A STRATIGRAPHICALLY CONTROLLED DOUBLE-DECKER MODEL FOR THE THIN- TO THICK-SKINNED FOLD-THRUST BELT OF EAST-CENTRAL IDAHO AND SOUTHWESTERN MONTANA

The North American Cordillera is often split into two structural domains: an interior thin-skinned belt where thrusts utilized weak lithologic contacts, and a foreland thick-skinned province where thrusts fundamentally involve mechanical basement. In southwestern Montana, the two structural domains overlap in space and time. However, thin-skinned thrusts occur within the otherwise thick-skinned foreland and extensive outcrops of mechanical basement occur within the otherwise thin-skinned belt that continues into east-central Idaho. These observations suggest that the relationship between the two structural domains is more complicated than a single curvilinear line in map view. Through mapping and structural analysis in the Beaverhead Mountains near the Idaho/Montana border, we document mutually crosscutting thin- and thick-skinned thrusts, within what has generally been considered a thin-skinned fold-thrust belt. Within the northern part of the map area, Neoproterozoic to Silurian passive margin strata are absent and Devonian strata thin dramatically atop the basement high of the Lemhi arch. Within this pre-thrusting basement high, initial detachment folding inferred to be Cretaceous in age shortened the sedimentary cover rocks. Basement and cover rocks were then tilted toward the southeast during low-magnitude, basement-involved thrusting. Subsequent east-directed, thin-skinned thrusting truncated the tilted section. Finally, thick-skinned thrusting of the tilted basement high truncated older thin-skinned thrusts. Integrating this sequence of local deformation events with similar results from across the region leads to a double-decker model, where the Idaho-Montana fold-thrust belt is composed of an upper thin-skinned and lower thick-skinned domain. Our working model predicts that the transition from thin- to thick-skinned thrusting occurs if the foreland-ward advancing critically-tapered wedge can no longer fit within sedimentary cover rocks and the basal detachment steps structurally downward into underlying mechanical basement. An important implication of our model is that the orogenic structural style within the North American Cordillera may be controlled by the cumulative thickness and availability of detachment horizons within rift, passive margin, and foreland basin strata.