LATE PALEOCENE-EARLY EOCENE ACCELERATED LARAMIDE DEFORMATION, EXHUMATION, AND ELEVATION GAIN IN WYOMING

The Laramide Rocky Mountains in the western U.S.A consist of approximately 30 isolated basement uplifts and arches separated by local basins, situated within the former foreland region of the Cordilleran thrust belt. Many of the uplifts are bounded by moderately dipping reverse faults. The region today is an orogenic plateau with high relief located ~1000-2000 km inland from the location of the nearest convergent plate margin during development of the Laramide structures. Although low-angle subduction of the Farallon oceanic plate during Late Cretaceous through Eocene time is generally considered to have controlled the foreland deformation, the means by which stress was transferred from the flat slab into the upper crust is not well understood. Moreover, the great duration of deformation (~50 Ma) is not consistent with the relatively simple models that have been proposed to link flat-slab subduction and the Laramide orogenic event. We combine paleoelevation and thermochronologic data with records of erosional unroofing and basin subsidence to assess the amounts and rates of rock uplift and regional shortening during Laramide deformation. Rock uplift in individual Laramide ranges and total shortening across Wyoming was ~3km and ~26 km, respectively, during the early Maastrictian-middle Paleocene. During late Paleocene-early Eocene time, rock uplift was ~5 km and total shortening was ~43 km. The early Maastrictian-middle Paleocene shortening rate (~2 mm/yr) was comparable with that of the Sevier thrust belt during the Late Cretaceous. Shortening accelerated to ~5 mm/yr during late Paleocene-early Eocene time. We suggest that the slow deformation during the early Maastrictian-middle Paleocene was related to eastward migration of the Cordilleran deformation front, which was coupled with flat subduction of the Farallon plate, whereas accelerated deformation and exhumation during the Late Paleocene-early Eocene may have been caused by rollback/foundering of the Farallon flat slab and associated inflow of the asthenosphere beneath the Laramide region. Post-Laramide regional uplift is required to raise basin floors from ~0.5 km elevation in early Eocene to present elevations.