LARAMIDE MID-CRUSTAL DETACHMENT IN THE ROCKIES: RESULTS FROM THE NSF/EARTHSCOPE BIGHORN PROJECT

The Bighorn Project documented the geometry and kinematics of the Bighorn Arch, a Laramide basement-involved arch in the NE Rocky Mountains. These foreland arches have been explained by (1) crustal block translations and rotations on through-going faults, (2) crustal buckling and detachment, (3) lithospheric buckling, and (4) pure shear thickening. We tested these hypotheses with detailed arch- and crustal-scale geophysical imaging and structural studies.

BASE, the Bighorn Arch Seismic Experiment, integrated passive seismic data (3 arrays with up to 850 seismometers) with active reflection/refraction data (24 shots, 1800 seismometers) collected over a 300 X 200 km area encompassing the central Bighorn Mountains and much of the adjoining Bighorn and Powder River basins. Complex shear wave splitting in the mantle below suggests intact Precambrian mantle lithosphere without wholesale Phanerozoic modification. Seismic tomography and gravity models show upper crustal thickening under the arch, with shallow low-velocity and low-density regions at arch margins that may correspond to Laramide fault zones. They reveal an unfaulted, continuous Moho and a heterogeneous lower crust with discontinuous patches of high velocity (>7 km/s) crust quite unlike the continuous slab of high velocity lower crust to the west. The Moho surface is arched under the northern Bighorn Arch, but this arch trends NE, at a high angle to NNW-trending Laramide arching defined by folded Phanerozoic strata. The Moho arch is probably dominantly Precambrian in age because it parallels Precambrian tends, not Laramide trends.

Minor faults, paleomagnetic data and structural balancing show regional Laramide ENE-WSW shortening, locally combined with gravity spreading away from the arch high. Despite a lack of a clear impedance contrast illuminating major fault surfaces, the ENE-verging arch geometry combined with depth-to-detachment calculations indicates Laramide crustal detachment at ~30 km depth. The Bighorn master thrust, with associated rotational fault-propagation folding, ramped off that detachment, thickening the upper crust and forming the Bighorn Arch. We propose that the Bighorn Arch, and much of the Laramide Rockies, formed above a mid-crustal detachment rooted in the hinterland of the Sevier collisional belt.