The Laramide is a broad region of thick-skinned, contractional deformation that developed far inboard of the North American Cordillera plate margin during the later Cretaceous to Paleogene and is characterized by a network of basement-cored arches and intervening basins. The Laramide developed in thick lithospheric mantle, Precambrian basement with pre-existing fabrics, and thin sedimentary cover. Laramide arches are bound by reverse faults that propagate into folded cover rocks and flatten into a lower crustal detachment or merge into diffuse lower crustal shortening and buckling. Layer-parallel-shortening preceded large-scale fault propagation and folding, with development of minor faults and subtle grain-scale fabrics. Arches define regional NW-SE to NNW-SSE trends, but individual arches are curved and vary in trend from N-E to E-W. 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. Ages and lithologies of synorogenic basin strata, and thermochron data from basement record protracted arch uplift, exhumation, and cooling, with deformation onset starting at ca. 80 Ma in the southern CO Plateau, then younging to ca. 60 Ma into northern WY and central MT, consistent with migration of a flat-slab segment. Basement-cored arches in SW MT, however, do not fit this pattern with deformation and inboard migration of igneous activity starting at ca. 80 Ma, possibly related to a slab window. Cessation of contractional deformation began at ca. 50 Ma in WY, followed by a southward migration to extension and an igneous flare-up, interpreted as rollback and removal of Farallon slab.

A model will be presented that combines development of a broad flat-slab related to subduction of an oceanic plateau and formation of a slab window, with stress transfer from flat slab to thick North American mantle lithosphere due to increased basal traction, end loading along a deep keel, and enhanced asthenosphere flow along slab margins. Diffuse mantle lithosphere and lower crustal shortening resulted in upward stress transfer, leading to focused mid- to upper crustal faulting influenced by pre-existing weaknesses, fault propagation, and linkage to form major arches and cover folds.