An Investigation of the Upper Mantle Compressional Velocity Distribution Beneath the Basin and Range Province

The compressional velocity distribution of the upper mantle beneath the Basin and Range province is interpreted from a large group of shortperiod, long-range recordings of nuclear explosions at the Nevada Test Site. The interpretation includes a low-velocity channel characterized by a top at a depth of 50 ± 10 km and a velocity increase in the depth range from 120 to 155 ± 10 km, with a discontinuity in velocity near the bottom of this depth range. The compressional velocity beneath the Basin and Range province apparently increases very slowly at depths between about 170 and 290 km, as inferred from weak arrivals along a 8·6±0·1 km s−1 refraction branch for distances of about 1520 km and greater. Sharp increases of velocity were found for depths of 315±15 km and 422±15 km; the velocity contrast at the deeper discontinuity is about twice as large as that near 315 km. Compressional velocities beneath the central United States are inferred to be similar to those beneath the Basin and Range province at depths greater than 170 km although small velocity differences may possibly persist to depths of the order of 400 km.