abstract: Understanding the evolution of the Himalayan-Tibetan orogen is important because of its purported effects on global geodynamics, geochemistry and climate. It is surprising that the timing of initiation of this canonical collisional orogen is poorly constrained, with estimates ranging from Late Cretaceous to Early Oligocene. This study focuses on the Ladakh region in the northwestern Indian Himalaya, where early workers suggested that sedimentary deposits of the Indus Basin molasse sequence, located in the suture zone, preserve a record of the early evolution of orogenesis, including initial collision between India and Eurasia. Recent studies have challenged this interpretation, but resolution of the issue has been hampered by poor accessibility, paucity of robust depositional age constraints, and disputed provenance of many units in the succession. To achieve a better understanding of the stratigraphy of the Indus Basin, multispectral remote sensing image analysis resulted in a new geologic map that is consistent with field observations and previously published datasets, but suggests a substantial revision and simplification of the commonly assumed stratigraphic architecture of the basin. This stratigraphic framework guided a series of new provenance studies, wherein detrital U-Pb geochronology, 40Ar/39Ar and (U-Th)/He thermochronology, and trace-element geochemistry not only discount the hypothesis that collision began in the Early Oligocene, but also demonstrate that both Indian and Eurasian detritus arrived in the basin prior to deposition of the last marine limestone, constraining the age of collision to older than Early Eocene. Detrital (U-Th)/He thermochronology further elucidates the thermal history of the basin. Thus, we constrain backthrusting, thought to be an important mechanism by which Miocene convergence was accommodated, to between 11-7 Ma. Finally, an unprecedented conventional (U-Th)/He thermochronologic dataset was generated from a modern river sand to assess steady state assumptions of the source region. Using these data, the question of the minimum number of dates required for robust interpretation was critically evaluated. The application of a newly developed (U-Th)/He UV-laser-microprobe thermochronologic technique confirmed the results of the conventional dataset. This technique improves the practical utility of detrital mineral (U-Th)/He thermochronology, and will facilitate future studies of this type. / Dissertation/Thesis / Ph.D. Geological Sciences 2011

This dissertation investigates the evolution of continental orogenic systems in Central Asia during and between pre-collisional plate convergence (Cordilleran-style orogenesis), syn-collisional plate convergence (collisional orogenesis), and post-collisional tectonic processes within the scope of closing Paleo-Asian and Tethyan ocean basins. A brief introductory chapter outlines the scope and context of the research. Appendix A focuses on the Late Paleozoic closure of the Turkestan ocean basin and subsequent collision between the Karakum–Tarim and Kazakh–Kyrgyz terranes in the South Tian Shan, within the scope of the final amalgamation of the Mesoproterozoic–Permian Central Asian Orogenic Belt. Appendix B focuses on late Cenozoic syn-collisional exhumation of gneiss domes in the India–Asia collision, which is a component of the Triassic–recent Alpine–Himalayan orogenic belt. Abstracts of the results are provided in the respective appendices.