Evidence of episodically accelerated denudation on the Namche Barwa massif (Eastern Himalayan syntaxis) by megafloods

Abstract

Megafloods cause widespread mobilization of sediments that have strong potential to control the geodynamic evolution of the Namche Barwa (NB) massif by changing the landscape morphology within a short time. However, the impact of catastrophic events on the denudation of the massif over the longer time scale has not been explored in detail. This contribution assesses the impact of catastrophic floods on the denudation style of the Namche Barwa (NB) massif over millennial time scales. Towards this, 21 optical ages and ∼1700 detrital thermochronometric ages of zircon and apatite minerals extracted from paleoflood sediments downstream of the massif, were integrated with decadal sediment load and aggradation data to estimate the minimum denudation rate during paleoflood events. The results indicate that the 7 ka flood eroded ∼71 m of material from the NB massif. This denudation is equivalent to the material that would be produced over ∼8000 years of erosion at a rate of ∼9 mm/a. The 3.5 ka flood eroded ∼24 m of material equaling ∼2500 years worth of erosion at a rate of ∼9 mm/a. Such relatively instantaneous denudations are three orders of magnitude higher than averaged denudation rates over million-year time scale. This study, therefore, provides the first evidence of the strong interplay of catastrophic flood events and tectonics for the rapid exhumation of the Namche Barwa massif. Megafloods from the failure of dammed lakes provide valuable information to understand the flood surge risk for human settlements in the Siang and the Assam valleys.