EARLY LARAMIDE DEFORMATION AND DEVELOPMENT OF DYNAMIC SUBSIDENCE IN MONTANA AND WYOMING: PRODUCTS OF LATE CRETACEOUS MULTI-MICROPLATE SUBDUCTION UNDER THE WESTERN UNITED STATES

The transition between Sevier (thin-skinned) and Laramide (thick-skinned) tectonism occurred during the Late Cretaceous in the Western Interior Basin. The shallowing of the down going slab and eventual flat slab subduction is commonly inferred to have caused the transition; however, the driver of flat slab subduction is still a point of contention/confusion. Subduction of the buoyant conjugate Shatsky Plateau is commonly called upon to explain the slab flattening. This model does not explain the Laramide deformation in southwestern Montana in the Early Coniacian, approximately 10 Ma earlier than predicted. Therefore, another model must be considered.

Late Cretaceous strata in basins in Wyoming, Colorado, and Montana were deposited synchronously with the transition from Sevier to Laramide deformation, providing the opportunity to examine the effects of the transition on accommodation development and depocenter movement along strike of the axis of the basin. Seven regional isopach maps spanning 100 to 65 Ma in five million-year-steps were constructed based on the integration of petroleum well logs, sequence stratigraphic correlations, and chronostratigraphic tops from public databases and published literature. The maps show depocenters moved from southwestern Wyoming to southern Montana by the late Coniacian (85 Ma), coeval with a depocenter geometry transition from narrow and deep to broad and shallow. Results support a model in which dynamic subsidence induced by flat subduction reached southwestern Montana by approximately 85 Ma, earlier than predicted by previous models.

This study suggests a multi-microplate model in which microplates, spreading ridges, and archipelagos are moving northeasterly to the North American craton and subducted in the Late Cretaceous prior to the subduction of the Farallon plate, as proposed by previous tomotectonic plate reconstructions. The overriding of the buoyant Mezcalera slab with a northeast trajectory induced end loading in Montana and Wyoming by approximately 90 Ma, which induced early Laramide deformation. Continued flat slab subduction led to hydration and weakening of the lithosphere under the flat slab. This process allowed for the development of dynamic subsidence.