The evolution of the Main Mantle Thrust in the Western Syntaxis, Northern Pakistan

Abstract

AbstractNeogene events in the Nanga Parbat-Haramosh massif have obscured much of its earlier evolution. However, structural mapping of the eastern margin reveals a ductile contact zone preserving many features of the original Main Mantle Thrust that emplaced the Ladakh island arc over the Indian margin in the late Cretaceous. The sequence of ductile deformation was controlled both by the contrasting rheologies of the Ladakh island arc and the Main Mantle Thrust footwall, and the changing thermal regime during subduction, collision and burial. Preliminary P-T estimates indicate conditions during southward thrusting on the Main Mantle Thrust of c. 650 °C and 9.5 kbar, with later deformation (post-dating garnet growth) in some units at c. 500 °C and 7.4 kbar. The concordant fabrics and lithological boundaries on either side of the contact are only disrupted by a NW-vergent, brittle thrust south of the village of Subsar (Indus gorge) which cross-cuts the steepened Main Mantle Thrust Zone. This thrust is related to the neotectonic Liachar Thrust on the western margin of the massif, and is an expression of the regional tectonics at the western termination of the Himalayan arc. This late thrusting followed formation of the syntaxial antiform in Neogene times.