A THERMAL RECORD OF GROWTH AND DESTRUCTION ACROSS THE THIN- TO THICK-SKINNED IDAHO-MONTANA FOLD-THRUST BELT

Growing fold-thrust belts propagate toward their forelands, promoting erosion of early, wedge-top strata. Although wedge-top deposits may be stripped, burial heating during sedimentation and cooling-related exhumation of these strata leave a thermal record archiving the competition between growth and destruction of fold-thrust belts. In the Idaho-Montana fold-thrust belt, the thin-skinned Sevier belt and thick-skinned Laramide belt overlap in space and time, suggesting that the transition in structural style may have resulted from fold-thrust belt propagation into a thin section of passive margin strata. Using Raman spectroscopy of carbonaceous material, we quantify maximum temperatures across the Idaho-Montana fold-thrust belt to constrain the dimensions of the early orogenic wedge and test the hypothesis that thin strata over the Lemhi arch caused a shift from thin- to thick-skinned thrusting. Sampling the low-relief unconformity that hosted the early décollement reveals a cold (<100°C) foreland in fault contact with a uniformly hot (~250°C) wedge interior. Most of the internal fold-thrust belt reveals comparable maximum temperatures, suggesting that the early décollement was everywhere buried to at least ~6.5 km depth, requiring an exceptionally low taper for the early orogenic wedge. Estimated maximum burial depths are ~1.5-5.0 km deeper than can be explained by observed pre-Cretaceous stratigraphic thicknesses, suggesting that the low-relief wedge was achieved by widespread wedge-top deposition and thin-skinned shortening above the Lemhi arch. In contrast, cumulative exhumation is irregular, varying by up to 5 km over large (~75 km) wavelength folds that involve mechanical basement. Our results support a tectonic inheritance model, where the maximum possible thickness of the thin-skin wedge was limited by thin strata over the low-relief Lemhi arch. Subsequent thickening required activation of a much deeper décollement, and a switch to a thick-skinned structural style. Sedimentation and burial heating in low-relief orogenic wedges may facilitate the transition from thin- to thick-skinned thrusting by bringing the plastic middle crust, which hosts the late décollement, closer to the early décollement near the basement/cover contact.