Laramide Orogenesis Driven by Late Cretaceous Weakening of the North American Lithosphere

This paper investigates the causes of the Late Cretaceous transition from "Sevier" to "Laramide" orogenesis and the spatial and temporal evolution of effective elastic thickness (EET) of the North American lithosphere. We use a Monte Carlo flexural model applied to 34 stratigraphic profiles in the Laramide province and five profiles from the Western Canadian Basin to estimate model parameters which produce flexural profiles that match observed sedimentary thicknesses. Sediment thicknesses come from basins from New Mexico to Canada of Cenomanian-Eocene age that are related to both Sevier and Laramide crustal loads. Flexural models reveal an east-to-west spatial decrease in EET in all time intervals analyzed. This spatial decrease in EET may have been associated with either bending stresses associated with the Sevier thrust belt, or increased proximity to attenuated continental crust at the paleocontinental margin. In the Laramide province (i.e., south of similar to 48 degrees N) there was a coeval, regional decrease in EET between the Cenomanian-Santonian (similar to 98-84 Ma) and the Campanian-Maastrichtian (similar to 77-66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET in the Western Canada Basin (north of similar to 48 degrees N), which is consistent with a lack of Laramide-style deformation or flat subduction. We conclude that the regional lithospheric weakening in the late Santonian-Campanian is best explained by hydration of the North American lithosphere thinned by bulldozing by a shallowly subducting Farallon plate. The weakening of the lithosphere facilitated Laramide contractional deformation by focusing end-loading stresses associated with flat subduction. Laramide deformation in turn may have further reduced EET by weakening the upper crust. Finally, estimates of Campanian-Maastrichtian and Paleogene EET are comparable to current estimates indicating that the modern distribution of lithospheric strength was achieved by the Campanian in response to flat subduction. Plain Language Summary The western United States and Canada share an organized mountain belt which runs their length. However, in the United States there is also a series of isolated mountain ranges to the east of that organized belt. This difference is often attributed to the shallow descent of an oceanic tectonic plate beneath the western United States between about 90 and 45 million years ago (Ma), and steeper descent beneath Canada. However, it is unclear why this would produce the differences observed at the surface. We examine this question by reconstructing the strength of the North American tectonic plate from 90-45 Ma, by modeling its rigidity (resistance to bending). In the model we apply random loads and stiffnesses to observe how the modeled plate bends to form sedimentary basins and accommodate sediment accumulation. We then compare the results of this Monte Carlo model to observed sediment thicknesses in real basins to determine which set of parameters match observations. Our results are most consistent with weakening of the USA portion of the North American plate by incorporation of fluids from the shallowly descending oceanic plate. This weakening focused deformation in the region directly above the shallow plate leading to spatially limited deformation.