BUILDING ON THE LEGACY OF DEEP PROBE AND CD-ROM: CRUSTAL STRUCTURE OF THE BIGHORN MOUNTAINS, WYOMING AND ITS RELATIONSHIP TO PRECAMBRIAN AND LARAMIDE TECTONICS

Results from the Deep Probe and CD-ROM experiments showed that the crustal structure of the Rocky Mountain west records a history of complex lithospheric evolution from Precambrian accretion to Cretaceous-Paleogene Laramide shortening. The NSF/EarthScope-funded Bighorn Arch Seismic Experiment (BASE) collected two active source P-wave velocity model profiles across north-central Wyoming in 2010. Analysis of these velocity models and single-fold reflection data, together with potential field modeling of regional gravity and magnetic signals, constrains crustal structure and thickness of the Bighorn region. We image a west-dipping reflection boundary and model a sharp magnetic contact east of the Bighorn Arch that together may be part of a previously undetected Precambrian suture zone. Localized patches of a high-velocity, high-density lower crustal layer (the ‘7.x layer’) occur across the study area, but are largely absent beneath the Bighorn arch culmination. Moho topography is relatively smooth with no large-scale offsets, with depths ranging from ~50-37 km, and is largely decoupled from Laramide basement topography. These observations suggest that: 1) the edge of the Archean Wyoming craton may lie just east of the Bighorn Mountains, approximately 300 km east of previous interpretations; and 2) Laramide deformation may be localized in areas with thin or absent 7.x layer, due to its relatively weak lower crust, leading to detachment faulting. Our findings provide important new details about Precambrian tectonics and crustal architecture in Wyoming that augment the discoveries made in the Deep Probe and CD-ROM projects for which G. Randy Keller served as a lead investigator. In addition they indicate that basement-involved foreland arches may be critically dependent on pre-existing lateral heterogeneities in crustal structure.