STRUCTURAL ANALYSIS OF PRESSURE SOLUTION CLEAVAGE IN THE MCCARTNEY MOUNTAIN FOLD-THRUST SALIENT, SOUTHWEST MONTANA

The McCartney Mountain salient is a distinct convex-east segment of the Sevier fold-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 syncline-anticline pairs in the hanging wall and footwall that display complex intra-formational duplexes and parasitic detachment folds at all scales. Pressure solution cleavage (PSC) occurs in the argillaceous carbonate members of the Cretaceous Kootenai and Triassic Dinwoody Formations throughout the area. PSC 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 carbonate member of the Kootenai. This PSC exhibits planar morphology with domainal spacing at the mm-cm scale, classified as strongly spaced-cleavage to slaty-cleavage. Structural orientation data divides the PSC into three populations. S1 is localized and fans with respect to axial surfaces of mesoscopic folds. S2 is defined by axial planar PSC that trends N-S, dips east and is inferred to have formed during late stage shortening (parallel to regional fold axes). S3 is recognized by E-W trending PSC that dips within 10^o of vertical, cross cutting hanging wall and footwall folds in the area. Later tectonic influences such as: a) impingement against foreland Laramide structures, b) Late Cretaceous magmatic inflation of the fold-thrust belt, and/or c) Eocene exhumation and extension, could provide mechanisms for S3 development.