EARLY EOCENE TECTONIC MODE SWITCHING IN THE HINTERLAND OF THE SEVIER OROGENIC BELT EVIDENCED BY THERMODYNAMIC MODELING AND KINEMATIC ANALYSIS OF GARNETS IN THE SCHIST OF UPPER NARROWS, RAFT RIVER MOUNTAINS, NORTHWESTERN UTAH

The Sevier-Laramide orogenic belt experienced a fundamental transition from shortening to extension during the Eocene. The timing of the transition has been well documented in the foreland fold-thrust belt between 50 and 48 Ma, but remains poorly constrained in the hinterland. Garnets from the schist of Upper Narrows in the western Raft River Mountains, for which we have previously determined Lu-Hf isochron ages of 51.0 ± 2.0 and 53.3 ± 2.2 Ma, provided an opportunity to constrain the timing of the transition from shortening to extension by combining PT paths with kinematic analysis. The study area lies in the footwall of the Basin-Elba fault, a complex fault zone that experienced both major thrusting (Proterozoic over Ordovician strata) and extensional reactivation.

Thermodynamic modeling was undertaken on 10 chemically zoned garnets using the method of Moynihan and Pattison (2013), which calculates PT conditions along a garnet’s chemical zonation profile. While there are slight differences in the resulting PT paths, overall they show that garnet growth initiated at ~500°C and 4000 bars. During the majority of garnet growth, both T and P increased to ~525°C and 4400 bars, followed by a decrease in P to ~3700 bars and an increase in T to 540°C. The P changes evident during garnet growth are consistent with a switch from net burial to exhumation. The rocks exhibit meso-scale kinematic indicators (e.g., shear bands, asymmetric quartz veins) showing top-to-NW shear, similar to the kinematics preserved within the extensional Middle Mountain shear zone, which is exposed along the western margin of the Grouse Creek, Raft River and Albion Mountains. Inclusion trails in garnet, while complex, show sigmoidal shapes consistent with top-NW shear. This interpretation is supported by NE-trending garnet rotational axes in two samples as determined through serial sectioning. Thus, the rocks experienced an increase followed by a decrease in P during continued top-NW shearing along the extensional Middle Mountain shear zone, which we interpret 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.