IMAGING THE ROOTS OF ROCKY MOUNTAIN ARCHES: THE NSF-EarthScope BIGHORN PROJECT

The Bighorn Project is an integrated seismological and structural geology investigation to understand how basement-involved foreland arches form, how they are connected to plate tectonics, and what they reveal about the rheology of the continental lithosphere. Prior hypotheses for basement-involved arches include subcrustal shear and/or mantle hydration during shallow subduction and lithospheric buckling, crustal detachment, and/or domino-style lithospheric faulting during the end-loading of continental lithosphere.

All of these hypotheses predict different lower crustal and Moho geometries beneath foreland arches. They will be tested by combining the near-surface geology of the optimally-exposed Bighorn Arch of northern Wyoming and southern Montana with new kinematic data and the results of the Bighorn Arch Seismic Experiment (BASE), an EarthScope Flexible Array experiment. Existing outcrop and industry subsurface data will be used to develop a 3D model of the upper crust. Data from minor faults, joints and fold geometries will be used to delineated the history of kinematic movements. The geometry of the deeper crust and uppermost mantle for the Bighorn Mountains and the adjoining Bighorn and Powder River basins will be determined by the integrated passive/active BASE experiment. During the summer of 2009, 35 broadband instruments were deployed to densify the EarthScope Transportable Array stations already in the region. They have already recorded several magnitude 7+ teleseismic events. This summer, this passive array will be supplemented by 200 short period seismometers and a several week deployment of 800 “Texan” instruments. In addition, an active refraction experiment involving 30+ shots recorded by 1600 “Texan” instruments will occur in late July. The combination of traditional geologic data with the results from the simultaneous inversion of active and passive seismological data will give a detailed structural crustal image of the Bighorn region at all levels of the crust. This will be combined with our kinematic data to define a complete 4D (3D space plus time) restorable lithospheric model of arch formation revealing the rheology of continental lithosphere and the mechanisms of basement-involved arch formation.