LARAMIDE KINEMATICS AND TECTONICS OF THE ROCKY MOUNTAINS

Geometric, kinematic and geophysical observations from the Rocky Mountains provide excellent tests of tectonic models for basement-involved orogens. Current hypotheses for Laramide deformation include (1) block tilting models with through-going, high-angle faults and (2) basement thrust models with (a) pure shear thickening of the lower crust under Laramide arches, (b) subcrustal shear during low-angle subduction, (c) lithospheric buckling, and (d) crustal buckling during NE-directed detachment in the lower crust.

Laramide minor faults are dominated by thrust and strike-slip faults indicating NE-SW to E-W horizontal shortening and compression. GIS analyses of map-scale folding show that fold orientations become progressively more north-trending toward the east, suggesting strain refraction during NE-SW transpression. These orientations falsify block tilting models invoking vertical or normal faults and suggest that strike-slip faulting is secondary to thrust faulting.

Gravity and deep seismic data indicate a relatively flat Moho under Laramide arches, which disproves block tilting models predicting reverse faults that cut the Moho. This evidence for rootless Laramide arches is also inconsistent with models that predict pure shear thickening of the lower crust under individual arches. Recent P-wave tomography documenting thick Rocky Mountain lithosphere (generally exceeding 200 km) contradicts models predicting major removal of mantle lithosphere by subcrustal shear during low-angle subduction.

Geophysical data showing a relatively flat Moho under individual Laramide arches contradict lithospheric buckling predictions. In addition, buckling wavelengths of thick Rocky Mountain continental lithosphere would be expected to exceed 200 kilometers, which is the observed buckle wavelength of thin, 40 Ma oceanic lithosphere. In reality, maximum wavelengths between Laramide arch culminations are closer to 100 km, with greater distances between arches in Wyoming and lesser distances in central Colorado near Laramide magmatism. In conclusion, Rocky Mountain crustal geometries are consistent with detachment of the upper and middle crust initiating arches by buckle folding followed by dip-slip and oblique-slip fault-propagation and rotational fault-bend folding.