Direct evidence for a steep geotherm under conditions of rapid denudation, Western Himalaya, Pakistan

Recent fluid-inclusion and 40 Ar/ 39 Ar cooling-age data show that currently exposed basement rocks in the Raikhot glacier valley of the Nanga Parbat-Haramosh massif, Pakistan Himalaya, were at temperatures of 350 ± 50 °C at depths of 6 ± 2 km (hydrostatic pressure correction). These data imply the presence of a steep thermal gradient in the upper crust at 1 Ma (29-100 °C/km) and denudation rates over the past 1.0 m.y. of 3-6 mm/yr, providing independent corroboration of previous estimates of rapid denudation at Nanga Parbat (4.5 mm/yr over 3.3 m.y.). Our data provide direct documentation of near-surface compaction of isotherms under conditions of rapid denudation, a result that has long been supported by thermal modeling.