TRACKING LARAMIDE EXHUMATION AS A PROXY FOR UPPER CRUSTAL RESPONSE TO SUBDUCTION PROCESSES

Western North American is a Cordilleran orogenic system and has long been studied in order to understand subduction zone coupling along plate margins through geological time. The Cretaceous-Paleocene tectonic evolution of the North American Cordillera is dominated by two major and temporally overlapping contractional events known as the Sevier and Laramide orogenies. The Sevier orogeny is typified by thin-skinned deformation exemplified by a well-developed fold-thrust belt and regional foreland basin system, whereas the Laramide orogeny is characterized by thick-skinned deformation, basement block uplift, and partitioning of the regional foreland basins. These Laramide structures or “uplifts” are hypothesized to have resulted from increased coupling between the North American and Farallon plates as a response to flat-slab subduction of buoyant oceanic plateaux or aseismic ridges. However, there remain major questions about the timing of onset, trajectory of the flat-slab, coupling mechanism(s), and the upper-plate deformational response. Central to the flat-slab hypothesis is the timing, rate, and distribution of exhumation experienced by the Laramide uplifts as recorded by low-temperature thermochronology as a proxy for deformation-driven exhumation. In this contribution, we address these questions by combining apatite fission-track (AFT) and (U-Th-Sm)/He (AHe) data from 21 new samples with 338 published AFT and AHe ages from documented Laramide structures in Arizona, Utah, Wyoming, Colorado, Montana, and South Dakota. Integrated with existing geological constraints on the timing of uplift and regional cross-sections, we present a spatial and temporal record of exhumation from the mid-Cretaceous to Paleogene. Our preliminary results suggest a two-stage exhumation of the Laramide province, with an early phase of exhumation occurring >80 Ma in Wyoming and Montana, followed by a more regional period of exhumation at ca. 55 Ma. Further, the sweep of exhumation broadly documents a more W-to-E or WSW-toENE pattern of younging, inconsistent with models that suggest a NNE-ward younging pattern of deformation and exhumation. These results have implications for testing proposed models of Farallon flat-slab and for models of the upper-plate response to flat-slab subduction zones globally.