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

摘要

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 degrees N-45 degrees N, 112 degrees W-114 degrees W), we identify crosscutting thin-skinned and thick-skinned thrusts within an otherwise thin-skinned map domain. This transition occurs within a thin (similar to 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.