Petrological constraints of the ‘Channel Flow’ model in eastern Nepal

The metamorphic architecture of eastern Nepalese Himalaya is characterized by a welldocumented inverted metamorphic field gradient, with metamorphic grade increasing northward from lower (LHS) to higher (HHC) structural levels across the north-dipping Main Central Thrust Zone (MCTZ). Peak metamorphic conditions experienced by units at different structural levels have been investigated extensively, but their P–T–(t) evolution could be constrained better. A synthesis of our recent petrological studies in eastern Nepal is based on selected geotraverses across the Dudh–Kosi, Arun, and Tamur tectonic windows, where the LHS is exposed beneath MCTZ and HHC. To define the entire P–T evolution experienced by lithotectonic units, detailed petrological investigations were focused on metapelites. P–T trajectories were constrained combining microstructural observations and isochemical phase diagrams modelling. The uniformity of the approach applied is a robust method to quantitatively compare the resulting P–T paths. These P–T paths are compared with the petrological constraints inferred from the ‘Channel Flow’ model, one of the most popular paradigms to explain the tectonometamorphic evolution and the first-order geological features of the Himalaya. The overall geometries of our P–T paths match the results of the numerical model, suggesting that ‘Channel Flow’ is compatible, from a petrological viewpoint, as the main process operating during the exhumation of eastern Himalaya.