TECTONO-STRUCTURAL EVOLUTION OF THE COLORADO PLATEAU

The Colorado Plateau is composed of late Proterozoic, Paleozoic, and Mesozoic sedimentary rocks overlying mechanically heterogeneous Proterozoic crystalline basement riddled with Precambrian shear zones and faults. The structure of the Plateau is dominated by ten major basement-cored uplifts (maximum structural relief of 2000m) constructed during the Late Cretaceous-early Tertiary Laramide orogeny. Each uplift is a highly asymmetric anticline, cored by a blind reverse fault. The geometric characteristics conform to trishear fold geometry. Master faults are rooted in basement and originate along reactivated Precambrian weaknesses. Some uplifts are linked to one another along strike.

This structural style of the Colorado Plateau was influenced strongly by its mechanical stratigraphy and a tectonically sheltered location east of the weak miogeocline. By the Late Cretaceous, the miogeocline had shortened and thickened enough to impart a SE-directed stress on the Colorado Plateau. At the same time flat-slab subduction created a viscous NE-directed drag along the base of the lithosphere. Simultantouesly stressed by the edge loading and the basal shear, the Colorado Plateau failed along its weakest links, the ancient faults. Subsequent rollback of the flat slab provided thermal support for plateau uplift.

The Colorado Plateau is one example on a continuum of tectono-structural styles defined by rheology and total strain, modulated by applied stress. Homogeneous miogeoclinal sequences shortened by sustained horizontally driven compressive stresses tend to form critically tapered wedges that are everywhere on the verge of failure. Increasing strain results in self-similar growth. Regions like the Colorado Plateau, composed of thin platform sequences atop crystalline basement with preexisting discontinuities of widely varying orientation and strength, respond to shortening via systems of basement-cored uplifts in which only the weakest elements ever come close to failure. A line drawn through the peaks of the resulting uplifts may describe a kind of critical taper, but the upper surface of the “wedge” is extremely serrate; and growth is not self-similar.