THE LATE EOCENE HOUSE RANGE FLORA, SEVIER DESERT, UTAH - PALEOCLIMATEAND PALEOELEVATION

Analysis of the late Eocene House Range flora can shed light on the cl imatic and tectonic history of the Sevier Desert, Utah. It is the firs t described flora from the middle Tertiary of western Utah, and it gre w on the hanging wall of both the Sevier-age Canyon Range thrust and t he middle-Tertiary Sevier Desert detachment fault. The flora, which is preserved in a lacustrine limestone on the east flank of the House Ra nge, is dated as Chadronian (late Eocene) on the basis of an associate d gastropod. This age determination is consistent with a single-crysta l Ar-40/Ar-39 date of 31.4 +/- 0.5 Ma from a nearby outcrop of the Win dous Butte tuff which contains clasts of the limestone Physiognomic an alysis of the fossil angiosperm leaves implies a mean annual temperatu re (MAT) of 13.2 +/- 2.9 degrees C and a growing season precipitation of 85 +/- 30 cm. The vegetation is physiognomically most similar to mo dern warm-temperate floras from Japan. MAT-based and enthalpy-based al timeters derive paleoelevations of 3.6 +/- 0.7 km or 2.9 +/- 1.5 km fo r the flora, compared to a modern elevation of 1.7 km. This paleoeleva tion, if taken to be representative of regional paleoelevation, implie s that the Sevier orogen was high, but then collapsed to modern elevat ions during middle and late Cenozoic extension.