Download or read book Structural Analysis of the Pre-Beltian Metamorphic Rocks of the Southern Highland Mountains, Madison and Silver Bow Counties, Montana written by Mack Stewart Duncan and published by . This book was released on 1976 with total page 444 pages. Available in PDF, EPUB and Kindle. Book excerpt: Approximately 240km2 of pre-Beltian metamorphic rocks are exposed in the southern portion of the Highland Mountains, Montana. At least six periods of deformation are indicated for these rocks, two of which are believed to have been accompanied by metamorphism. Three major lithologic units, quartzo-feldspathic gneiss, garnetiferous gneiss, and micaceous gneiss, serve as a crude stratigraphic sequence and help delineate two macroscopic structures, the Rochester antiform and the Gilfoy Hills synform. The axes of both of these folds trend NE-SW. The absence of a more detailed stratigraphy and the scarcity of exposures make structural analysis the most effective means of evaluating the deformational styles and orientations present in the map area. Lineation and foliation diagrams were prepared by computer using fabric element orientation data from over 1800 stations. Differences in structural style and orientation indicate that the two types of quartzo-feldspathic gneiss, leucocratic gneiss and gray gneiss, have undergone different deformational histories. The leucocratic gneiss, which is the lowermost unit exposed in the Rochester antiform, exhibits a similar style of folding, many reversals in dip direction, and a statistical axis of folding which plunges to the east-northeast. This geometry is attributed to a metamorphic/deformational event (M1/D1) which occurred prior to the accumulation of the gray, garnetiferous, and micaceous gneisses. These overlying units exhibit a concentric style of folding and a statistical axis of folding which plunges to the southwest. A subsequent metamorphic/deformational event (M2/D2) is believed to have produced this geometry. Mineral assemblages in the garnetiferous and micaceous gneisses indicate that temperatures between 650-700°C and pressures between 3/5kb were reached during M2/D2. Later deformational events, D3, D4, D5, and D6, all involved faulting of some type and are not believed to have been accompanied by metamorphism. A Laramide-age, metamorphic event, M3, is indicated by the occurrence of cordierite near contacts with the Laramide-age Hells Canyon pluton and Boulder batholith. A northeast-southwest trending fold system is present throughout most of the Precambrian metamorphic terrain in the Highland Mountains, the Tobacco Root Mountains, the Ruby Range, the Gravelly Range, and the Madison Range. The structural styles present within these areas indicates that the northeast-southwest trending fold system in southwestern Montana was produced by a primary stress coming from the southeast. Nappe-like overthrusting is suggested as a possible explaination for the anomalous structural sytle present in the northern Tobacco Root Mountains.