Dynamic topography and vertical motion of the U.S. Rocky Mountain region prior to and during the Laramide orogeny

Abstract

Dynamic topography of Earth’s surface occurs in response to hydrodynamic stresses due to mantle flow beneath a flexible lithosphere. Here, we compare the predicted dynamic topography from an inverse-convection model that includes flat slab subduction with the known geologic history of the western United States from Late Cretaceous through Paleocene time to evaluate the validity of the model results. Downwarping behind the evolving Cordilleran volcanic arc took place after its inception in Late Triassic time, culminating in the formation of the Western Interior Seaway in Late Cretaceous time. Subsidence behind arcs is a consistent prediction of most models of dynamic topography. A more-detailed convection model is evaluated here by comparing it to the geologic history of the central Rocky Mountain region from 100 to 50 Ma. The match of predicted and observed tectonic subsidence is good up until the time that local deformation by the Laramide orogeny begins. This is most likely due to local flexural effects of mountain building overwhelming regional dynamic effects. Maps for successive time intervals show that the model matches the geologic history quite well through time and space; in particular, the subduction of the previously postulated oceanic plateau—the conjugate Shatsky Rise—has a significant impact on surface movements. (1) From 95 to 88 Ma, the locations of most significant regional unconformities match the eastward migration of a zone of high topography. (2) From 90 to 85 Ma, the zone of maximum subsidence coincides with the motion of the leading edge of the conjugate Shatsky Rise. (3) From 78 to 60 Ma, the site of initiation of Laramide deformation migrates coincidently with the position of the center of the conjugate Shatsky Rise. (4) From 70 to 60 Ma, the time-transgressive deposition of thin fluvial conglomerate units in the southern part of the study area is generally coincident with surface uplift caused by the trailing part of the conjugate Shatsky Rise. These results suggest that the inverse model approximates the vertical motion history quite well. Synchroneity of the position of the center of the passing conjugate Shatsky Rise, the landward limit of the evolving Cordilleran volcanic arc, and the initiation of Laramide deformation suggests that the Farallon plate became coupled with the overlying North American plate as the subducted oceanic plateau passed beneath. Progressively enhanced mechanical coupling between the plates was likely the impetus for Laramide shortening. This comparison of model results with surface geologic history provides a means to validate, but not verify, predictions of dynamic mantle-flow models.