THE LARAMIDE OROGENY: MODERN UNDERSTANDING OF THE STRUCTURAL STYLE, TIMING, AND SPATIAL DISTRIBUTION OF A CLASSIC FORELAND THICK-SKINNED TECTONIC SYSTEM

The Laramide belt represents the archetype example of foreland basement-involved deformation associated with continent-oceanic plate convergence. Although the Laramide has been investigated for over 150 years and inspired multiple tectonic models, its temporal evolution, lithospheric structural style, and linkage to plate dynamics remain debated. Herein we present a regional series of crustal cross sections and structural data sets to develop an integrated model for evolution of the Laramide belt. The Laramide is characterized by a network of basement-cored arches and intervening basins that developed in thick lithospheric mantle and Precambrian basement with pre-existing fabrics during the later Cretaceous to Paleogene. Laramide arches are bound by reverse faults that propagate into folded cover rocks and may flatten into a lower crustal detachment or merge into diffuse lower crustal shortening and buckling. Layer-parallel-shortening with development of minor faults and subtle grain-scale fabrics preceded large-scale fault propagation and folding. Arches define a regional NW-SE trend, but individually are curved and vary from N-E to E-W trending. Regional Laramide shortening was oriented overall WSW-ENE, similar to relative motion between North America and Farallon plates but was locally refracted along curved and obliquely trending arches. Synorogenic basin strata and thermochronologic data record protracted arch uplift, with deformation onset starting at ca. 80 Ma in the southern CO Plateau, and younging to ca. 60 Ma in northern WY, consistent with migration of a flat-slab segment. Basement-cored arches in southwest MT, however, do not fit this pattern with deformation and inboard migration of igneous activity there starting at ca. 80 Ma, possibly related to development of a slab window. Cessation of contractional deformation began at ca. 50 Ma in WY, followed by SW migration of extension and an igneous flare-up, likely related to rollback and removal of the Farallon slab.

We present a model combining development of a broad flat-slab during subduction of an oceanic plateau and formation of a slab window, with stress transfer to North American lithosphere from increased basal traction, end loading along a deep keel, and enhanced asthenosphere flow along slab edges. Diffuse mantle lithosphere and lower crustal shortening resulted in upward stress transfer, which led to localized upper crustal shortening influenced by pre-existing weaknesses, fault propagation, and linkage to form major arches and cover folds.