A major, abrupt, southward decline in elevation (~800 m) with an accompanying increase in Bouguer gravity anomaly (~80 mGal) crosses a seismically active region of southern Nevada at about 37 ¿N. The steepness of the gravity gradient requires significant crustal density contrast, some of which is probably caused by the plutonic roots of voluminous Tertiary volcanism to the north, but the amplitude of the anomaly requires additional contrast within the mantle. The topographic step probably developed in mid-Miocene time, coincident with the arrival of the starting head of the Yellowstone thermal plume at the base of the lithosphere. A plausible combination of crustal and deep buoyancy sources, related to the heat and melt input of an anomalously hot asthenospheric source, is consistent with gravity, seismic, heat flow, and isotopic observations and explains the origin of the topographic step. ¿ American Geophysical Union 1995