DETACHMENT FOLDING AND THE ROCKIES: NEW DATA FROM THE EarthScope BIGHORN PROJECT

Laramide arches of the Rocky Mountains in the western U.S. resemble detachment folds due to their anastomosing network of symmetric and asymmetric culminations separated by elliptical basins. In Wyoming, synorogenic sediments give northeasterly-younging initiation ages, consistent with detachment from west to east. Minor fault analyses show unidirectional, subhorizonal Laramide shortening and compression trending ENE-WSW. Arch spacings and depth-to-detachment calculations suggest crustal buckling above a lower crustal detachment, but the large scale of the deformation complicates excess area determinations due to isostatic effects. Thus, a variety of alternative hypotheses have been proposed, including lithospheric buckling due to detachment below the lithosphere, pure shear thickening of lithospheric mantle due to subduction-related hydration, and domino-style faulting of the entire lithosphere. All of these hypotheses predict different, seismically-resolvable Moho and lower crustal geometries.

The NSF/EarthScope-funded Bighorn Project is integrating kinematic studies to document Laramide slip directions with 3 passive (totaling over 1000 seismometers) and 2 active (up to 1850 seismometers and 17 ~1 ton blasts) seismic experiments to define the deep crustal geometry of the Bighorn Arch in northern Wyoming. This presentation will use preliminary results from these recently-concluded experiments to test hypotheses for basement-involved foreland arches. This project will allow a better understanding of the plate tectonic connections of basement-involved foreland deformation and the rheology of the continental lithosphere.