DEVIATORIC STRESS VS. STRENGTH IN CONTINENTAL DEFORMATION, WITH EXAMPLES FROM THE WESTERN U.S

The nature of plate tectonics has always suggested that stresses were applied to continents at plate boundaries; heterogeneity of deformation can be explained as a decay of force away from a finite-length boundary interacting with continental material with heterogeneous strength. It is the absence of deformation that most routinely evokes the inference of strength. Stability could just as easily reflect the absence of deviatoric stress as the presence of strength. One such stable domain, the Colorado Plateau, is often identified as a strong element in the western U.S. It is not undeformed. During the Laramide orogeny the Plateau was shortened at monoclines. Most of these structures are poorly dated, but in the instances where they can be dated, they originated early in the Laramide orogeny. If the Plateau was strong, why did it deform before most of the foreland more distant from the plate margin? Strong regions should deform last if the whole orogen experiences the same deviatoric stress. Conversely, at the opening of the Laramide orogeny the Wyoming craton probably seemed quite strong as it was Archean lithosphere undeformed since Middle Proterozoic time, yet it experienced far more severe deformation than the Proterozoic Colorado Plateau. One explanation could be temporal variations in strength, but a more parsimonious explanation is that the deviatoric stresses being applied to Wyoming were larger. This logic was the physical basis for flat slab models of the Laramide orogeny, which generated deviatoric stresses through application of basal shear, as well as a new proposal that generates stresses through subsidence driven by loading at the base of the lithosphere. At a broader scale, this interpretation reinforces the notion that global inferences of strong and weak lithosphere may be colored by the difficulty of creating high deviatoric stresses far into a continental interior. Suggestions of strong regions should be treated as hypotheses in need of testing.