Early Inception of the Laramide Orogeny in Southwestern Montana and Northern Wyoming: Implications for Models of Flat‐Slab Subduction

Abstract

AbstractTiming and distribution of magmatism, deformation, exhumation, and basin development have been used to reconstruct the history of Laramide flat‐slab subduction under North America during Late Cretaceous‐early Cenozoic time. Existing geodynamic models, however, ignore a large (~40,000‐km2) sector of the Laramide foreland in southwestern Montana. The Montana Laramide ranges consist of Archean basement arches (fault‐propagation folds) that were elevated by thrust and reverse faults. We present new thermochronological and geochronological data from six Laramide ranges in southwestern Montana (the Beartooth, Gravelly, Ruby and Madison Ranges, and the Tobacco Root and Highland Mountains) that show significant cooling and exhumation during the Early to mid‐Cretaceous, much earlier than the record of Laramide exhumation in Wyoming. These data suggest that Laramide‐style deformation‐driven exhumation slightly predates the eastward sweep of magmatism in western Montana, consistent with geodynamic models involving initial strain propagation into North American cratonic rocks due to stresses associated with a northeastward expanding region of flat‐slab subduction. Our results also indicate various degrees of Cenozoic heating and cooling possibly associated with westward rollback of the subducting Farallon slab, followed by Basin‐and‐Range extension.