Geological structure of the southern part of the Nanga Parbat massif, Pakistan Himalaya, and its tectonic implications

Abstract

Abstract The Nanga Parbat massif lies in the core of the major north-south trending, broadly upright antiform that marks the NW syntaxis of the Himalayan arc. However, this antiformal structure is not evident in the trend of foliation and banding within the central and southern parts of the massif. Reconnaissance field studies in this region (Astor, Rama and Rupal areas) have delineated an important shear zone with top-to-the-south overthrust kinematics. This Rupal Shear Zone carries the migmatitic core of the massif onto nonmigmatitic metasediments locally termed the Tarshing Group. The shear zone traces north into a broad high strain zone of steep foliation with gently plunging mineral elongation lineations with no consistent sense of shear. A tentative model is proposed whereby top-to-the-south overshear in the Rupal area passes northwards into a steep belt of apparently constrictional N-S elongation. This type of large-scale transpression may record the early growth of the syntaxis. However, relating these structures to Himalayan orogenesis and the amplification of the NW syntaxis is problematic. The Nanga Parbat massif displays a long and complex history of polyphase deformation, metamorphism and magmatism, as might be expected of a terrane derived from the basement of the Indian sub-continent. Although at least the later part of the constrictional steep belt developed with syn-kinematic leucogranite intrusions (< 10 Ma), the old age limit on the Rupal Shear Zone remains unconstrained.