THE PERSISTENCE OF RAPID, FOCUSED EXHUMATION IN THE EASTERN HIMALAYAN SYNTAXIS

The eastern Himalayan syntaxis is one of the most dynamic regions of Earth’s crust where rapid rock uplift and efficient surface erosion are linked over highly localized spatial scales. After flowing eastward >1000 km along the southern margin of Tibet, the Yarlung-Siang-Brahmaputra River turns abruptly southward through the syntaxis, plunging through a 2 km knickzone across a rapidly exhuming crustal scale antiform. This coincidence of fluvial incision and active tectonic uplift has been hypothesized to result from an extreme positive feedback between surface and crustal processes (i.e., a ‘tectonic aneurysm’). Feedback initiation may be linked to a hypothetical river capture event wherein the ancestral Yarlung River was captured by a headward eroding Himalayan tributary of the Brahmaputra. This explanation is complicated by observations of Tibetan detritus in Neogene foreland basin (Siwalik Group) deposits that indicate a fluvial connection from Tibet to the foreland preceded the onset of rapid rock exhumation. Alternatively, the Tibetan detritus may have been delivered via a Yarlung-Subansiri River connection nearly 200 km downstream of the Siang-Brahmaputra confluence. We evaluate this possibility using new detrital zircon U-Pb data to constrain sedimentary provenance of Siwalik detritus more proximal to the Siang-Brahmaputra confluence, and document the onset of rapid exhumation within the syntaxis using zircon fission track and muscovite ⁴⁰Ar/³⁹Ar cooling ages from the same deposits. The presence of Gangdese-age (Tibetan) zircons throughout the entire depositional sequence establishes the connection of the Yarlung-Siang-Brahmaputra River in the Middle Miocene, likely by 12 Ma. Thermal modeling of muscovite and zircon lag times indicates an increase in source exhumation rates in the Late Miocene with rapid exhumation rates persisting since 5 Ma. Antecedent drainage of the Yarlung-Siang-Brahmaputra River through the syntaxis prior to this time rules out river capture as a mechanism to initiate rapid exhumation. We suggest that localized tectonic uplift associated with syntaxial deformation initially increased rock exhumation rates by steepening the antecedent river channel, leading to a thermo-mechanical feedback that has remained focused in the syntaxis through the Plio-Quaternary.