Data from the EarthScope USArray, combined with new approaches to converted body wave and surface wave imaging, have improved resolution of the lithosphere-asthenosphere boundary (LAB) and discontinuities internal to the lithosphere across the contiguous U.S. In the tectonically active western U.S., the LAB corresponds to a vertically-localized negative velocity gradient (<40 km) that is consistent with ponded partial melt, embedded in a broader thermal gradient.
In contrast, beneath the thick cratonic lithosphere in the central U.S., a gradual shear velocity decrease over >80 km is typical and can be explained by a purely thermal gradient; patches with a sharper gradient are spatially intermittent. Middle-aged lithosphere in the eastern U.S. manifests an LAB velocity gradient of intermediate sharpness, and partial melt is required in zones with anomalously low mantle velocities. Within the cratonic mantle lithosphere, shallow sub-horizontal negative velocity gradients can be explained by lithospheric alteration due to partial melt and metasomatism.