Sixty million years ago, the Rocky Mountains mysteriously emerged from a shallow sea. Old crystalline rocks were thrust upward, bending overlying sedimentary layers into large arches, producing many of the west’s signature mountains. How this happened so far from plate tectonic boundaries and so near the middle of an until-then stable continent are unresolved questions.

The EarthScope Bighorn Project is an integrated geological and geophysical investigation of these contractional basement-involved foreland arches. The geophysical portion of this project is called BASE: the Bighorns Arch Seismic Experiment. The Bighorn Project addresses how these foreland arches form and how they are linked to plate tectonic processes. The research in the Rocky Mountain Bighorn Arch of northern Wyoming and southern Montana combines geological investigations of surface geometries and kinematic indicators with geophysical imaging of 3D crustal and upper mantle geometries from an active/passive seismic experiment. The resulting 4D (3D spatial and temporal), lithospheric-scale model of foreland arch deformation tests current hypotheses for basement-involved foreland thrust belts in the Rockies, outlined on our Project Design page. Our investigation to determine the mechanism driving basement-involved arch formation is advancing our understanding of continental lithospheric rheology.