LARAMIDE UPLIFTS IN WESTERN U.S. AS FAR-FORELAND DEFORMATION ASSOCIATED WITH THE INSULAR SUPERTERRANE COLLISION

Continent-continent collisions involve deformation far away from the plate boundary, often of basement-cored block uplifts (e.g., the Tien Shan uplift of the Himalayan collision). Paleomagnetic data indicate that the Insular superterrane was located from northern Mexico to Idaho (and possibly northward) at 100 Ma. Geological constraints within both the Insular superterrane and North America record collision at the corresponding latitudes. The evidence from North America includes dextral transpressional deformation in the magmatic arcs, thermochronology-constrained exhumation on Laramide block uplifts, and changes in foreland sedimentation patterns. Far-foreland deformation does not stop at the Rocky Mountain front, but extends eastward to Kansas and Nebraska, buried by younger Paleogene-Neogene sedimentation. The Lewis and Clark zone, recording sinistral motion in the Late Cretaceous-Paleocene, constrains the northernmost extent of Laramide-style block uplifts. Movement on the sinistral Lewis and Clark zone is hypothesized to result from clockwise rotation of the Blue Mountains and continental section of the central Idaho, itself caused by Insular superterrane collision and dextral translation of both the Insular and Intermontane superterranes along the western edge of North America.

Seismic results from the Bighorn uplift, WY, may indicate how Laramide deformation was translated into the continental interior. The mantle appears to bend upward, from a deeper Moho under the Bighorn basin to a shallower Moho under the Powder River basin. The Bighorn uplift is located over the area of maximum curvature of the Moho, a result also seen in scaled physical experiments of lithospheric-scale deformation. The bending is hypothesized to be the result of lithospheric folding, in which large-wavelength (150-350 km) lithospheric folds developed in response to an end-load (i.e., collision of the Insular superterrane). Importantly, there is no evidence for a throughgoing detachment below the Bighorn uplift in the active seismic data.