New Timing Constraints for the Onset of Laramide Deformation in Southwest Montana Challenge our Understanding of the Development of a Thick-Skinned Structural Style During Flat-Slab Subduction

Abstract

In the Rocky Mountain region of the western United States, Laramide-style reverse faulting involving Proterozoic-Archean basement is interpreted to be a result of flat-slab subduction of oceanic lithosphere beneath the continent. Garber et al. (2020, https://doi.org/10.1029/2019TC005944) present new provenance data from the Upper Cretaceous Beaverhead Group in southwest Montana, which suggest the basement-cored Blacktail-Snowcrest uplift to be active earlier than previous estimates. Combined with other studies from the region, an earlier age for onset of Laramide-style deformation requires reevaluation of how the interior of the upper plate deforms during subduction.