INTERACTIONS OF FLUVIAL EROSION, CLIMATE AND TECTONICS IN THE EASTERN HIMALAYAN SYNTAXIS (Invited Presentation)

Rapid, localized rock uplift and efficient surface erosion make the eastern Himalayan syntaxis of northeastern India and southeastern Tibet one of the most dynamic regions of Earth’s crust. In the syntaxis, the Yarlung-Siang-Brahmaputra River cuts a steep knickzone through high Himalayan peaks and a rapidly exhuming crustal-scale antiform, raising questions about the possible role of focused fluvial erosion in initiating and maintaining rapid rock uplift and exhumation. We examine these questions using detrital zircon provenance, detrital zircon and white mica thermochronology, and geomorphological observations and numerical modeling of outburst floods. Results document the relative timing of two key events: establishment of the river course from Tibet through the syntaxis to the Himalayan foreland, and the onset of rapid exhumation within the syntaxis. Detrital zircon U-Pb, clast provenance and paleocurrent data for Neogene foreland basin (Siwalik Group) deposits indicate that the river course was established by at least ~11 Ma, while thermal modeling of double-dated zircon fission track and white mica ⁴⁰Ar/³⁹Ar cooling ages from the same deposits indicate exhumation rate increased 5-10 fold significantly later at ca. 7-5 Ma. The relative timing suggests tectonics rather than river capture triggered a positive feedback between crustal processes and erosion. Glacial damming and outburst floods may play an important role in halting knickzone migration and focusing erosion in the syntaxis through the Quaternary. We investigate the latter using simulations of a modern landslide-dam-break flood and of a reconstructed ancient glacial outburst flood through the syntaxis. Simulated peak flow speeds correlate with landslides observed directly after the modern event, and inundation patterns and flow depths are consistent with the style of deposition observed far downstream, displaying a clear link between flood hydraulics and geomorphic change. Preliminary results for simulations of ancient glacial outburst floods are consistent with field observations of slackwater flood deposits, and with intense erosion potential in the gorge region. These results shed light on the interactions of fluvial erosion and exhumation through the Late Cenozoic tectonic evolution of the syntaxis.