In late Cenozoic time the Sierra Nevada was uplifted to twice the height of the Appalachian Range, although crustal thickness beneath the two ranges is the same. Moreover, the Sierran root is probably much too old to have caused the late uplift. Rayleigh-wave phase velocities along the axis of the Sierra Nevada are low and, combined with other evidence, indicate that thinning of mantle lithosphere produced the uplift. The former subducted slab of lithosphere beneath the Sierra Nevada may have caused not only the present low heat flow but also shielded the Sierra Nevada from a high regional asthenospheric heat flux. Progressive warming followed the Mendocino triple juction northward, converted sub-Sierran lithosphere to asthenosphere, and raised the southern Sierra Nevada highest because they have been rising for the longest time.