PN TOMOGRAPHY BENEATH THE SOUTHERN GREAT-BASIN

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