Evolution of the Central Asian Orogenic System and Himalayan-Tibetan Orogen: Constraints from Uranium-Thorium-Lead Geochronology, Thermobarometry, and Geochemistry

The focus of this dissertation is on the Central Asian Orogenic System (CAOS)--- an accretionary province encompassing a large fraction of the Asian continent including southern Siberia, eastern Kazakhstan, western China and all of Mongolia. The CAOS is known to consist of predominantly juvenile materials, yet the tectonic processes responsible for the creation and assembly of the juvenile components are poorly constrained. U-Pb geochronology and geochemistry of granitoids in western Mongolia reveals that arc magmatism migrated southwestward during the Neoproterozoic and early to middle Paleozoic, followed by a magmatic lull in the early Paleozoic. Widespread arc magmatism accompanies the end of the Paleozoic. U-Pb detrital zircon age spectra of the Gorny Altay flysch complex, which extends from southern Siberia, through NW China and into western Mongolia, show generally unimodal distributions and are consistent with deposition during the Ordovician-Silurian in a forearc/accretionary wedge setting. The results of U-Th-Pb monazite geochronology and petrographic analyses of rocks from the Altay metamorphic belt of NW China and western Mongolia indicate amphibolite-facies metamorphism in the Middle to Late Devonian followed by retrograde recrystallization in the Permian. The Altay metamorphic belt was exhumed along the Irtysh-Ertix-Bulgan fault, which was likely a major intra-arc thrust.;Additional results from this work come from the Himalayan-Tibetan orogen and pertain to the evolution of the Yalu-Brahmaputra River system of NE India and southern Tibet. Currently, the Yalu-Brahmaputra River is one of the largest rivers draining the Himalaya and its drainage extends north of the range, connecting the Lhasa terrane of Tibet with the eastern Himalayan foreland. U-Pb and Lu-Hf analyses of modern river sand and Neogene sandstones from the foreland basin were used to test whether this connection has existed throughout the river's history or was more recently established. The detrital signatures of the Neogene sandstones reveal the presence of material derived from the Gangdese batholith of southern Tibet in the foreland of the Arunachal Himalaya and imply a connection between the foreland basin and southern Tibet throughout Late Miocene and Pliocene time.