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

• Main Author: Shoshitaishvili, Elena
• Other Authors: Johnson, Roy A.
• Language: en_US
• Published: The University of Arizona. 2002
• Subjects: Geophysics.
• Online Access: http://hdl.handle.net/10150/280205

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.