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Three basement-cored anticlines in the southern Bridger Range and Canyon Mountain anticline and Squaw Creek fault in the northern Gallatin Range were analyzed by comparing foliations, mesoscopic faults, and slickenside lineations in basement rocks to Laramide structures in Phanerozoic cover strata. In regions where the pre-Laramide angle of discordance between Archean metamorphic foliation and bedding in Phanerozoic cover rocks was low, less than 25° (Bridger Range anticlines and at Canyon Mountain anticline), the basement was rotated congruently with cover strata during Laramide folding. This was accomplished by localized flexural slip on basement foliation surfaces. However, the more commonly observed style of deformation involved segmentation of the basement into internally undeformed, rigid, macrogranular (up to 12 m thick) domains that were displaced relative to one another along zones of cataclastic deformation. This style of basement deformation commonly occurred in regions where complex Archean fold patterns in the basement impeded foliation-parallel slip.

At Squaw Creek, the angle of discordance between Phanerozoic cover strata and basement foliation was high (up to 80°) prior to Laramide folding. The absence of rotated foliation surfaces concordant with cover rocks and the presence of a planar basement-cover interface indicate a nonrotational basement response. Therefore, the structural attitude of Archean metamorphic foliation with respect to the basement-cover contact appears to have been a major factor in controlling the geometry of Laramide basement deformation in the cores of these anticlines. Congruent rotations of rigid, macrogranular basement blocks and cover rocks predominated where the angle of discordance was low (e.g., western anticline, Bridger Range), whereas a nonrotational response was observed where the angle of discordance was high (Squaw Creek).

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