Abstract
The northwestern flank of the Beartooth uplift exposes the Snowy shear zone, a thick belt of NW-dipping, ductilely-deformed Archean rocks. In the South Snowy Block north of Gardiner, Montana, the shear zone separates high-grade sillimanite + cordierite-bearing biotite gneisses and augen gneisses (to the southeast) from andalusite-bearing biotite-staurolite schists (to the northwest). The shear zone contains fine-grained, mylonitized, and retrograded equivalents of these rocks interfingering with lenses of actinolite amphibolite, metagabbro, and quartzite.
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 normal slip component is consistent with the differences in metamorphic equilibration conditions recorded in the biotite gneisses and the biotite-staurolite schists. These results contradict earlier hypotheses that suggested lateral shortening in these mylonites. 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.
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Erslev, E.A. (1992). Precambrian geology and ductile normal faulting in the southwest corner of the Beartooth Uplift, Montana. In: Bartholomew, M.J., Hyndman, D.W., Mogk, D.W., Mason, R. (eds) Basement Tectonics 8. Proceedings of the International Conferences on Basement Tectonics, vol 2. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-1614-5_21
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DOI: https://doi.org/10.1007/978-94-011-1614-5_21
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