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Mylonite-Montana, Mylonite-Idaho, Geology, Structural-Montana, Geology, Structural-Idaho, Cataclastic rocks-Ductility, Shear zones (Geology)-Montana, Shear zones (Geology)-Idaho
Mylonites, protomylonites, and cataclasites occur in thin, widely spaced, NE trending, NW dipping, SE verging (Fig. 5 - 7) shear zones in the Henrys Lake Mountains, the field area for the 2011 Montana, Big Sky Keck Project. Differences in protolith composition are interpreted to control position of shear zones and textural differences between mylonites, protomylonites, and cataclasites. Cataclasites likely have the highest concentrations of retrograde minerals because their highly fractured texture provided a conduit for water flow. Symplectite grew during shear, potentially as a means for volume reduction (Simpson and Wintsch, 1989). Mineral assemblages typical of greenschist facies and phengite equilibrium with k-feldspar, quartz and phlogopite suggests a minimum pressure during shear of 5 kb (0.5 GPa). As shear zones in the Henrys Lake Mountains are found to be the along strike equivalents of the 1.8 Ga Madison mylonite zone in the Southern Madison Range (Erslev and Sutter, 1990), it is believed that they are the result of shearing as part of a foreland thrust zone inboard of a major compressional orogen at 1.8 Ga (Erslev and Sutter, 1990). It is suggested that this event was the Big Sky Orogeny.
Walker, Alianora, "Structural analysis of Precambrian mylonite zones, Henry's Lake Mountains, southwest Montana and Idaho" (2011). Honors Project, Smith College, Northampton, MA.
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