Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana, USA

Abstract

The Cordilleran thrust belt of northwestern Montana (United States) has received much less attention than its counterparts in the western interior of USA and Canada. The structure of the thrust belt in this region is well preserved and has not been strongly overprinted by Cenozoic extension, providing an opportunity to reconstruct its geometry and to relate it to the foreland basin system. The thrust belt in this region consists of a frontal part of highly deformed Paleozoic, Mesozoic, and Paleocene sedimentary rocks, and a western region dominated by a >15-km-thick succession of Proterozoic Belt Supergroup strata underlain by faults of the Lewis thrust system. The frontal part can be subdivided into the foothills and the Sawtooth Range. At the surface, the foothills show deformed Mesozoic and Paleocene rocks; at depth, refl ection seismic data indicate numerous thrust faults carrying Paleozoic strata. The Sawtooth Range, south from the Lewis thrust salient, is defi ned by steeply dipping imbricate thrusts that detach at the basal Cambrian stratigraphic level. The Sawtooth Range plunges northward beneath the Lewis thrust salient and diverges into a pair of independent thrust systems that form the Flathead and Waterton duplexes in Canada. The relatively minor internal deformation in the western part of the thrust belt resulted from the great rheological strength of the Belt Supergroup rocks and initial high taper of the preorogenic stratigraphic wedge. A new ~145-km-long balanced cross section indicates ~135 km of shortening, a value similar to that in the southern part of the Canadian thrust belt. Previous work and new conventional and isotopic provenance data from the foreland basin and U-Pb ages from crosscutting intrusive rocks establish a preliminary kinematic model for this segment of the Cordilleran thrust belt. The emerging pattern is a relatively simple forelandward progression of thrusting events. Most shortening in the Lewis thrust system, Sawtooth Range, and foothills occurred roughly between mid-Campanian and Early Eocene time (ca. 75‐52 Ma), yielding a shortening rate of ~5.9 mm/yr. This pattern differs from the pattern of shortening in the better known Sevier thrust belt to the south, where regional far-traveled Proterozoic quartzitebearing thrust sheets were mainly active during Early Cretaceous time. From Middle Eocene to Early Miocene time, this sector of the Cordillera collapsed, generating a number of extensional depocenters.