SEISMIC EVIDENCE FOR ACTIVE MAGMATIC UNDERPLATING BENEATH THE BASIN AND RANGE PROVINCE, WESTERN UNITED-STATES

Near-vertical and wide-angle seismic reflection data provide evidence for the presence of a magma body at the base of the crust beneath Buen a Vista Valley in northwestern Nevada. The seismic response of this hy pothesized magma body is characterized by high-amplitude, near-vertica l P wave reflections and a comparably strong P-to-S converted phase. T he magma body, referred to here as the Buena Vista Magma Body, is prob ably a single sill with thickness no greater than 200 m and length no greater than 1.8 km. The melt fraction in the sill is undoubtedly grea ter than 20-30%, and probably exceeds 50%. Melt composition is unconst rained. Although the age of the Buena Vista Magma Body is difficult to determine precisely, it is probably no older than 500,000 years. This suggests that magmatism in the Basin and Range Province is an ongoing process, despite the relative paucity of volcanic rocks erupted at th e surface during the last 6 m.y.