A seismic refraction profile was recorded along the southern Wasatch front in Utah to investigate crustal structure in the transition zone between the Great basin and Colorado plateau provinces. Interpretation of both refracted and reflected phases indicates a thin (~25-km) crust, a low Pn velocity (~7.5 km.s), and a crustal low-velocity layer in the approximate depth range of 8--15 km. Correlation with previous geophysical studies suggests that these features are present throughout the transition zone and that a mantle upwarp extending at least 50 km east of the boundary between these provinces (Wasatch front) is present. This upwarp may be of considerable tectonic significance, since it approximately coincides with a zone of high seismicity, the intermountain seismic belt.