Seismic Constraints on Temperature, Partial Melt, and Lithosphere‐Asthenosphere Dynamics in the Southwestern United States

The southwestern United States hosts well‐documented and dramatic variations in topography, seismicity, heat flow, magmatism, and seismic velocity structure. This region offers opportunities to investigate how processes such as temperature and partial melt are related to the Lithosphere‐Asthenosphere Boundary (LAB), and how they ultimately shape lithospheric evolution. Using interpretive toolkits and a seismic wavespeed model that offers new high‐resolution constraints on upper mantle absolute velocities and gradients, we model the thermal structure and melt fraction throughout the upper mantle in the southwestern United States, with an emphasis on resolving the LAB. In the northern interior of the Colorado Plateau and in the Wyoming Craton, the LAB (as defined from the thermal models) is relatively deep and upper‐mantle melt is not likely. In the Basin and Range and southern Colorado Plateau, in contrast, the lithosphere is thin, and temperature is near the solidus in the upper mantle. Small fractions of partial melt are potentially stable in the asthenosphere over large tracts of the Basin and Range, and within the southern margin of the Colorado Plateau. Furthermore, the best‐fit thermal models are consistent with a transition layer between the lithosphere and asthenosphere, in which temperatures exceed the solidus. Such a scenario could be accommodated by small‐scale irregularities in LAB depth and/or channelized melt infiltrating the lithosphere. This work underscores the sensitivity of modeled melt to volatile content, and the significant consequences of melt stability on the long‐term evolution of the lithospheric mantle.