ALLUVIAL GRAVEL TRANSPORT AS EVIDENCE OF CONTINENTAL TILTING AND DYNAMIC UPLIFT OF THE U.S. CORDILLERA

Sediment transport theory can be used to quantitiatively constrain paleoslopes of ancient large-scale river systems, providing a datum against which the tilt history of continental lithosphere can be compared. The U.S. Cordillera contains at least three thin, but widespread, fluvial, conglomeratic units that occur at times of major tectonic transitions. These units include: the Shinarump Conglomerate (Upper Triassic) in Arizona, Utah and neighboring states; the Buckhorn/Lytle/Cloverly (BLC) units (Lower Cretaceous) exposed in Utah, Colorado and Wyoming (respectively); and the Ogallala Fm (Oligocene) found along the western Great Plains. Paleoslope modeling, coupled with isopach data and flexural modeling provides an approach to calculating paleotilts of the Cordillera over long wavelengths (many hundreds of kilometers). Paleoslopes for these systems range from 10^-4 to 10^-3. Isopach patterns for these units indicates that such slopes required tilting of the basement by >10^-4 over similarly long wave lengths. In addition, we note that each of these units was deposited during times of significant tectonic transitions in the Cordillera — the Shinarump Conglomerate coincides with the initiation of the Andean-type margin along the west edge of the Cordillera; the BLC units coincide with the expansion of the continental arc system and associated fold-thrust belt towards the eastern backbone of the Cordillera and the Ogallala Fm. parallels the opening Rio Grande Rift that is propagating northward up the eastern side of the Cordillera.

Modeling of the coupling between tectonic transitions and wavelength of deposition of the gravel units suggests a mantle depth cause for, or at least involvement with, the generation of slopes along which these rivers flowed. Dynamic topography predicted by slab subduction over the past 200 MY has been shown to produce km scale topography over 1000+ km wavelength. We suspect that these gravel sheets are intimately tied to relatively short-lived dynamic uplift mechanisms coupled to the onset of new tectonic regimes in the U.S. Cordillera.