First evidence for high-grade, Himalayan-age synconvergent extension recognised within the western syntaxis—Nanga Parbat, Pakistan

Abstract

We present evidence for horizons of medium to high grade (400–600°C) deformation accompanying normal sense displacement in high-strain Himalayan gneisses and schists from the southeastern Nanga Parbat massif, the western syntaxis in the Pakistan Himalaya. In their present orientation, the broadly N–S trending, steeply-dipping gneisses show microstructural and outcrop scale evidence for dextral and sinistral shear in discrete layers. We interpret these as horizons of thrust and normal motion within the footwall of the original Main Mantle Thrust, once Neogene antiformal folding is removed. This is the first report of significant normal motion in the Main Mantle Thrust footwall in the Nanga Parbat syntaxis and may indicate that synconvergent extension in the Himalaya extended to the western syntaxis. This episode of extension could correspond to a period of similar normal motion in the central Himalaya, or represent a separate event earlier in the orogeny.