocity structure is attributed to temperature. Partially molten mantle is inferred beneath Yellowstone and the eastern Snake River Plain (SRP), the Salton Trough, and the Clear Lake volcanic field. The inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern SRP and Yellowstone. A northeast trending swath of relatively high-velocity mantle extends from the Colorado Plateau to northeastern Wyoming suggesting that considerable compositional heterogeneity in the lithosphere is necessary to reconcile the high mean elevation and negligible geoid anomaly of the region. This swath of high-velocity mantle is juxtaposed against generally low-velocity mantle beneath the Basin and Range, Rio Grande Rift, and Colorado Rockies. Despite more than 150 My of continuous subduction, the distribution of sub-lithospheric high-velocity anomalies is dissected (similar to other recent studies). Based the new tomography models, western U.S. geologic history, and plate-tectonic reconstructions, we infer patchy and incomplete removal of the flat-subducting Laramide slab and slab tearing associated with Eocene accretion in the northwestern U.S. Broader geodynamic implications of our results include the potential for subducted ocean lithosphere to remain in the upper mantle for tens of millions of years and that the spectrum of convection in the shallow upper mantle has high power at short wavelengths (~100 km).