Seismic Thermometers: Vs Structure and Constraints on Mantle Temperature and Melting Beneath the Southwestern United States

Seismic wavespeed within the lithosphere and asthenosphere is influenced by multiple properties and processes, including temperature and melt fraction. These parameters are of particular interest in the southwestern United States where recent intraplate volcanism has occurred and inferred temperatures are high. Constraining the distribution and fraction of partial melt can help elucidate mechanisms of melt formation and percolation through the asthenosphere, the controls melt exerts over processes at the Lithosphere-Asthenosphere Boundary (LAB), and the interactions between melt and lithospheric structure. We introduce a new model of shear wavespeed obtained through a Bayesian joint inversion of common-conversion-point stacked Sp receiver functions (periods of 2-100 s) and fundamental mode Rayleigh wave phase velocities from earthquakes (periods of 25-180 s; Babikoff and Dalton, 2019) and ambient noise (periods of 6-40 s; Shen and Ritzwoller, 2016). From this inversion we generate 300 1-D profiles of Vs with depth across the southwestern US, quantifying absolute wavespeed as well as the depth and sharpness of discontinuities within the mantle, including the LAB and intralithospheric discontinuities. We observe strong negative velocity gradients at depths of 60-80 km below the Basin and Range region, interpreted as a shallow LAB. In the Colorado Plateau this decrease in Vs is generally weaker, deeper and distributed over a wider depth range. This contrast implies spatial variability in the thermochemical state of the lithosphere and/or asthenosphere, and that different mechanisms may control physical properties at the LAB. Using multiple approaches that relate seismic velocity to mantle properties (Havlin et al., 2021; Shinevar et al., 2022), we are modeling the Vs profiles to determine how seismic velocity provides constraints on temperature and partial melt fraction. We are also comparing the estimated melt fractions and geotherms with other geophysical and geochemical thermobarometers, including from xenolith and magmatic samples. This study provides an opportunity to explore agreement between seismic and geochemical modeling, to test theoretical models of anelasticity within the mantle, and to gain insight on variations in mantle melting processes in a complex tectonic setting.