Estimates of surface extension in the southern Basin and Range province and transition into the Colorado Plateau range from a few percent to several hundred percent locally, yet the crustal thickness varies perhaps only 10-15 km across these provinces. Within the southern Basin and Range and the metamorphic core complex belt, extremely extended crust is directly juxtaposed against equally thick (or thinner) crust that underwent far milder extension. Unless preextension crustal thickness varied dramatically over a short distance, the crust must have maintained its thickness during extension, through mechanisms that involve crustal flow and magmatism. We employ a 300-km-long profile of seismic refraction and coincident vertical-incidence reflection data to investigate the geophysical signature of these processes from the extended southern Basin and Range province to the unextended Colorado Plateau. By integrating the seismic velocity with the pattern of reflectivity along the profile, we estimate the amounts of Tertiary magmatism and flow that have occurred. We estimate an upper bound of 8 km of mafic material intruded beneath the metamorphic core complex belt and 4 and 5 km of intruded material beneath the Transition Zone and southern Basin and Range province, respectively. We emphasize that this 8-km estimate is strictly an upper bound, and that the actual amount of magmatism was probably less (3 to 4 km). We further speculate that several kilo-meters of silicic rock was added to the metamorphic core complex belt via ductile flow. As suggested by numerous numerical models of crustal extension, we conclude that a mobile, felsic midcrustal layer accommodated most of this crustal flow. This ductile midcrustal layer appears to be thickest beneath the most extended terranes and thinnest beneath the less extended Transition Zone and Colorado Plateau. In contrast, the lowermost crust appears to have thinned passively in an amount that corresponds more directly to the regional surface extension.