A reversed seismic refraction profile was recorded in the southern Basin and Range Province between Parker and Globe, Arizona. Interpretation of both refracted and reflected phases indicates a two-layered crust 23-25 km thick, and a low Pn velocity (7.67 km/s). PmP/Pn amplitude analysis on individual traces and synthetic seismogram modeling indicate that a positive velocity gradient may exist in the upper mantle. The low Pn velocity is indicative of partial melting in the upper mantle and implies that the crust is identical with the lithosphere. The heat flow data suggest that the lithosphere was about 40 km thick approximately 5 m.y. ago. On the basis of this and a negative free air gravity anomaly we propose that the lithosphere has been thinned from about 40 to 24 km over the past 5 m.y. and will probably thin and uplift further in the southern Basin and Range in Arizona.