P-wave first arrival times recorded on the Southern Great Basin Seismic Network were used to tomographically image lateral variations in both crustal thickness and Pn velocity beneath the southern Great Basin. The tomographic image indicates that the southern Great Basin has a flat Moho at about 30 km depth. We attribute the flat Moho to both underplating by partial melt from the uppermost mantle and to ductile flow in the crust. Magmatic underplating probably dominates where Pn velocities are low (<7.8 km/s) and ductile flow should dominate where Pn velocities are high (>7.9 km/s). Low velocity regions occur predominantly beneath the east margin of the Walker Lane Belt and the Las Vegas Shear Zone suggesting that these shear zones overlie a zone of relatively weak mantle. A region of high Pn velocity northeast of Nevada Test Site (NTS) may correspond to old lithosphere or to a region that has experienced extensive basalt extraction.