Structural analysis of cleavage at Sandy Hollow, McCartney Mountain fold-and-thrust salient, Southwest Montana
Helmke, Elizabeth Ashley
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The McCartney Mountain salient is a distinct convex-east segment of the Sevier fold-and-thrust belt in southwest Montana, lying east of the Pioneer batholith and west of basement-cored Laramide uplifts. Prominent features of the central part of the salient include the in-sequence Sandy Hollow thrust fault (displacing Permian over Lower Cretaceous strata), and hanging wall and footwall syncline-anticline pairs that display complex intra-formational duplexes and parasitic detachment folds at all scales. Cleavage occurs in the argillaceous carbonate members of the Cretaceous Kootenai and Triassic Dinwoody formations throughout the area. Cleavage has been systematically mapped and characterized according to morphology, domainal spacing and orientation for all exposed outcrops of a distinctive yellowish dolomitic unit in the lower limestone member of the Kootenai. Cleavage within this unit exhibits planar morphology with domainal spacing at the mm-cm scale, classified as moderate to very strong spaced cleavage. Structural orientation data divides the cleavage into two populations. S1 is defined by cleavage that trends N-S, dips east, and is inferred to be reoriented axial planar cleavage that may have been modified during late-stage shortening as the frontal fold-and-thrust system impinged against basement uplifts in the foreland. S2 is recognized by E-W striking cleavage that dips within 10° of vertical, cross cutting hanging wall, and footwall folds in the area. S2 is inferred to have formed during the final stages of deformation in the Sandy Hollow fold-and-thrust system. S2 may reflect N-S shortening caused by lateral forces along the salient boundary as the fold-and-thrust system propagated eastward into the salient nose.