KINEMATIC CONSTRAINTS ON THE EMPLACEMENT OF VERY HOT RHEOMORPHIC IGNIMBRITES IN THE SOUTHERN SNAKE RIVER PLAIN, IDAHO

Voluminous rheomorphic ignimbrites dominate the stratigraphy in the mountains south of Twin Falls, Idaho. The ignimbrites are outflow products of massive, caldera forming eruptions from mid-late Miocene volcanic centers now buried beneath the central Snake River Plain (SRP). The largest ignimbrites comprise extensive welded sheets that were unusually hot when erupted, with pre-eruptive magma temperatures commonly in excess of 900 °C. They ductiley flowed prior to cooling and effectively enameled the Earth’s surface. A variety of rheomorphic structures can be observed in many of the ignimbrites including open to-isoclinally folded flow banding, sheath folds, refolds and small-scale kinematic indicators.

In this study, we present structural data from several large ignimbrite units exposed on the Brown’s Bench massif and the Shoshone Hills along the southern margin of the SRP. Upper Brown’s Bench ignimbrites contain NE-SW trending stretched vesicle lineations and microstructures that record SW-directed flow during ignimbrite emplacement and cooling. In the Shoshone Hills, previously unstudied ignimbrites exposed in large escarpments on the east side of the range contain open-tight SE-vergent folds and NW-trending lineations. Outcrop and micro-scale kinematic indicators and the consistent sense of fold overturning indicate SE-directed flow during ignimbrite emplacement and cooling. Lithological comparisons and kinematic data indicate that upper ignimbrite units in Brown’s Bench and the Shoshone Hills do not correlate across the intervening Rogerson Graben. Brown’s Bench structural data suggest an eruptive source to the NE, whereas Shoshone Hills data suggest a NW eruptive source. Assuming a time-progressive trend of caldera volcanism from SW to NE along the SRP, then the Shoshone Hills sequence represents an older eruption event(s) most likely correlative with the proposed 12.5-11.3 Ma Bruneau-Jarbridge caldera, whereas the ignimbrite sequence exposed on upper Brown’s Bench was most likely derived from the younger 10-8.6 Ma Twin Falls eruptive center directly to the N-NE. Our study places new constraints on stratigraphic correlations between Brown’s Bench, Shoshone Hills and Cassia Hills ignimbrite successions and the magnitude and frequency of Miocene “super-eruptions” along the SRP.