Overview of hydrothermal activity associated with active orogenesis and metamorphism: Nanga Parbat, Pakistan Himalaya

Abstract

The Nanga Parbat Massif is located in northwestern Pakistan and occurs in the interior of the western syntaxis of the Himalaya. Recent studies have shown that the massif exposes an active metamorphic anomaly developed in Indian- Plate rocks exhumed from beneath and surrounded by rocks of the Kohistan/Ladakh island-arc. Geochemical and geophysical data from Nanga Parbat show that an active hydrothermal system exists within the massif. In the upper 5 to 6 kilometers of the crust, above a shallow brittle/ductile transition constrained by microseismic data, there is a shallow hydrothermal system with meteoric fluids channelized along active fault zones. In contrast, below the brittle/ductile transition fluids are metamorphic in origin and restricted to unconnected packets along grain boundaries. The shallow system is stratified with water-rich fluid inclusions near the surface grading to vapor-rich inclusions at the brittle/ductile transition, as evidenced from fluid inclusion studies. These fluids are meteoric in origin and enter the massif along shear and fracture zones and exit the massif along the bordering fault zones. Evidence for these channelized meteoric fluids comes from stable isotopic values of minerals and rocks collected from fault zones that show that these rocks have interacted with meteoric fluids, as well as magnetotelluric (MT) data that show that the fault zones are electrically conductive and embedded in very resistive rocks. The deeper portions of the hydrothermal system consist of unconnected metamor- phic fluids. Stable isotopic and fluid inclusion data show that fluids were present during the recent metamorphism but the rocks did not extensively interact with these fluids. Existence of an unconnected fluid system is supported by the MT studies showing that below the brittle/ductile transition the rocks are relatively resistive (values ranging from 1000-10,000 ohm-m), ruling out an interconnected fluid network. Seismic and MT data also show no evidence of a large magma body at depth. We suggest that the type of hydrothermal system observed at Nanga Parbat can be an important feature of late-stage orogenic evolution, especially near the tectonically complex edges of orogens. The tectonism in these areas exposes deep, relatively dry rocks that have previously lost their volatile content during the earliest stages of tectonic thickening and metamorphism. These relatively dry rocks are rapidly uplifted and exposed within the massifs where they interact with surface waters at high temperatures and shallow depths.