UNDEFINING THE SNOWY SHEAR ZONE --- A TECTONOMETAMORPHIC APPROACH TO UNDERSTANDING POLYPHASE DEFORMATION WITHIN THE WESTERN ABSAROKA-BEARTOOTH MOUNTAINS OF MONTANA

Southwest Montana records a long-lived geologic history extending from the formation of protocontinents in the Archean to neotectonic deformation. Exposures throughout this region document processes that operated at a variety of crustal levels under a multitude of petrologic conditions, forming a valuable geologic microcosm that provides an important venue to test hypotheses concerning the evolution of the continental lithosphere. In this contribution we present details concerning the structural and petrologic architecture of the western Absaroka-Beartooth mountains. Central to this investigation is the Snowy shear zone, which defines the western flank of the Beartooth uplift. Despite over a century of detailed work within the region, the geometry, kinematics, timing, and tectonic significance of the Snowy shear zone is equivocal. Early hypotheses suggest a Proterozoic origin for the Snowy shear zone based primarily on the occurrence of andalusite within hanging-wall exposures of Crystal Cross Mountain. These exposures are juxtaposed against footwall rocks of high-grade gneiss from Yankee Jim Canyon. In this contribution, we present petrologic constraints on hanging-wall rocks of the Snowy shear zone that indicate peak-metamorphic temperatures between 600°C and 700°C, with pressures between 500 MPa and 1000 MPa. THERMOCALC AvPT estimates of peak metamorphic conditions indicate temperatures of 630 ± 91°C and pressures of 830 ± 220 MPa. Previously reported data from rocks of Yankee Jim Canyon record peak conditions of approximately 780°C and 500 MPa. As such, evidence suggesting a major thermobarometric gradient across the Snowy shear zone is equivocal, if not erroneous. Structural analyses from exposures near Sheep Creek indicate the formation of five superimposed fabrics, indicating a polyphase history that must be considered in the use of thermobarometric data as a kinematic indicator for the Snowy shear zone. Petrologic and structural analysis of samples collected along a transect through the North Snowy block aim to better establish controls on temperatures of deformation and peak metamorphism within the foot-wall of the Snowy shear zone. Resolution of such structural and petrologic details is a critical step towards establishing controls on the tectonic evolution of the Wyoming Province.