LATE CRETACEOUS EVOLUTION OF PALEO-RELIEF AND LITHOSPHERIC STRENGTH IN THE NORTH AMERICAN CORDILLERA FROM MONTE CARLO-BASED FLEXURAL MODELING

In the Late Cretaceous, deformation and subsidence in the western USA transitioned from an organized “Sevier” thrust belt with ramp-flat thrust geometries and a unified foreland basin to a partitioned mosaic of “Laramide” basement uplifts bounded by crustal-scale thrust ramps and associated, isolated flexural basins. The coeval thrust belt and foreland basin in Canada witnessed no similar change. Nevertheless, previous research has documented a northward younging in exhumation and widely varying paleoelevation estimates in the Canadian Cordillera. We investigate the causes of the change in deformation style, the spatial and temporal evolution of North American lithosphere’s effective elastic thickness (EET), and paleorelief using a Monte Carlo flexural model applied to 34 stratigraphic profiles. Modeling provides estimates of flexural parameters including load height and EET which produce flexural profiles that match observed stratal thicknesses in Sevier and Laramide basins from New Mexico to Canada.

Results show that south of ~48°N there was a coeval, regional decrease in EET between the Cenomanian–Santonian (~98–84 Ma) and the Campanian–Maastrichtian (~77–66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET north of ~48°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 which was also 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.

Comparison of paleorelief based on flexural models and stable isotope-based paleoelevation reconstructions indicate a close correspondence, although paleorelief estimates are slightly lower than paleoelevation estimates. This may reflect a difference in their datums, where paleoelevation estimates are referenced to sea level whereas paleorelief estimates indicate height above the local basin floor. We conclude that flexural modeling provides an independent and complementary means of assessing the topographic evolution of orogenic belts.