The composition of the lower continental crust is well-studied but poorly understood because of the difficulty of sampling large portions of it. Petrological and geochemical analyses of this deepest portion of the continental crust are limited to the study of high grade metamorphic lithologies, such as granulite. In situ lower crustal studies require geophysical experiments to determine regional-scale phenomena. Since geophysical properties, such as shear wave velocity (Vs), are nonunique among different compositions and temperatures, the most informative lower crustal models combine both geochemical and geophysical knowledge. We explored a combined modeling technique by analyzing the Basin and Range of the United States, a region for which plentiful geochemical and geophysical data is available. By comparing seismic velocity predictions based on composition and thermodynamic principles to ambient noise inversions, we identified three compositional trends in the southwestern United States that reflect three different geologic settings. The composition of the lower crust depends heavily on temperature because of the effect it has on rock mineralogy and physical properties. In the Basin and Range, we see evidence for a lower crust that overall is intermediate-mafic in composition (53.7 +/- 7.2 wt.% SiO2), and notably displays a gradient of decreasing SiO2 with depth.