TEMPORAL VARIATIONS IN THE COOLING AND DENUDATION HISTORY OF THE HUNZA PLUTONIC COMPLEX, KARAKORAM BATHOLITH, REVEALED BY AR-40 AR-39 THERMOCHRONOLOGY/

The Ar-40/Ar-39 thermochronology of the Late Cretaceous Hunza plutonic complex reveals an episodic cooling and denudation history for this r egional-scale pluton. The Ar-40/Ar-39 analyses of biotites from a vert ical relief section of >3200 m reveal a pulse of rapid cooling at simi lar to 20 Ma. In the interval of 110-27 Ma, age-elevation distribution s suggest denudation rates of the order of 0.02 +/- 0.003 mm/yr. At si milar to 20 Ma, denudation rates increased significantly to 2.7 +/- 0. 7 mm/yr, then returned to much slower rates until 12 Ma. A second puls e of rapid cooling beginning at 12 Ma is revealed by inverse numerical modeling of multidiffusion domain alkali feldspars from a vertical se ction of 1700 m. Decreasing in elevation, these samples record the ons et of rapid cooling at 12, 9, and 7 Ma, respectively. All of the alkal i feldspars record a period of nearly isothermal conditions prior to t he onset of rapid cooling when rates increased to 30 degrees C/m.y. As suming a geothermal gradient of 30 degrees C/km, these cooling rates t ranslate into denudation rates of 1.0 mm/yr. Apatite fission track ana lysis indicates denudation rates of 0.7 +/- 0.1 mm/yr over the interva l of 6.6 Ma to 2.4 Ma in agreement with the alkali feldspar data. Thes e data suggest denudation of 2.9 +/- 0.4 km since the Plio-Pleistocene . Together, the alkali feldspar and apatite data indicate that a minim um of 10 km of overburden has been removed since the mid-late Miocene.