A kinematic model linking the Sevier and Laramide belts in the Idaho-Montana fold-thrust belt, U.S. Cordillera

Abstract

The Sevier and Laramide belts of the U.S. Cordillera are differentiated based on thin- and thick-skinned structural domains, commonly inferred to have formed under different plate-boundary conditions. However, spatial and temporal overlap in the Idaho-Montana fold-thrust belt suggests that thin- and thick-skinned thrust systems are kinematically linked. We present the first balanced and sequentially restorable cross section that integrates the Sevier and Laramide belts. Encompassing most of the width of the Cordilleran retroarc, our kinematic model accounts for at least 244 km of horizontal shortening, linking thin- and thick-skinned thrust systems. We hypothesize that thin strata overlying the Lemhi arch basement high determined the geometry and relative timing of the later thrusting. Early shortening (pre–ca. 90 Ma) was thin skinned, with the décollement of the Medicine Lodge–McKenzie thrust system following Devonian shales on top of the Lemhi arch unconformity. Displacement on upper thin- and lower thick-skinned thrusts overlapped between ca. 90 and 70 Ma as a mid-crustal décollement was activated, efficiently transmitting strain through the Lemhi arch to the Blacktail-Snowcrest uplift in the foreland. A regional-scale duplex (Patterson culmination) linked the lower and upper décollements, internally thickening and increasing the basal slope of the orogenic wedge. Thick-skinned thrusts of the Dillon cutoff (Hawley Creek, Cabin, and Johnson thrusts) eventually thickened the wedge and exhumed the abandoned upper décollement. Following this, the thick-skinned wedge advanced in-sequence from ca. 70 to 55 Ma. This kinematic model establishes continuity between thin- and thick-skinned thrust systems by a mid-crustal décollement. In this model, the stratigraphic thicknesses of sedimentary cover rocks limit the availability of décollement horizons, determining the style of mountain building and triggering a slow transition from thin- to thick-skinned thrusting.