Geology of the Nanga Parbat Syntaxis Along the Astor Valley Transect, Northern Pakistan

Abstract

The Nanga Parbat-Haramosh syntaxis represents the youngest phase of thermal, tectonic and metamorphic activity in the Himalaya. The syntaxis is an active crustal-scale antiformal structure comprising Indian plate basement gneisses and cover sequence. The Nanga Parbat rocks in the Indus gorge section have previously been divided into grey Iskere gneiss and pink Shengus gneiss. Along the Astor valley transect, south of Indus valley, several new lithological units have been identified. These include 1) Muskhin grey gneiss 2) Harchu pink gneiss and 3) Rattu formation, The Mushkin gneiss is a medium-grained, locally-porphyroclastic gneiss of grey colour probably representing the basement complex. Harchu pink gneiss is clearly derived from a sedimentary protolith (as suggested by the preserved relicts of quartzite, calc-silicate components and preserved sedimentary structures). The Rattu formation comprises metapellites, marbles, calc-silicates pods and lenses and is considered to represent a metamorphosed cover sequence. Structural cross-section from north-west to south-east reveals that the Nanga Parbat sequence consists of two crustal-scale antiformal structures with a tight faulted synform in between. It is noticeable that the Mushkin grey gneiss exhumes with respect to the cover sequence at the margins. Field and petrographic data show that the Rattu formation and Harchu pink gneiss reaches upto kyanite and sillimanite metamorphic grades, whereas the Mushkin grey gneiss lacks these minerals. The Mushkin grey gneiss exhibits a great variety of migmatite structures, indicating that the Mushkin grey gneiss has experienced more severe metamorphic conditions than any other rock unit in the Astor section. Our preliminary studies indicate possibilities of correlation between Mushkin and Harchu gneisses from the Astor valley with the Iskere and Shengus gniesses from the Indus valley, respectively.