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People | George Davis
The basement uplifts, arches, and monoclines have cohesive geometries that reflect fault-propagation folding in general and “trishear” fault-propagation in particular. Detailed and repeated applications of trishear inverse- and forward-modeling for each of two of the major Colorado Plateau uplifts suggest to us that the uplifts require 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 approximately 100?. Expressions of the shear zones in uppermost basement may in some cases be neo-formed shear zones that broke loose as “footwall short cuts” from the deeper reactivated zones. Structural analysis of outcrop-scale structures, including stylolites, Eshelby fractures, en echelon arrays of semi-brittle structures (gash veins and stylolites), deformation bands, and meso-scale faults permitted determination of principal stress directions in the Paleozoic and Mesozoic sedimentary cover of the uplifts. Principal stress directions arrange themselves in two groupings of uplifts, one which revealed NE/SW-directed compressive stresses, and a second which revealed NW/SE-directed compressive stresses. Because the strain in cover is localized to the upward projections of the blind shear zones, and because the measured stress 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 deduced for the cover must be interpreted as a reflection of basement strain. The basement strain (expressed as thrust-shear and thrust-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. Tectonically, the Colorado Plateau was caught in a bi-directional tectonic vise. On the northwest side of the Plateau, the Charleston-Nebo salient of the Sevier thrust belt imparted a southeast-directed compressive stress. At the same time, the shallowly subducting Farallon slab created a profound viscous northeast-directed undershearing along the base of the lithosphere. Impacted by these two superposed conditions of loading, the Colorado Plateau deformed systematically along its weakest links in the deeper basement crust, i.e., along ancient shear zones. It is the combination of these two tectonic drivers, together with the variable orientations of basement shear zones having capacity to be reactivated, that generated a disparate array of uplifts that to date have eluded interpretation by a simple, cohesive Laramide kinematic plan. [Davis, G. H., and Bump, A. P., in review, Tectono-structural
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