Antecedence of the Yarlung–Siang–Brahmaputra River, eastern Himalaya

At the eastern terminus of the Himalayan orogen, distortion and capture of southeast Asian drainage basins reflects regional patterns of crustal strain due to the indentation of the Indian Plate into Eurasia. After flowing eastward >1000 km along the southern margin of Tibet, the Yarlung–Siang–Brahmaputra River turns abruptly southward through the eastern Himalayan syntaxis rapidly exhuming a crustal scale antiform in an impressive >2 km knickpoint. This conspicuous drainage pattern and coincidence of focused fluvial incision and rapid rock exhumation has been explained by the capture of an ancestral, high-elevation Yarlung River by headward erosion of a Himalayan tributary. However, recent observation of Tibetan detritus in Neogene foreland basin units complicates this explanation, requiring a connection from Tibet to the foreland prior to the estimated onset of rapid rock exhumation. We constrain the sedimentary provenance of foreland basin units deposited near the Brahmaputra River confluence in the eastern Himalayan foreland basin using detrital zircon U–Pb geochronology. We interpret the significant presence of Gangdese-age detritus in each foreland basin unit to indicate that connection of the Yarlung–Siang–Brahmaputra River was established during, or prior to foreland deposition in the Early Miocene. Our results indicate that connection of the Yarlung–Siang–Brahmaputra River precedes exhumation of the syntaxis, demonstrating the potential for the progressive coevolution of rock uplift and rapid erosion of the Namche Barwa massif. •We sampled Neogene Siwalik units from the easternmost Himalayan foreland basin.•We used Detrital zircon U–Pb geochronology to determine sedimentary provenance.•Geochronology indicates a mixed Tibetan–Himalayan provenance in all Neogene units.•This indicates connection of the Yarlung–Siang–Brahmaputra River since the Neogene.•Coevolution of rock uplift and fluvial incision may explain rapid regional exhumation.