Abstract

Minor tectonic structures and related metamorphic mineral growths in the Archean crystalline rocks (circa 2.7 b.y.) of the Beartooth Mountains indicate the effects of at least four successive phases of deformation. The most intense metamorphic recrystallization was associated with the formation of intrafolial passive flow folds in the earliest recognized phase. Flexural-flow folds, which are probably parasitic on the major structures of the region, and an associated secondary foliation, were developed subsequently. At least two successive sets of flexural folds, with related axial-planar structures, were formed in later deformational phases.

The common granitic rocks of the region, the quartzo-feldspathic gneisses, are of early development. Cross-cutting pegmatitic veins were emplaced after the formation of the structures of the second phase while xenolith-bearing granite is of late formation.

The tectonic, metamorphic, and igneous events recognized are considered to have been developed during a major erogenic episode which is named the Beartooth orogeny.

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