WHY IS IT DOWNHILL FROM TONOPAH TO LAS-VEGAS - A CASE FOR MANTLE PLUME SUPPORT OF THE HIGH NORTHERN BASIN AND RANGE

A major, abrupt, southward decline in elevation (similar to 800 m) wit h an accompanying increase in Bouguer gravity anomaly (similar to 80 m Gal) crosses a seismically active region of southern Nevada at about 3 7 degrees N. The steepness of the gravity gradient requires significan t crustal density contrast, some of which is probably caused by the pl utonic roots of voluminous Tertiary volcanism to the north, but the am plitude of the anomaly requires additional contrast within the mantle. The topographic step probably developed in mid-Miocene time, coincide nt with the arrival of the starting head of the Yellowstone thermal pl ume at the base of the lithosphere. A plausible combination of crustal and deep buoyancy sources, related to the heat and melt input of an a nomalously hot asthenospheric source, is consistent with gravity, seis mic, heat flow, and isotopic observations and explains the origin of t he topographic step.