Preliminary Results of the Active Source Portion of the Bighorns Array Seismic Experiment (BASE), North-Central Wyoming, USA

The Bighorn Arch Seismic Experiment (BASE), is designed to image the Bighorn Arch in North Central Wyoming using active and passive seismic methods. The Bighorn Mountains formed as a basement involved foreland arch during the Laramide Orogeny. Whereas such arches are major features of both modern and ancient contractional orogens, the manifestation of arch shortening at depth has been highly contested due to the absence of detailed geophysical imaging of crust and mantle beneath the arches. BASE is a component of an integrated geological-geophysical program, the Bighorns Project, focused on discovering the nature of features that accommodate shortening at depth in such orogens. The active component of BASE was conducted in two deployments during the summer of 2010 and was designed to image the crust and upper mantle of the region. During the first deployment, ca. 1800 single channel Texan seismographs were deployed at 100 and 500 m spacings along a north-south profile located on the west flank of the Bighorn Mountains and along an east-west profile that extended across the arch from near Cody, Wyoming to north of Gillette, Wyoming. The seismograph array recorded blasts at 16 locations over two nights. The second deployment recorded 8 shots on ~800 Texan seismographs deployed at 200 and 1000 m spacings along the same north-south and east-west profiles. These shots were also recorded by ~400 additional Texans seismographs with 1 km spacing on a larger grid that encompassed the entire arch. Preliminary review of the data shows that shot energy propagated to offsets as great as 270 km and that large delays and advances in first arrival times correlate with the location of basins and exposed basement rocks respectively. Early one-dimensional velocity models show that the crystalline crust has velocities of 6.1 to 6.5 km/s to depths of ca 22 km, below which velocities increase from 6.8 to 7.0 km/s Crustal thickness appears to vary between 35 and 45 km. Future work includes developing velocity models from seismic tomography that will show what type of basement structures exist beneath the mountains, and, after integration with other datasets, how their geometries change in two- and three-dimensions.