GEOLOGIC MAPPING AND STRUCTURAL ANALYSIS NORTH OF LEADORE IN EAST-CENTRAL IDAHO REVEALS STACKED THIN- AND THICK-SKINNED FOLD-THRUST BELTS

The North American Cordillera is often separated into the western thin-skinned Sevier belt and eastern thick-skinned Laramide province, with a frontal thrust marking the boundary between the two. The relationship between the two provinces has long been the starting point for developing structural and geodynamic models of the North American Cordillera. In east-central Idaho near the Idaho-Montana border, mechanical basement crops out >50 km west of the frontal Sevier thrust, demonstrating that thick-skinned thrusts occur at deeper structural levels within the otherwise thin-skinned fold-thrust belt. Geologic mapping in the central Beaverhead Mountains near the Idaho/Montana border documents a previously unrecognized carbonate mylonite that we interpret as the exposed basal detachment of a major regional, thin-skinned thrust system (Thompson Gulch and Railroad Canyon thrusts). The transport direction of the thin-skinned thrust system is toward the east, in accord with the regional shortening direction. Three structurally deeper, thick-skinned thrusts (Baby Joe Gulch, Radio Tower and Italian Gulch thrusts) truncate the older thin-skinned basal detachment horizons, with shortening oblique to the regional transport direction. In the northern part of the study area, a thinner than average (~1.8 km) package of Devonian to Pennsylvanian passive margin strata rests in angular unconformity on previously-tilted quartzite of likely Mesoproterozoic age. In contrast, in the southern part of the map area, Ordovician to Devonian passive margin strata—which are missing to the north—rest in nonconformity on the Cambro-Ordovician Beaverhead pluton. The contrasts between the lower Paleozoic stratigraphy of these faulted packages constrains the position of the Lemhi arch, a prominent basement high in the passive margin. Early thin-skinned thrusts utilized weak lithologic units in the passive margin stratigraphy. With continued deformation, the basal detachment stepped structurally down into the mechanical basement of the Lemhi arch. Lateral thinning and shallowing of passive margin strata over the basement high of the Lemhi arch limited the depth of the thin-skinned portion of the fold-thrust belt, resulting in a transition from thin- to thick-skinned thrusting beneath the former thin-skinned belt.