The new issue of Geology has a cover article on mantle variations below the Colorado Plateau. Authors Ryan Crow, et al, found evident that suggests "a process by which upwelling asthenosphere is progressively infiltrating and replacing lithospheric mantle, especially where Proterozoic boundaries exist."
[right, GSA cover: "False color Landsat (bands 7,4,2) image, perspective view looking east, of the western edge of the Colorado Plateau, showing the Grand Canyon, and Lake Powell (upper left). The dark patches north (left) of the Grand Canyon are the Shivwits (foreground) and Uinkaret (background) volcanic fields. The dark area south (right) of the Grand Canyon is the San Francisco volcanic field (upper right). Crow et al. use Nd isotopic data and geochronology from these and other basalts to show progressive replacement of lithosphere by asthenosphere around the edges of the Colorado Plateau. Image by: Ryan Crow, with public domain LANDSAT images from NASA and elevation data. Three-dimensional model built in Blender, an open source animation software package."]
“Shrinking of the Colorado Plateau via lithospheric mantle erosion: Evidence from Nd and Sr isotopes and geochronology of Neogene basalts” by Crow et al., v. 39 no. 1 p. 27-30