Early Onset and Late Acceleration of Rapid Exhumation in the Namche Barwa Syntaxis, Eastern Himalaya

The Namche Barwa massif, occupying the eastern syntaxis of the Himalaya, is characterized by young metamorphism, rapid and highly localized exhumation, and extreme river-incision and erosion rates. Both the timing and mechanism of rapid exhumation in the massif, as well as the potential links with the evolution of drainage patterns in the eastern Himalayan region, remain controversial, leading to numerous models and inferences on potential coupling between tectonic and surface processes. Existing bedrock and detrital thermochronology studies suggest rapid exhumation since as early as 8-10 My ago (Ma) or as late as ~3 Ma. Here we present new detrital zircon fission-track (ZFT), mica ⁴⁰Ar-³⁹Ar (Mar), and rutile U-Pb (RUPb) data from a newly dated composite Siwalik sedimentary section located directly downstream of the syntaxis, which currently constitutes the most proximal detrital archive of material eroded from the syntaxis. These data are incorporated together with published detrital data from the syntaxis region into a 1D version of the thermokinematic model Pecube to invert for the most probable rate of exhumation and timing of onset of rapid exhumation in the Namche Barwa. Our best-fit inverse thermo-kinematic model shows that rapid exhumation (~4 km My^-1) of the massif started ~8 Ma, but accelerated to ~9 km My^-1 after ~2 Ma. These results reconcile apparent contradictions in previous studies, are consistent with geomorphological observations, and support Quaternary operation of the long-debated "tectonic aneurysm" model.