AGE AND REGIONAL TECTONIC IMPLICATIONS OF LATE CRETACEOUS THRUSTING AND EOCENE EXTENSION, CABINET MOUNTAINS, NORTHWEST MONTANA AND NORTHERNIDAHO

Understanding the kinematic and temporal relationships between the Cor dilleran magmatic belt and the foreland fold and thrust belt can provi de insights into the development of thrust systems in retro-arc settin gs. Combining data from field mapping and Ar-40/Ar-39 geochronometry a nd thermochronometry, we show that Late Cretaceous thrusting in the Ca binet Mountains of the northwest Montana thrust belt coincided both sp atially and temporally with hinterland magmatism. Thrusting and magmat ism in the hinterland of the northwest and southwest Montana thrust be lt were coeval with development of the foreland fold and thrust belt d uring the Late Cretaceous. Movement on the Moyie thrust, a principal t hrust in the western part of the Purcell anticlinorium, occurred betwe en ca. 71 and 69 Ma. In the Cabinet Mountains, the anticlinorium devel oped by systems of east- and west-directed thrusts, with a southward d ecrease in slip on the Moyie thrust that helped contribute to less ove rall shortening in the anticlinorium in Montana than in British Columb ia. The lithosphere underlying the anticlinorium may have been thermal ly weakened by magmatic heating, promoting the development of both the Late Cretaceous thrusts and early to middle Eocene normal faults.