A stratigraphically controlled, double-decker model linking thin- and thick-skinned thrust domains of the North American Cordillera

Abstract:

In thin-skinned orogenic belts, upper crustal shortening is generally accommodated by folding and thrusting above a regional stratigraphic décollement, the depth of which is determined by the available weak layers within the crust. In contrast, thick-skinned thrust belts fundamentally involve mechanical basement and are inferred to form above mid to lower crustal décollements that reflect a temperature-dependent decrease in compressive strength of the crust. We describe new results from field-based investigations in the Cretaceous-early Cenozoic Cordilleran fold-thrust belt of east-central Idaho and southwestern Montana. Near the Idaho-Montana border in the central Beaverhead Mountains, contractional structures exhibit a continuum from a thin- to thick-skinned structural style and overlap in space and time. Early shortening was accommodated above weak carbonate rocks that constitute the base of the miogeoclinal succession in the region, with intermittent tilting inferred to reflect low-magnitude basement-involved shortening. Subsequent thrusts involving mechanical basement offset overlying low-angle thrusts. Thus, this region of the Cordilleran orogen represents a double-decker fold-thrust-belt, with kinematically coupled upper thin-skinned and lower thick-skinned domains. This region of overlapping structural styles occurs within an area of significantly thinner pre-orogenic strata above a regional basement high. Our working model involves early shortening facilitated by weak lithologies in pre-orogenic strata. During continued shortening, the basal décollement horizon stepped downward into mechanical basement. In response, mechanical basement and the overlying thin-skinned thrust belt were transported along a mid to lower crustal décollement, linking the overlapping Sevier belt and Laramide province. This double-decker model is applicable to other fold-thrust belts and predicts that the transition from thin- to thick-skinned thrusting occurs where the growing critically-tapered wedge can no longer fit within the sedimentary cover rocks and the basal décollement steps down into structurally lower mechanical basement. Thus, the availability of pre-orogenic stratigraphic detachment horizons is hypothesized to exert a fundamental control on structural style.