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The Late Cretaceous and Paleocene structural pattern in southwestern Montana is comprised of three spatially and temporally overlapping sets of structures whose positions strongly reflect the influence of previous basement faults and whose kinematics imply dominantly west-to-east compressive forces.
The first of these patterns is a set of steep northwest-trending faults involving the metamorphic basement rocks of the Rocky Mountain foreland. Movement on these faults was oblique (left-reverse) and occurred as a reactivation of faults which developed initially in middle Proterozoic time. Net horizontal shortening by faulting and associated large-scale folding is 10-20 km (6-12 mi).
Another fault pattern within the foreland is a set of widely spaced, gently west-dipping thrusts. These thrusts also involve basement rocks and appear to have been controlled by previous zones of weakness within the crust. Movement is principally dip-slip and latest movement appears to involve tearing on the northwest-trending fault set. Shortening by basement thrusting and associated folding is 20-25 km (12-15 mi).
The other structural pattern is that of the frontal fold and thrust zone on the western boundary of the foreland and the zone of transverse thrusting on the northern boundary. These zones of impinging thrusts also involve basement rocks principally as slices that were "picked off" from underlying basement highs during ramping. As the thrusts of the frontal zone advanced to the east they impinged on northwest-trending foreland anticlines and were torn on the northwest-trending faults. The transverse thrust zone on the north involves right-reverse movement with about 20 km (12 mi) of west-east displacement. The zone is fundamentally a transverse ramp produced by deflection along the east-trending Willow Creek fault zone of Proterozoic age.
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