LOCATION, LOCATION, LOCATION: THE VARIABLE LIFESPAN OF THE LARAMIDE OROGENY AND IMPLICATIONS FOR THE ANCIENT GRAND CANYON HYPOTHESIS

Orogenic events are generally considered to affect a particular region over a set duration of time. The Laramide orogeny, a principal example of this convention, most commonly refers to mountain building in the southern Rockies from ~80 to 40 Ma. We show that the Laramide orogeny had a variable lifespan that depended strongly on location, with deformation driven by shallow subduction initiating in SE CA at about 90 Ma, sweeping to the NE, arriving in the Black Hills, SD at about 60 Ma, and concluding in SD within ~10 m.y. During subsequent slab rollback, the areal extent of Laramide deformation decreased as the eastern edge of active deformation swept back to the SW rapidly from ~55 to 50 Ma and more slowly from 50 to 40 Ma, with deformation ultimately ceasing in NM, TX, AZ, and UT at about 30 Ma. Geodynamic modeling of this process suggests that changes in the strength of the NA plate and densification of the subducted conjugate Shatsky rise played important roles in controlling the areal extent of the Laramide orogen and hence, the lifespan of the orogenic event. We suggest that surface uplift of the southern Rockies tracked deformation in a similar fashion to that farther north and that an orogenic plateau, which included both the Nevadaplano province and the Laramide province, extended further south than is commonly discussed. The distribution of synorogenic sedimentary rocks south of the Colorado Plateau is consistent with development of high elevations attendant upon Laramide shortening. These coarse-grained deposits were deposited in isolated intermontane basins in Sonora, SE AZ and SW NM in Late Cretaceous-Paleogene time.

Northeastward advance of a broad Laramide plateau is consistent with erosion of an ancient Grand Canyon, whereby the western part of the canyon was carved to within a few 100 m of its current depth by a N-flowing river as early as ~70 Ma. Subduction-generated uplift of the western Grand Canyon region evidently initiated downcutting by the headwaters of a Laramide drainage network potentially unrelated to the present course of the Colorado River in the eastern Grand Canyon. Post-Laramide exploitation of part of the Laramide drainage by a Miocene Colorado River could then have established the modern Colorado River drainage, resulting in a modern Grand Canyon that was a product of both old and new stages of formation.