NEW RECORD OF MID-MIOCENE LARGE-SCALE EXPLOSIVE SUPER-ERUPTIONS AND ASSOCIATED SUBSIDENCE ON THE YELLOWSTONE HOTSPOT TRACK

The central Snake River Plain (cSRP) region, southern Idaho, preserves records of numerous, high-temperature and voluminous rhyolitic explosive eruptions that devastated the area during the Mid-Miocene. The ~2 km-thick Cassia Formation, exposed on the southern margin of the cSRP, comprises 12 refined and newly described rhyolitic eruption units, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. The formation records a period of voluminous high-temperature, explosive eruptions between ~11.3 Ma and ~8.1 Ma that emplaced intensely welded rheomorphic Snake River-type ignimbrites and associated ash-fall layers. The distinct characteristics of the units have allowed several correlations to be made over large distances between previously identified local successions. One ignimbrite records the ~8.1 Ma Castleford Crossing super-eruption (~1900 km³), which covers 14,000 km² and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Correlations such as this have shown that eruptions originating from the cSRP were less numerous but considerably larger than previously thought, and serve to further refine the more regional volcanic stratigraphy. The revised stratigraphy allows the recognition of three successive temporal geochemical trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a mid-crustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. This incremental loading and modification of the crust also contributed to the development of a regional NE-trending monocline, here termed the Cassia monocline, which formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, off-lap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The large eruptions probably had different source locations all within the subsiding basin, therefore the proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau.