Mylonitic granitic rocks from a large shear zone within the southern Ruby Mountains in southwest Montana record evidence for a multiple tectonic movement history. The shear zone is 500 meters thick with ductile shear fabrics generally striking northeast and dipping to the northwest. Foliations in the adjacent country rocks are sub-parallel to the foliations within the mylonites suggesting shear zone development was contemporaneous with the regional deformation and metamorphism of the country rock. A well-developed mineral stretching lineation, defined by aligned sillimanite crystals, records predominantly dip-slip movement, but locally, oblique-slip is also preserved. Sheath-type folds developed in thin calc-silicate layers in calcitic marble have axial orientations similar to the down-dip mineral stretching lineations of the shear zones. Mesoscopic and microscopic analysis of the mylonites reveals the presence of asymmetric kinematic indicators that record a strong top-to-the-south sense of shear associated with regional compression, thrusting, and crustal thickening. However, in places an opposite shear sense can be identified indicating a period of top-to-the-north sense of shear corresponding with extension, normal faulting, and crustal thinning. The thrust episode is consistent with tectonic shortening and associated high-grade metamorphism reaching 750-800°C and 7-11 kb. Normal movement appears to postdate thrusting, suggesting either a period of post-orogenic collapse due to topographic collapse resulting from an overthickened metamorphic pile (which can be seen today in the Himalayas) or a discretely younger tectonic event. However, the high temperature minerals that define the mineral lineation suggest they were closely related temporally. The timing of deformation, metamorphism, and mylonite formation in this area is unknown, although regional tectonic arguments suggest that high-grade metamorphism is associated with the Proterozoic, 1.78-1.71 Ga Big Sky Orogeny.
Kinematic Analysis of Mylonitic Rocks, Southern Ruby Mountains, SW Montana: Evidence for Proterozoic Orogenic Crustal Thickening and Topographic Collapse.
Undergraduate Review, 4, 57-63.
Available at: https://vc.bridgew.edu/undergrad_rev/vol4/iss1/13
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