Onset of Rapid Exhumation in the Namche Barwa Syntaxis Constrained by Detrital Thermochronology

International audienceThe Himalayan syntaxes have intrigued geoscientists for decades because of their young (~10 Ma) high-grade metamorphism and exceptionally high rates of exhumation (up to 10 mm yr-1), leading to numerous models and inferences on potential coupling between tectonic and surface processes. Several models have been proposed to explain the rapid exhumation of the eastern Namche Barwa syntaxis that invoke different controlling factors, including compressive crustal-scale folding orthogonal to the strike of the mountain belt, the subduction geometry in the indentor region, or focused fluvial incision following river capture and ensuing crustal channel flow. These models predict different times of peak metamorphism and rapid exhumation, which remain poorly constrained. Existing bedrock and detrital thermochronology studies, based on zircon fission-track (ZFT) and mica 40Ar-39Ar (Mar) dating, suggest rapid exhumation since as early as 8-10 Ma or as late as ~3 Ma. Here we report detrital ZFT, MAr and rutile U-Pb data from a newly dated composite Siwalik section located directly downstream of the syntaxis and currently constituting the most proximal detrital archive of material eroded from the syntaxis by the paleo-Siang River. We incorporate these new data 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 syntaxis. Our results suggest onset of rapid exhumation of the syntaxis before 7 Ma, and possibly as early as ~13 Ma, at high but not extreme rates (~4 mm yr-1). In contrast to earlier modeling predictions, our best-fit model does not require exhumation rates to have slowed down over time. The detrital rutile U-Pb ages are particularly important in constraining our inverse models and we highlight the relevance of this thermochronological system for studying rapid exhumation of high-grade metamorphic massifs from the detrital recor