HIGH COOLING AND DENUDATION RATES AT KONGUR-SHAN, EASTERN PAMIR (XINJIANG, CHINA) REVEALED BY 40AR/39AR ALKALI FELDSPAR THERMOCHRONOLOGY

Orthogneiss samples taken from the Kongur antiform show ages varying from 2 Ma to 1 Ma for 40Ar/39Ar ages of biotites and muscovites and fission tracks on apatites, leading to cooling rates of 150-degrees-C/m.y. Modeling of K-feldspars highlights the effect of a range of diffusion domains with contrasting diffusion characteristics, yielding closure temperatures from 400-degrees to 150-degrees-C. The feldspar data document the cooling history since 5 Ma and indicate a sudden change in cooling rates of the antiform at 2 Ma. At that time, cooling increases by a factor of 5, from an average of 20-degrees-C/m.y. to a minimum of 150-degrees-C/m.y. Consideration of the regional thermal history, ongoing uplift, and erosional history of the antiform during the Quaternary suggests that denudation rates have been of the order of 5-7 km/m.y. since 2 m.y. ago and could be associated with significant upward surface movement triggered by major normal faulting. The antiform is interpreted to have formed during thrusting at the Pamir front as a result of the development of thrust ramps and normal faulting at the crustal scale. Ramp stacking is an important process of mountain building, and normal faulting in this context must be regarded as a very efficient way of building high relief.