Geophysical investigation of Archean and Proterozoic crustal-scale boundaries in Wyoming and Colorado with emphasis on the Cheyenne Belt

This work presents geophysical investigation of the rock properties of crustal boundaries in Colorado and Wyoming that were established during Proterozoic continental amalgamation. I used multicomponent seismic reflection/refraction data to determine seismic velocities, Poisson's ratios and geometries of shallow subsurface structures across the Cheyenne Belt, an Archean-Proterozoic boundary in southeastern Wyoming, and high-frequency geoid data for modeling density contrasts associated with crustal boundaries in Wyoming and Colorado.; I adapted a time-domain-based filtering technique described by Butler and Russell (1993) to filter the multicomponent seismic data because high-amplitude harmonic noise obscured P- and S-wave first arrivals. The travel-times of filtered P-wave first arrivals were inverted to obtain a model of both P-wave velocity and subsurface geometry. Since S-wave data quality was inferior to that of the P-wave data and S-wave ray coverage of the subsurface was discontinuous, I proposed a method to estimate Poisson's ratio using SiO₂ concentration and the average atomic weight (AAW) of a formation with known mineral and oxide compositions. Subsequently, the final P-wave velocity model was converted into an initial S-wave model using Poisson's ratios estimated by this method. The S-wave data were inverted for velocities only, keeping the subsurface geometry derived from P-wave inversion constant. The dependence of Poisson's ratio on AAW and SiO₂ concentration, and measured mineral Poisson's ratios, permitted estimation of two- or three-mineral compositions of formations in the vicinity of the seismic line from the Poisson's ratio model calculated using final P- and S-wave velocity models.; Geoid data were modeled along four north-south profiles with positive density contrasts in the crust compensated by deeper negative density contrasts. The modeled crustal-scale bodies were correlated to regional geological features based on their relative locations. Thus, out of an infinite number of possible models explaining the geoid anomalies, I obtained one that fits both the geoid data and current tectonic models.