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Two-dimensional thermal models of the lithosphere under the Basin and Range Province, the Rio Grande rift and the Colorado Plateau have been constructed using a numerical method for solution of the equation of conservation of energy for the transient temperature field. It is established that the crust in the Basin and Range Province and the Rio Grande rift is still heating on a regional scale. We suggest that intrusion of asthenospheric material in the shape of wide upwellings was a cause of lithospheric heating. The time elapsed since asthenospheric material reached the base of the crust of the Basin and Range Province and the Rio Grande rift is estimated as 2–4 m.y. About the same time a great dyke intrusion into the crust beneath the Rio Grande rift occurred. The asthenospheric surface under the Colorado Plateau reached the present depth (about 60 km) not less than 30 m.y. ago.