Precambrian geology and ductile normal faulting in the southwest corner of the Beartooth Uplift, Montana

Abstract

The northwestern flank of the Beartooth uplift exposes the Snowy shear zone. In the South Snowy Block north of Gardiner, Montana, the shear zone separates high-grade biotite gneisses and augen gneisses (to the southeast) from biotite-staurolite schists (to the northwest). Foliation and fold axis orientations within the biotite-gneisses and the biotite-staurolite schists are nearly identical, suggesting a common geologic history and limited displacement in the Snowy shear zone since prograde metamorphism. Asymmetrical fabrics are well developed in the shear zone, showing one stage of oblique slip with normal and right-lateral components. The Snowy shear zone probably formed during intra-cratonic extension during a late-stage, possibly Proterozoic, orogen that uplifted an ancestral Beartooth block by ductile normal faulting. -from Author