Roy et al. (2005) analysed Bouguer gravity data, crustal thickness, xenolith data and seismic speed data, and they found that a low density uppermantle anomaly correlated with regioins of middle to late Tertiary magmatism (Jemez Lineament). The crust thickness variations do not explain long wavelength variations in Bouguer gravity across the Colorado Plateau-Rio Grande Rift. This Bouguer gravity anomaly is explained by the low lithosphere density structure underneath the boundary of Colorado Plateau, the Hemez Lineament, the western margin of the Great Plains. This model with lithosphere density anomaly is supported by the in xenolith data in the Colorado Plateau and the latest seismic speed model.