The nature and timing of deposition and deformation of Proterozoic siliciclastic metasedimentary cover sequences in the southwestern U.S is important for understanding Paleoproterozoic and Mesoproterozoic tectonism in the context of crustal accretionary episodes. These thick, ultramature siliciclastic successions are important orogenic markers that record regional sedimentation events that occurred between orogenic pulses. Their tectonic connections to these orogenic events remains an important issue. New geologic mapping and structural analysis of the siliciclastic Uncopmpaghre Group and the Vallecito Conglomerate in the Needle Mountains, southwestern Colorado, provide insight into the depositional and deformational history of one of the best exposed of these Paleoproterozoic cover successions. This study documents a gradational contact between the basal Vallecito Conglomerate and the overlying Uncompahgre Group and offers a transgressive depositional model for regional basin evolution. This new stratigraphic connection also helps elucidate deformational geometry. The succession is interpreted to be ~ 1.70 Ga based on new detrital zircon data. Structural analysis indicates that the two cover units underwent a similar polyphase deformation sequence involving D1 north-directed thrusting, D2 macroscopic folding and south-directed thrusting and D3 macroscopic folding-associated high-temperature low-pressure metamorphism of ~ 500°C and 2-3 kbars. D1 thrusting is interpreted to be related to the Mazatzal orogeny and resulted in distributed shearing along the basement-cover contact and duplexing of basement and cover. D2 deformation, the main folding event in the quartzites, is interpreted to be synchronous with andalusite growth throughout the area but is undated and could be related to either ~1.65 or ~1.43 Ga events. This event is associated with 50% shortening of the cover sequence via south-vergent folding and south-directed shearing. D3 deformation is interpreted to be temporally related to 1.43 Ga plutonism and resulted in doming and broad refolding of earlier structures. This structural synthesis reconciles previous models by reinforcing some aspects of each: early north-directed thrusting and multiple shear directions, south-directed shear zones and related macroscopic folds as the main structural elements of the Uncompahgre synclinorium, partial basement-cover detachment, and basement-cored thrust duplexes. Our interpretations do not strongly support the previous cuspate-lobate basement-cover infolding model.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Second Committee Member
SEMP, Rocky Mountain Section and GSA.
"Polyphase deformational history", Needle Mountains, Stratigraphy, Thrust Belt Model, Paleoproterozoic tectonism, Mesoproterozoic tectonism
Austin, Zinsser. "New stratigraphy, polyphase deformational history and basement-involved thrust belt model for the Proterozoic Uncompahgre Group and Vallecito Conglomerate, Needle Mountains, Colorado." (2006). https://digitalrepository.unm.edu/eps_etds/2