Metamorphic constraints on the tectonic evolution of the High Himalaya in Nepal; the art of the possible

This review presents an objective account of metamorphic, microstructural and geochronological studies in the Greater Himalayan Sequence (GHS) and adjacent units in Nepal, in the light of recent research. The importance of integrated, multidisciplinary studies is highlighted. A personal view is presented of strategies for determining P-T evolution, and of petrological processes at the micro scale, particularly in relation to departures from equilibrium and the behaviour of partially-melted rock systems. Evidence has accumulated for the existence within the GHS of a High Himalayan Discontinuity, marked by differences in timing of peak metamorphism in hanging wall and footwall, and changes in P-T gradients and paths. Whether or not this is a single continuous horizon, it forms at each location the lower boundary to a migmatitic zone capable of ductile flow, and separates the GHS into an upper division in which channel flow may have operated in the interval 25-18 Ma, and a lower division characterized by an inverted metamorphic gradient, and by metamorphic ages that decrease down-section and are best explained by sequential accretion of footwall slices between 20 and 6 Ma. An overall model for extrusion of the GHS is still not resolved.