Complex subduction and small-scale convection revealed by body-wave tomography of the western United States upper mantle

Abstract

New high-resolution P- and S-wave tomography of the United States 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 EarthScope Transportable Array and more than 1700 additional temporary and permanent stations for 3-D velocity perturbations to a depth of 1000 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 V P and V S models, we jointly invert the two datasets for V P/V S perturbations by imposing a smoothness constraint on the δ lnV S/δ lnV P field. The joint inversion helps us identify regions where partial melt is probable. The amplitude of V P, V S, and V P/V S 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. 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. Despite continuous subduction since the Cretaceous, the distribution of sub-lithospheric high-velocity anomalies is dissected (similar to other recent studies). Based on our 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.