The extraordinary Namche Barwa sediment factory in the Eastern Himalayan Syntaxis

The Namche Barwa Syntaxis (NBS) is one of the most productive detrital factories on Earth. Previous studies have shown that the NBS supplies large amounts of sediment to the Brahmaputra River, although the sources and controlling factors of sediment production have not been ascertained in detail. This study presents petrographic and heavy-mineral data for 43 sand samples collected in the Yarlung and Parlung river catchments covering the entire NBS and surrounding areas. Combined with U–Pb ages of detrital zircons, our data indicate that 89 ± 11% of Yarlung River sediments downstream of the NBS are produced in the Yarlung and Parlung gorges. The annual sediment flux of the Yarlung River increases by a factor of 20 within ca. 250 km from upstream of the NBS (ca. 10 Mt) to downstream (ca. 200 Mt/a). The Yarlung and Parlung gorges, representing only ca. 1% of the Yarlung-Brahmaputra catchment area, contribute 74 ± 9% of the total Brahmaputra sediment flux. Average interannual erosion rates in the Yarlung and Parlung gorges corresponding to these fluxes are calculated to be 9.2 ± 1.2 mm/a and 6.5 ± 2.1 mm/a respectively. Focused erosion of the Namche Barwa Complex and Yarlung Suture Zone in the gorge, where high-grade metamorphic rocks are exposed, is a consequence of high channel steepness (ksn values up to 800–1800), high stream power and extreme events including frequent earthquakes and landslides. The coupling between surface erosion and tectonic uplift in the Yarlung Gorge is in full agreement with the tectonic aneurism model of NBS evolution.