EARLY EOCENE METAMORPHISM IN THE SEVIER HINTERLAND CONSTRAINED BY LU-HF GARNET GEOCHRONOLOGY

The hinterland of the Sevier orogen experienced alternating shortening and extension during development of the foreland fold-thrust belt, prior to a terminal transition from shortening to extension that affected the entire orogen. The timing of the terminal transition between shortening and extension, while well documented in the Idaho-Utah-Wyoming salient of the Sevier fold-thrust belt between ca. 50 and 48 Ma, remains poorly constrained within the hinterland. To address the timing of this kinematic transition in the hinterland, we take advantage of a rare opportunity to determine a P-T-t-d path provided by the occurrence of garnet within the schist of Upper Narrows in the western Raft River Mountains, where it is metamorphosed to amphibolite facies. The 1-2 cm diameter garnets are datable and preserve growth zoning, and are thus amenable to thermodynamic modeling to produce pressure-temperature (P-T) paths. They also preserve sigmoidal inclusion trails indicating synkinematic growth. Here we present results of Lu-Hf isochron dating of two samples of garnet schist in the schist of Upper Narrows, from the type locality in the Upper Narrows of the Raft River in the western Raft River Mountains. Lu-Hf isochron ages of 51.0 ± 2.0 and 53.3 ± 2.2 Ma are each defined by 3 garnet fractions + whole rock on two samples. Garnets grew during top-to-NW shear, similar to the kinematics preserved within the Middle Mountain shear zone, which is exposed along the western margin of the Grouse Creek, Raft River and Albion Mountains. Prior studies have interpreted a polyphase extensional history for the Middle Mountain shear zone, with earliest extension responsible for 45 Ma cooling ages in its footwall. Thermodynamic modeling including pseudosection and Gibbs Method approaches are currently in progress to constrain P-T changes associated with Early Eocene top-to-NW shear. The results will better constrain the timing of the terminal transition phase from contraction to extension of the Sevier orogeny.