Abstract

Understanding the kinematic and temporal relationships between the Cordilleran magmatic belt and the foreland fold and thrust belt can provide insights into the development of thrust systems in retro-arc settings. Combining data from field mapping and 40Ar/39Ar geochronometry and thermochronometry, we show that Late Cretaceous thrusting in the Cabinet Mountains of the northwest Montana thrust belt coincided both spatially and temporally with hinterland magmatism. Thrusting and magmatism in the hinterland of the northwest and southwest Montana thrust belt were coeval with development of the foreland fold and thrust belt during the Late Cretaceous. Movement on the Moyie thrust, a principal thrust in the western part of the Purcell anticlinorium, occurred between ca. 71 and 69 Ma. In the Cabinet Mountains, the anticlinorium developed by systems of east- and west- directed trusts, with a southward decrease in slip on the Moyie thrust that the heloped contribute to less overall shortening in the anticlinorium in Montana than in British Columbia. The lithosphere underlying the anticlinorium may have been thermally weakened by magmatic heating, promoting the development of both the Late Cretaceous thrusts and early to middle Eocene normal faults.

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