Garnet dating, pressure-temperature time paths and kinematic analysis of the schist of Upper Narrows, Raft River Mountains, northwestern Utah: Tectonic implications of pressure-temperature-time-deformation paths

The hinterland of the Sevier orogenic belt underwent multiple episodes of synconvergent extension, prior to a brief transitional phase from shortening to overall extension that affected the entire orogen during the Early Eocene. The timing of the final transition from contraction to extension, while well documented in the Sevier fold-thrust belt, remains poorly constrained within the hinterland. The study of metamorphic rocks within the hinterland region provides a unique perspective on the nature of deformational events as well as the timing of the transition from contractional to extensional processes. Garnets from the schist of Upper Narrows in the western Raft River Mountains provide a rare opportunity to determine a P-T-t-d path. The 1-2 cm diameter garnets are datable and preserve chemical 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 and allowing kinematic analysis. Garnets from two samples yielded Lu-Hf isochron ages of 51.0 +/- 2.0 and 53.3 +/- 2.2 Ma, each defined by 3 garnet fractions + whole rock. Thermodynamic modeling of chemically zoned garnets indicates garnet growth initiated at P-T conditions of ~505 °C and ~3500 bars. During the majority of garnet growth, both pressure and temperature increased to ~525 °C and 4400 bars, followed by a decrease in pressure to ~3700 bars and an increase in temperature to 540 °C. The pressure changes evident during garnet growth are consistent with a switch from net burial to exhumation. This is significant in that garnet growth in the western Raft River Mountains is recording evidence of the regional kinematic change associated with the transition from contraction to extension in the hinterland of the Sevier orogen.;The study area is known as the Upper Narrows and is located in the western Raft River Mountains. It lies in the footwall of the Basin-Elba fault, a complex fault zone that underwent both major thrusting (overturned Proterozoic over upright Ordovician strata), and extensional reactivation. The study area is also located within the Middle Mountain shear zone, which is exposed along the western margin of the Grouse Creek, Raft River and Albion Mountain. The schist of Upper Narrows exhibits meso-scale kinematic indicators (e.g., shear bands and asymmetric quartz veins) showing top-to-NW shear, similar to the kinematics preserved within the Middle Mountain shear zone. Inclusion trails in garnet, while complex, show sigmoidal shapes consistent with top-to-NW shear. This interpretation is supported by NE-trending garnet rotational axes in two samples as determined through serial sectioning. Thus, the rocks underwent an increase, followed by a decrease, in pressure during continued top-to-NW shearing along the extensional Middle Mountain shear zone, which is interpreted to result from a complex interplay between the Basin-Elba fault (thrust) and the Middle Mountain shear zone. This is significant in that it may be directly linked to a regional tectonic change from contraction to extension within the hinterland of the Sevier orogenic belt.