The Colorado Plateau is composed of Neoproterozoic, Paleozoic, and Mesozoic sedimentary rocks overlying mechanically heterogeneous latest Paleoproterozoic and Mesoproterozoic crystalline basement containing shear zones. The structure of the plateau is dominated by ten major basement-cored uplifts and associated monoclines, which were constructed during the Late Cretaceous through early Tertiary Laramide orogeny. Structural relief on the uplifts ranges up to 2 km. Each uplift is a highly asymmetric, doubly plunging anticline residing in the hanging wall of a (generally) blind crustal shear zone with reverse or reverse/oblique-slip displacement. The master shear zones are rooted in basement, and many, if not most, originated along reactivated, dominantly Neoproterozoic shear zones. These can be observed in several of the uplifts and within basement exposures of central Arizona that project “down-structure” northward beneath the Colorado Plateau. The basement shear zones, which were reactivated by crustal shortening, formed largely as a result of intracratonic rifting, and thus the system of Colorado Plateau uplifts is largely a product of inversion tectonics. The overall deformational style is commonly referred to as “Laramide,” and this is how we use this term here.
In order to estimate the dip of basement faults beneath uplifts, we applied trishear modeling to the Circle Cliffs uplift and the San Rafael swell. Detailed and repeated applications of trishear inverse- and forward-modeling for each of these uplifts suggest to us that the uplifts require initiation along a low-angle shear zone (between ~20° and ~40°), an initial shear-zone tip well below the basement-cover contact, a propagation (p) to slip (s) ratio that is higher for mechanically stiffer rocks and lower for mechanically softer rocks, a planar shear-zone geometry, and a trishear angle of ~100°. These are new results, and they demonstrate that the basement uplifts, arches, and monoclines have cohesive geometries that reflect fault-propagation folding in general and trishear fault-propagation in particular. Expressions of the shear zones in uppermost basement may in some cases be neoformed shear zones that broke loose as “footwall shortcuts” from the deeper reactivated zones.
Structural analysis of outcrop-scale structures permitted determination of horizontal-shortening directions in the Paleozoic and Mesozoic sedimentary cover of the uplifts. These arrange themselves into two groupings of uplifts, one that reveals NE/SW-directed shortening, and a second that reveals NW/SE shortening. Because the strain in cover strata is localized to the upward projections of the blind shear zones, and because the measured shortening directions are uniform across a given uplift (independent of variations in the strike of the bounding monocline), it seems clear that the regional stresses ultimately responsible for deformation were transmitted through the basement at a deeper level. Thus, the stresses responsible for deformation of the cover may be interpreted as a reflection of basement strain. The basement strain (expressed as oblique shear displacements into cover driven by reactivations of dominantly Neoproterozoic normal-shear zones) was a response to regional tectonic stresses and, ultimately, plate-generated tectonic stresses.
The driving mechanism for the Sevier fold-and-thrust belt was coupled to subduction of the Farallon plate, perhaps enhanced by slab flattening and generation of higher traction along the base of the lithosphere. However, the disparate shortening directions documented here suggest that two tectonic drivers may have operated on the Colorado Plateau: (1) gravitational spreading of the topographically high Sevier thrust belt on the northwest side of the plateau adjacent to an active Charleston-Nebo salient of the Sevier thrust belt, which imparted northwest-southeast shortening; and (2) northeast-southwest shortening driven by the flat slab. The effect of the two drivers tended to “crumple” part of the region in a bidirectional vise, creating added complications to structural patterns. Testing of this idea will require, among other things, very precise age determinations of progressive deformation of the Colorado Plateau within the latest Cretaceous to early Tertiary time window, and sophisticated finite-element modeling to evaluate the nature of the deformation gradients that would be induced by the two drivers.
