CROSSCUTTING THIN- AND THICK-SKINNED THRUSTS WITHIN THE IDAHO-MONTANA FOLD-THRUST BELT: A DETACHMENT CONTROL ON STRUCTURAL STYLE

The Sevier-Laramide fold-thrust belt within the North American Cordillera contains an internal region of thin-skinned thrusting and an outboard region of thick-skinned thrusting. Thin-skinned thrusts, with sub-horizontal detachment horizons in weak strata, are often attributed to steep subduction; in contrast, thick-skinned structures, with generally deeper detachment horizons in mechanical basement, are generally attributed to later flat-slab subduction. The two styles overlap in southwestern Montana and east-central Idaho. Here, in contrast to much of the orogenic belt to the southeast, the two styles are contemporaneous with ongoing arc magmatism and occur both in the interior and exterior portions of the fold-thrust belt; also, early thick-skinned shortening here is thought to be older than the inception of Late Cretaceous flat-slab subduction beneath southern California. Regional-scale map compilation and new detailed mapping in the interior of the fold-and-thrust belt near the Idaho-Montana border reveal a progressive downward shift of detachment level and a transition from a thin- to thick-skinned structural style. Initial thrusting utilized regionally extensive shales and siltstones at the base of the Mississippian section. Above here, fold trains and imbricate thrusts shortened strata in a thin-skinned style. In contrast, underlying Devonian rocks exhibit major changes in thickness and facies above a regional angular unconformity, which is generally underlain by Mesoproterozoic quartzite. Structurally deeper thrusts involve Mesoproterozoic quartzite and crystalline basement; in several localities, thick-skinned thrusts crosscut older thin-skinned faults. We interpret that progressive deformation within the fold-thrust belt resulted in down stepping of the basal décollement and a transition in structural style. These changes are easily explained by the mechanical strength profile of the pre-orogenic stratigraphy: the limited continuity of décollement horizons within structurally deeper strata resulted in crosscutting of structurally shallower thrusts by younger, structurally deeper thrusts. This detachment-controlled model provides a self-organizing mechanism for a thin- to thick-skinned fold-thrust belt in the absence of flat-slab subduction.