Laramide crustal detachment in the Rockies: Cordilleran shortening of fluid-weakened foreland crust

What causes previously stable continental crust in the forelands of Cordilleran orogenic systems to shorten during low-angle subduction? The National Science Foundation/EarthScope Bighorn Project combined seismic imaging of the crust and Moho with kinematic modeling of Laramide (Late Cretaceous–Paleogene) basement-involved deformation to address this question. In north-central Wyoming, asymmetrical ENE-verging upper-crustal folds are highly discordant with broader, N-trending warps in the Moho, indicating crustal detachment. Restorable cross sections of ENE-directed detachment at a depth of ~30 km, combined a smaller component of NNW–SSE shortening due to the east-narrowing shape of the crustal allochthon, can explain the anastomosing network of Laramide basement-cored arches without major deformation of the underlying mantle lithosphere. Thrust-related fold geometries and west-to-east initiation of deformation in the Laramide and Sevier thrust belts point to Cordilleran end-loading from the west. Differences between Laramide (~N65E) and plate (~N25E) convergence directions, along with the fanning of Laramide shortening directions from nearly E–W to the south to NE–SW to the north, indicate slip partitioning during end-loading west of the Rockies. Sub-horizontal detachment with a near-zero critical taper within cratonic crust suggests an extremely weak Laramide detachment zone during deformation. Analogous lower-crustal deformation in subduction forearcs is associated with slow earthquakes and slab dehydration. We hypothesize that low-angle subduction of the Farallon Plate suppressed fluid-consuming melting and corner-flow processes that characterize higher-angle subduction. This allowed subduction-generated fluids to escape upward into the overlying continental lithosphere, causing retrograde metamorphism and increased fluid pressure that facilitated crustal detachment. This hydration-based hypothesis predicts that crustal detachment will accompany major earthquakes in active analog orogens.