INTEGRATING THE LARAMIDE FORELAND STRAIN FIELD

A critical test of tectonic models for the late Cretaceous to early Tertiary Laramide Orogeny is whether they can produce an integrated strain field consistent with foreland geometries and kinematic indicators. Current uncertainties regarding the controls on basement-involved foreland deformation is shown by the variety of proposed mechanisms, including NNE-directed transpression, ENE-directed thrusting, linkage with the Sevier Thrust Belt, and rotation of the Colorado Plateau. The latter has been variously envisioned as either a rotating microplate or a stress guide transmitting compression to the foreland.

Map-scale folds and arches show a concentric pattern of deformation, with arch axes suggesting average shortening directions oriented N55E in Wyoming, N60E in Colorado, and N85E in New Mexico. Related minor fault analyses show a similar pattern of single stage shortening, except in northwestern Colorado, which shows distinct NE-directed Laramide and SE-directed Sevier shortening. An increase in shortening along the eastern margin of the Colorado Plateau suggests a minor rotational component consistent with clockwise (<5°) regional deflections of Paleozoic paleomagnetic data within the plateau. To explore the consequences of rigid plateau models, a simple, but properly georeferenced, Euler pole model was used to predict strain fields associated with rotational Colorado Plateau models. Specifically, Euler poles east of the Colorado Plateau produce a pattern of shortening that is parallel to the eastern margin in central New Mexico and are increasingly oblique to the east toward the north. Euler poles south of the plateau predict continuously-increasing slip to the north that is strongly east directed along the entire eastern margin. In the context of the observed pattern of deformation, Laramide basement-involved deformation probably resulted from general NE-directed shortening with contributions from the rotation of the Colorado Plateau and the adjacent shortening in the Sevier critical taper fold-and-thrust belt.