Two geophysical informations constrain the mechanical state of the lithosphere in the Basin and Range Province. COCORP reflection seismic structure across Nevada shows a rather smooth reflector at about 30 km depth, which can be interpreted as indicative of a rather flat Moho. On the other hand, the Bouguer map exhibits two marked troughs in the same region, and filtering techniques reveal a predominant 200-km wavelength of the deep compensating masses of the topography. This paper attributes these deep heterogeneities to lateral thermal variations created by boudinage-type instabilities of the stretched mantle lithosphere.