ABSTRACT The Laramide foreland belt comprises a broad region of thick-skinned, contractional deformation characterized by an anastomosing network of basement-cored arches and intervening basins that developed far inboard of the North American Cordilleran plate margin during the Late Cretaceous to Paleogene. Laramide deformation was broadly coincident in space and time with development of a flat-slab segment along part of the Cordilleran margin. This slab flattening was marked by a magmatic gap in the Sierra Nevada and Mojave arc sectors, an eastward jump of limited igneous activity from ca. 80 to 60 Ma, a NE-migrating wave of dynamic subsidence and subsequent uplift across the foreland, and variable hydration and cooling of mantle lithosphere during slab dewatering as recorded by xenoliths. The Laramide foreland belt developed within thick lithospheric mantle, Archean and Proterozoic basement with complex preexisting fabrics, and thin sedimentary cover. These attributes are in contrast to the thin-skinned Sevier fold-and-thrust belt to the west, which developed within thick passive-margin strata that overlay previously rifted and thinned lithosphere. Laramide arches are bounded by major reverse faults that typically dip 25°–40°, have net slips of ~3–20 km, propagate upward into folded sedimentary cover rocks, and flatten into a lower-crustal detachment or merge into diffuse lower-crustal shortening and buckling. Additional folds and smaller-displacement reverse faults developed along arch flanks and in associated basins. Widespread layer-parallel shortening characterized by the development of minor fault sets and subtle grain-scale fabrics preceded large-scale faulting and folding. Arches define a regional NW- to NNW-trending fabric across Wyoming to Colorado, but individual arches are curved and vary in trend from N-S to E-W. Regional shortening across the Laramide foreland was oriented WSW-ENE, similar to the direction of relative motion between the North American and Farallon plates, but shortening directions were locally refracted along curved and obliquely trending arches, partly related to reactivation of preexisting basement weaknesses. Shortening from large-scale structures varied from ~10%–15% across Wyoming and Colorado to <5% in the Colorado Plateau, which may have had stronger crust, and <5% along the northeastern margin of the belt, where differential stress was likely less. Synorogenic strata deposited in basins and thermochronologic data from basement rocks record protracted arch uplift, exhumation, and cooling starting ca. 80 Ma in the southern Colorado Plateau and becoming younger northeastward to ca. 60 Ma in northern Wyoming and central Montana, consistent with NE migration of a flat-slab segment. Basement-cored uplifts in southwest Montana, however, do not fit this pattern, where deformation and rapid inboard migration of igneous activity started at ca. 80 Ma, possibly related to development of a slab window associated with subduction of the Farallon-Kula Ridge. Cessation of contractional deformation began at ca. 50 Ma in Montana to Wyoming, followed by a southward-migrating transition to extension and flare-up in igneous activity, interpreted to record rollback of the Farallon slab. We present a model for the tectonic evolution of the Laramide belt that combines broad flat-slab subduction, stress transfer to the North American plate from end loading along a lithospheric keel and increased basal traction, upward stress transfer through variably sheared lithospheric mantle, diffuse lower-crustal shortening, and focused upper-crustal faulting influenced by preexisting basement weaknesses.

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.