We present new evidence on the seismic velocity and density of the crust and upper mantle along a 200-km-long transect across the eastern Basin and Range and western Colorado Plateau at 37¿N latitude. Receiver functions computed from the P waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations. The crust is 30--35 km thick within the eastern Basin and Range and increases over a distance of ~100 km at the western edge of the Colorado Plateau, reaching a maximum of approximately 45 km east of the Hurricane fault. The timing of crustal multiples within the receiver functions were used to estimate the Vp/Vs of the crust along the profile, and we found that the western Colorado Plateau crust is characterized by a high Poisson's ratio (0.28--0.29) indicative of a crust with an average mafic composition. We estimated the upper mantle lid thickness along our profile based on teleseismic P wave travel times and constraints provided by gravity data. Our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the Basin and Range to 100 km beneath the western Colorado Plateau, although the Basin and Range lithosphere may have significant thickness variations. The thick, strong mafic crust and thicker mantle lid under the Colorado Plateau can account for the relative geologic stability and subdued magmatism of the plateau during Laramide compression and Cenozoic extension compared to surrounding regions. The crustal and lithospheric thinning across the tectonic boundary occurs over a short distance (~100 km), suggesting it is a geologically young feature produced by a predominantly mechanical response to late Cenozoic extension. Our new lithosphere model at 37¿N latitude is consistent with the existence, in early Cenozoic time, of a flat subducted slab at 100 km depth and a relict Sevier-Laramide 50--60 km thick crustal welt, and 60--100% pure shear extension (β values of 1.6--2.0) during the late Cenozoic. ¿ American Geophysical Union 1995