Geochronologic Constraints on the Tectonic Evolution and Exhumation of Nanga Parbat, Western Himalaya Syntaxis, Revisited

We examine the timing of deformation and exhumation of the Nanga Parbat-Haramosh massif in the western syntaxis of the Himalaya. This study presents geochronologic and thermochronologic data obtained from basement, shear zone, and intrusive units within the massif to reveal the Cenozoic tectonic evolution of the massif and to document the extent of the Plio-Pleistocene tectonic activity. These results combined with structural and petrologic observations indicate that the western Himalayan syntaxis was tectonically active for a protracted length of time and that the deformational succession was punctuated by three episodes since the beginning of the India-Asia collision: (1) The western syntaxial Indian plate rocks (future Nanga Parbat) underwent metamorphism, melting, and deformation during the initial collision of India with Asia and the associated island arc, the result of which is recorded as Eocene to Early Miocene metamorphic and magmatic ages and Oligocene cooling preserved in the Indian cover metasedimentary sequences that flank the younger, high-grade core. (2) Transpression along the South Karakorum fault to the north during the Late Miocene resulted in an episode of crustal scale doming and associated tectonometamorphic processes within the Himalaya syntaxis concurrent with the formation of other Late Miocene domes that developed in the Karakorum to the north and east. (3) At the core of the massif during the Plio-Pleistocene, the latest evolutionary stage of Nanga Parbat produced granulite-grade metamorphism, anatectic melting, and rapid cooling with deformation manifested as a pop-up structure, overprinting the Late Miocene doming.