Abstract Early Oligocene partial melting and prolonged low-pressure–low-temperature (low-P/T) metamorphism were investigated in migmatites and orthogneisses from the upper High Himalaya Crystalline Sequence (HHCS) in the far east of Nepal. The migmatites were formed by biotite dehydration melting at c. 800°C from 33 to 25 Ma. Cordierite was only produced at shallow crustal levels at pressures <6 kbar. After Early Oligocene partial melting, the low-P/T metamorphism continued until 17 Ma during exhumation of the cordierite-bearing migmatites. Early Oligocene biotite dehydration melting in the upper HHCS occurred at different times and locations from the Early Miocene muscovite dehydration melting in the underlying HHCS and the metamorphic discontinuity was accompanied by thrusting of the High Himalayan Discontinuity at c. 27–19 Ma. Pervasive partial melting and prolonged low-P/T metamorphism in the upper HHCS is more compatible with a lateral southwards channel flow of the upper HHCS along the High Himalayan Discontinuity, whereas current channel flow models explaining the exhumation of the HHCS as driven only by the coupled activity of the Main Central Thrust and South Tibetan Detachment have faced difficulties in explaining the timing of the low-P/T metamorphism observed in the upper HHCS.

Abstract The Kosi River in India is well known in the fluvial fan literature because of its well-documented avulsive dynamics and because of the relationships between changes in the course of the river and megafan aggradational processes. The radial configuration of the Kosi drainage network was instrumental in the recognition of large, low-gradient, fluvial-dominated counterparts of alluvial fans, commonly defined as megafans, and the system forms a well-constrained example contributing to the recent development of the concept of distributive fluvial systems. A major flood inundated the Kosi Megafan in August 2008. The available data on the temporal evolution of the flood inundation patterns illustrate how the exceptional discharge event travelled across the megafan surface using the pre-existing distributary channel network, how the anthropogenic infrastructure affected the flood path and propagation, and the type of geomorphic changes that were induced by this catastrophic hydrological event. In spite of the large discharge involved in the flood, the fan drainage network appears not to have been significantly modified by the event, probably because the flood wave followed a pre-existing channel network for avulsion and down-fan propagation. Most of the evident aggradation took place on the proximal and medial domains along a well-defined, radially oriented sector of the fan. The observed pattern of flood propagation and associated sedimentation provide important clues to understanding the processes operating during exceptional discharge events, which are an integral part of the long-term, avulsion-dominated evolution of such systems.