BASEMENT-INVOLVED LARAMIDE THRUSTING AND LOWER-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. Foreland arches have been explained by 1) crustal block translations and rotations on mantle-cutting faults, 2) crustal buckling and detachment, 3) lithospheric buckling, and 4) pure shear thickening. These hypotheses were tested with crustal-scale geophysical imaging and structural studies.

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 low-velocity and low-density regions at arch margins that co-locate with Laramide fault zones. Seismic tomography reveal a continuous, unfaulted Moho and a heterogeneous lower crust with discontinuous patches of high velocity (>7 km/s) crust quite unlike the more continuous high velocity lower crust to the west. The Moho surface is arched under the northern Bighorn Arch, but this NE-trending arch is at a high angle to the NNW-trending Laramide arch defined by folded Phanerozoic strata. This suggests that the Moho arch is largely Precambrian in age and that Laramide deformation was driven by crustal detachment.

Minor faults, paleomagnetic data and structural balancing show Laramide ENE-WSW shortening, locally complicated slip partitioning at the arch's northern plunge. Serial cross-sections show symmetric to ENE-verging arch geometries and reveal that the Bighorn Arch developed progressively by ENE-WSW layer-parallel shortening and detachment folding followed by ENE-directed slip on the Bighorn master thrust and associated folding. Depth-to-detachment calculations indicate Laramide crustal detachment at ~30 km depth. This structural sequence is identical to that of many thin-skinned thrust belts, suggesting that the Laramide Rocky Mountains formed as a crustal-scale thrust belt rooted in the hinterland of the Cordilleran orogen.