BOILING FLUIDS IN A REGION OF RAPID UPLIFT, NANGA PARBAT MASSIF, PAKISTAN

The Nanga Parbat massif of northern Pakistan is currently undergoing rapid uplift (similar to 5-10 mm a(-1)), resulting in near-surface elevated temperatures. Numerous quartz veins cut geologically young structures (< 2 Ma), attesting to widespread young fluid flow. Fluid inclusions in quartz veins are predominantly low density water vapour (down to 0.05 mg m(-3)), with some low density carbon dioxide vapour, and the fluid is predominantly meteoric in origin. Fluid inclusions provide evidence for boiling near to the critical points for water and for 5 wt% NaCl solution (up to 410 degrees C). Head-driven meteoric water was convecting in fracture permeability under hydrostatic pressures which followed the boiling point-depth curve and near-boiling springs emanate from the surface. Hydrostatic pressures persisted to depths of about 6 km below the topographic surface, or near to sea level, where the brittle-ductile transition is inferred to lie. Numerical modelling of conductive heat flow in an area of high relief during rapid uplift indicates that the shape of the near-surface conductive geotherm is significantly influenced by topographic relief. Reasonable approximations for topography at Nanga Parbat produce a conductive geotherm which implies high, near-surface geothermal gradients (> 100 degrees C km(-1)), and the isotherms describe a giant pillar of heat. Above about 4 km, fluid temperature is greater than conductive rock temperature in permeable zones which carry convecting boiling meteoric fluid.