A new dataset of P-wave seismograms and travel times has been analyzed to determine features of the compressional velocity structure of the crust and upper mantle of northern Mexico and the border region. Features of compressional velocity profiles of the region include an average thickness that is approximately 36 km, a Pn or lid velocity of 7.80 to 7.95 km/sec, and a lithospheric thickness of 70 to 76 km. There is an upper-mantle low-velocity zone (LVZ) that is approximately 45 km wide, has a velocity decrease of no more than a few per cent and a gradual transition at its base. Models of the structure in the southwestern United States have smaller crustal and lithospheric thicknesses and lower lid velocities. The mantle LVZs are significantly broader and have sharper gradients at their bases. Models of shear and compressional LVZs of Mexico and the southwestern United States provide strong evidence that partial melting occurs in the upper mantle. The results of this paper enable us to document a systematic southward decrease in geophysical characteristics that are indicative of extensional thinning, high temperatures, and/or anomalous mantle composition. This is consistent with a southward decrease in tectonic activity (seismicity, volcanism) and supports the idea that some large scale process, with its origins in the upper mantle, is shaping both Mexico and the southwestern United States. It is plausible that such a process is related to the prior subduction, and possible present existence, of a remnant Farallon plate in the mantle beneath all of western North America.