The explosive rhyolitic eruptions that define the track of the Snake River Plain-Yellowstone volcanics have produced a large volume of tephra found in late Miocene and younger basin-fill sediments throughout the western United States. We focus on tephra deposits in three Neogene basins; Grand Valley, in eastern Idaho; the Jackson Hole area, and the Granite Mountains area, in central Wyo. Within these basin we have discovered a large number of previously unidentified post-middle Miocene major SRP silicic eruptive events. We correlate these tephra between basins and to known and newly discovered ash-flow tuffs on the margin of the SRP (some new 40Ar/39Ar dates are listed below). Including these newly discovered events into calculations for rates of major silicic eruptions yields a roughly constant eruption rate between ~ 14 Ma and 4.49 Ma. This fairly constant rate is interrupted by a substantial gap in major silicic eruptions between 4.49 Ma and 2.06 Ma, which we call the "post-Heise eruptive gap." We found for the interval from 10.35 Ma to 4.49 Ma an average eruption rate of one every 195 k.y. These rates are consistent with previously published rates for older SRP silicic eruptions. Subsequent to the Heise gap the rate of eruption has significantly slowed. We suggest the gap and reduced rate are the result of a new position of the hotspot under Laramide thickened crust intruded by Eocene Absaroka Volcanics. The constant rate of silicic eruptions and the constant migration rate of 2.2 cm/yr of the Y-SRP deformation field for the same 10 m.y. interval are suggestive of movement of the North American Plate over a fixed and constant thermal source. This pattern is consistent with a classical hotspot model but not necessarily unique to it.

4.49 Ma tuff of Heise, 5.37 Ma tuff of Elkhorn Springs, 5.81 Ma tuff of Wolverine Creek, 5.94 Ma Conant Creek Tuff, 6.16 Ma tuff of Inel (in WO-2 borehole), 6.19 Ma tuff of Blue Creek, 6.19 Ma Walcott Tuff, 6.57 Ma tuff of Edie School, 7.31 Ma tuff of Phillips Ridge, 7.48 Ma tuff of American Falls, 8.75 Ma tuff of Lost River Sinks, 9.15 Ma tuff of Timbered Dome, 9.17 Ma tuff of Kyle Canyon, 9.34 Ma tuff of Little Chokecherry Canyon, 10.09 Ma and 10.27 Ma tuff of ArbonValley A & B