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Abstract Continental fold‐thrust belts display a variety of structural styles, ranging from thin‐skinned thrusts following weak lithologic contacts to thick‐skinned thrusts that deform mechanical basement. The common practice of splitting fold‐thrust belts into thin‐skinned and thick‐skinned map domains has not yielded a predictive model of the primary controls on structural style. Within the Mesozoic‐Paleogene Idaho‐Montana fold‐thrust belt (44°N‐45°N, 112°W‐114°W), we identify crosscutting thin‐skinned and thick‐skinned thrusts within an otherwise thin‐skinned map domain. This transition occurs within a thin (∼2.5 km) portion of the western Laurentian passive margin, where lower strata pinch out over a prominent basement high (Lemhi arch). Early fold‐thrust belt shortening of sedimentary cover rocks was accommodated through detachment folding, followed by east‐directed, thin‐skinned thrusting along regional‐scale faults (Thompson Gulch and Railroad Canyon thrusts). Later, basement and cover rocks were tilted toward the southeast and a basement‐involved normal fault was reactivated during thick‐skinned thrusting (Radio Tower‐Baby Joe Gulch‐Italian Gulch thrusts), which accommodated shortening at an oblique angle to and truncated the basal detachment of the older thin‐skinned thrusts. This progression from thin‐skinned to thick‐skinned thrusting occurred >50 km from the foreland, coincident with a regional basement high. Thus, the Idaho‐Montana fold‐thrust belt is a double‐decker system, with upper thin‐skinned and lower thick‐skinned domains. This double‐decker model is applicable to other fold‐thrust belts and predicts that the transition from thin‐skinned to thick‐skinned thrusting occurs where the growing critically tapered wedge can no longer fit within the sedimentary cover rocks and the basal detachment steps down into the structurally lower mechanical basement. 

Abstract The Laramide province is characterized by foreland basin partitioning through the growth of basement arches. Although variable along the western U.S. margin, the general consensus is initiation of this structural style by the early Campanian (~80 Ma). This has been linked to flat‐slab subduction beneath western North America, but the extent and cause for a flat slab remain debated, invoking the need for better constraints on the regional variations in timing of Laramide deformation. We present new conglomerate clast composition, sandstone petrographic, and detrital zircon U‐Pb geochronologic data from the Upper Cretaceous Beaverhead Group in southwestern Montana that suggest a pre‐Campanian history of basement‐involved deformation. During the early stages of deposition (~88–83 Ma), two separate depositional systems derived sediment from the Lemhi subbasin and distal thrust sheets to the west as well as Paleozoic strata eroding off the exhuming Blacktail‐Snowcrest arch to the east. Our data provide the first conclusive evidence for the longitudinal transport of gravel via Cordilleran paleorivers connecting sediment sources in east central Idaho to depocenters in southwestern Montana and northwestern Wyoming. Furthermore, erosion of Paleozoic strata by this time requires that the Blacktail‐Snowcrest arch was exhuming prior to ~88 Ma in order to remove the Mesozoic overburden. Later (~73–66 Ma) sediment flux was entirely from the foreland‐propagating fold‐thrust belt to the west. These results suggest that Laramide‐style deformation in southwestern Montana preceded initiation elsewhere along the margin, requiring revision of existing models for Laramide tectonism.