Constraining Kinematics of the Helena Salient, Montana

The Helena Salient is a convex-to-the-foreland section of the Rocky Mountains in west-central Montana. It is comprised of Mesoproterozoic to Neoproterozoic sedimentary rocks overlain by a package of Paleozoic and Mesozoic sections. The salient contains three large east-directed thrust systems bounded by strike-slip zones to the north and south. This research explores the kinematic evolution of the Helena Salient and specific controls on the geometry of the salient. Three forward models and cross sections were built to evaluate variations in shortening along the salient in order to investigate kinematic evolution. These models represent defining zones of salient development; the northern syntaxis, the central apex, and the southern portion. The northern cross section across the syntaxis has 25 km of shortening, the central cross section has 12 km of shortening, and the southern cross section has 14.5 km of shortening. Restoration of all three cross sections was used to construct a finite displacement diagram for the Helena Salient and depict the initial position of the thrust faults within the salient. The measured magnitudes of displacement of the Helena Salient thrusts demonstrate that the initial position of the thrust faults coincides with deeper portions of the Belt Supergroup; suggesting pre-existing stratigraphic thicknesses were the primary control of initial formation of the Helena Salient. However, the larger shortening estimate from the northern syntaxis suggests a component of regional rotation of the cordilleran thrust belt may have exerted a significant control over the salient’s kinematic evolution.