THE POWDER RIVER PASS SHEAR ZONE - ROOTS OF AN ARCHEAN FORELAND THRUST FAULT?

A NE-SW trending, steeply-dipping shear zone in the southern gneiss terrain of the southern Bighorn Mountains of north-central Wyoming may be a foreland thrust-fault related to the Oregon Trail structural belt (OTSB) of the Granite Mountains, approximately 150 km to the south. Preliminary U-Pb apatite dates (450°C cooling dates) document unroofing of the southern Bighorn Mountains during the Archean, ca. 2.65 Ga, coeval with movement on the OTSB. U-Pb dates of syntectonic sphene collected from a sheared mafic dike suggest the Powder River Pass shear zone developed at ca. 2.64 Ga. Microstructures analyzed in thin-section exhibit a strong fabric, syntectonic growth of sphene, and sub-grain rotation of feldspar. Both sigma and delta-augens analyzed from hand-sample suggest a south-side up sense of movement. An alternative model of south-side up bulk tilting of the basement without movement on a discrete fault will be tested by high temperature thermal histories on either side of the fault through U-Pb dating of apatite. The OTSB transects the Granite Mountains, an east-west trending uplift in south-central Wyoming and is interpreted as a deformation front related to continent-arc collisions ca. 2.65-2.62 Ga. This study provides important insights into the tectonic evolution of the Wyoming province and southern Bighorn Mountains during the Neoarchean ~2.65 Ga and may lead to better tectonic models for thick-skinned basement uplifts associated with continent-arc collisions.