Structural Evolution of the Southwestern Tobacco Root Mountains, Montana

In the Sheridan district, southwestern Tobacco Root Mountains, Montana, Precambrian metasediments and metavolcanics are complexly folded and refolded. The exposed metamorphic bedrock is divided into the following major mappable units: quartzo-feldspathic gneiss, amphibolite-hornblende gneiss, anthophyllite-garnet gneiss, intermediate gneiss, and marble. Mapping delineates a series of slightly overturned, isoclinal antiforms and synforms which plunge N. 10° W., 24° NW. The nose of an earlier recumbent fold is also preserved.

Primarily, all folding occurred during two major deformational episodes; an early episode of recumbent folding around northeast axes and a second, intense period of refolding around nearly north-trending axes that produced the dominant isoclinal folds. During the second folding phase, the nose of a recumbent fold became detached, flowed to the east, and was refolded. The degree of metamorphism produced during recumbent folding is unclear. Progressive regional metamorphism of Barrovian type occurred during the second folding episode and produced mineral assemblages indicative of the amphibolite and granulite facies.

Petrofabric evidence substantiates inferences based on regional analysis. Calcite, dolomite, and quartz petrofabrics also reveal a third, less intense phase of folding which was related to the second phase, but which occurred around a distinct axis.

Joint systems within the Sheridan district are a result of both Precambrian and Laramide deformations. Dominant, nearly vertical, joint planes with average trends of N. 75° W., N. 45° W., N. 2° E., and N. 65° E. are thought to be Precambrian in origin on the basis of close agreement with theoretical trends.

Dominant fracture systems probably controlled the intrusion of the Tobacco Root batholith and the orientations of the Noble and other post metamorphic faults, all of which are associated with the Laramide orogeny.