The Laramide orogeny: Current understanding of the structural style, timing, and spatial distribution of the classic foreland thick-skinned tectonic system

Abstract

ABSTRACTThe 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.