Over the past several decades, contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long-term stability of the Colorado Plateau (CP) and crustal thinning beneath the Basin and Range Province (BRP) in the southwestern United States. Here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15,500 P-to-S receiver functions recorded at 72 USArray and other broadband seismic stations on the southwestern CP and the southern BRP. Our results reveal that the BRP is characterized by a thin crust (28.2 ± 0.5 km), a mean Vp/Vs of 1.761 ± 0.014 and a mean amplitude (R) of P-to-S converted wave (relative to that of the direct P wave) of 0.181 ± 0.014 that are similar to a typical continental crust, consistent with the model that the thin crust was the consequence of lithospheric stretching during the Cenozoic. The CP is characterized by the thickest crust (42.3 ± 0.8 km), largest Vp/Vs (1.825 ± 0.009) and smallest R (0.105 ± 0.007) values in the study area. In addition, many stations on the CP exhibit a clear arrival before the P-to-S converted phase from the Moho, corresponding to a lower crustal layer of about 12 km thick with a mafic composition. We hypothesize that the lower crustal layer, which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines, contributed to the long-term stability and preuplift low elevation of the Colorado Plateau.