Contractional, basement-involved foreland deformation in the Rocky Mountains of the conterminous United States occurred during the latest Cretaceous to Paleogene Laramide orogeny. Current kinematic hypotheses for the Laramide orogeny include single-stage NE- to E-directed shortening, sequential multidirectional shortening, and transpressive deformation partitioned between NW-striking thrust and N-striking strike-slip faults. In part due to this kinematic uncertainty, the links between Laramide deformation and plate-margin processes are unresolved, and proposed driving forces range from external stresses paralleling plate convergence to internal stresses due to gravitational collapse of the Cordilleran thrust belt.
To determine the tectonic controls on Laramide deformation, kinematic data from minor faults (n = 21,129) were combined with a geographic information system (GIS) database quantifying Rocky Mountain structural trends. Minor fault data were collected from a variety of pre-Laramide units to calculate average Laramide slip (N67E-01) and maximum compressive stress (N67E-02) directions for the Rocky Mountains. These largely unimodal, subhorizontal slip and compression directions vary slightly in space; more E-W directions occur in the southern and eastern Rockies, and more NE-SW directions are found near the Colorado Plateau.
This kinematic framework was extended to the entire orogen using map data for faults, folds, arches, and Precambrian fabrics from Wyoming, Colorado, northern New Mexico, southeastern Utah, and northeastern Arizona. Vector mean calculations and length-weighted rose diagrams show that Precambrian fabrics are at a high angle to most larger Phanerozoic structures but are commonly reactivated by smaller structures. Ancestral Rocky Mountain structures are subparallel to Laramide structures, suggesting similar tectonic mechanisms.
Laramide faults, defined by their involvement of Mesozoic and Paleogene strata but not Neogene strata, are complex, and preexisting weaknesses and minor strain components commonly predominate. In contrast, Laramide fold (avg. N24W) and arch (avg. N23W) axis trends are oriented perpendicular to minor fault slip and compression directions due to their generation by thrust-related folding.
Laramide deformation shows the primary external influence of ENE-directed shortening paralleling published convergence vectors between the North American and Farallon plates. Slightly radial shortening directions, from more NE-directed to the north to more E-directed to the south, suggest focused contraction originating near current-day southern California. A slight clockwise rotation of shortening directions going from west to east is consistent with proposed changes in Farallon–North American plate trajectories as the orogen expanded eastward. Additional complexities caused by localized preexisting weaknesses and impingement by the adjoining Cordilleran thrust belt have provided structural diversity within the Laramide province. Obliquities between convergence directions and the northern and southeastern boundaries of the Laramide province have resulted in transpressive arrays of en echelon folds and arches, not major through-going strike-slip faults.
