Structural geology of the Armstead anticline area, Beaverhead County, Montana

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Abstract
The Armstead anticline is an Archean-cored, asymmetrical anticline located within the hanging wall of the leading edge thrust along the central portion of the southwestern Montana reentrant. Structural analysis, consisting of detailed mapping of megascopic structures, was undertaken to determine the extent and effects that the foreland deformation had on subsequent thrusting. Based on this analysis, the structural history of the area is interpreted to be the result of a dominantly horizontal compressive mode of deformation involving a model of overthrusting. In this model, uplift of the Precambrian-cored Armstead anticline and thrusting are genetically related through the development of a “Rich-Model” anticline, thus indicating the presence of a subsurface thrust ramp that is structurally lower and westward of the Armstead anticline. Other structural features that point to the existence of a subsurface ramp include backthrusts (Serra, 1977, Mode III) along the eastern edge of Hans Peterson Flats, listric normal faults along the western edge of the Armstead anticline, and the Cedar Creek syncline. Additionally, a previously unmapped duplex structure involving layered Precambrian rocks is identified along the eastern limb of the Armstead anticline. The duplex structure, which probably records earliest thrusting, has been rotated along with bedding by later thrusting.

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