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Pre‐Thrusting Stratigraphic Control on the Transition From a Thin‐Skinned to Thick‐Skinned Structural Style: An Example From the Double‐Decker Idaho‐Montana Fold‐Thrust Belt

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.