EXHUMATION HISTORY OF THE WESTERN CHINESE ALTAI MOUNTAINS, CENTRAL ASIA, FROM SINGLE-CRYSTAL APATITE U-TH/HE THERMOCHRONOLOGY

It is proposed that late Cenozoic topographic growth in central Asia intensified aridity in the region and forced increased dust production, but direct constraints on the timing of mountain building in the Chinese Altai remain scarce. The NW striking Chinese Altai Mountains form a ~5.4*10⁵ km² area of high elevation west of the Mongolian Plateau in central Asia. The Tian Shan, Altai Mountains, and Himalayan-Tibetan orogen bound a ~3.2*10⁶ km² arid to hyperarid zone in central Asia. The Chinese Altai are bound by Cenozoic transpressional strike-slip faults; many of these faults exhibit Quaternary activity. Paleozoic granitic bedrock samples were collected along four transects in the western Chinese Altai between 43.1° N and 48.3° N and between 906 m and 3078 m in elevation for single-crystal apatite U-Th/He dating. The U-Th/He dates range from 272 Ma to 56 Ma (n= 82). The mean of 121 Ma (53 Ma SD) is skewed by a population (n=10) of >200 Ma crystals. The largest population of dates cluster at ~94 Ma. This pattern is consistent with >2–3 km of exhumation during the Cretaceous followed by minimal exhumation since the Late Cretaceous. The cluster of early Late Cretaceous ages reported here from the western Chinese Altai are younger than most AFT and AHe ages reported for the Mongolian Altai and Hangay Mountains to the east which are mostly Jurassic–Early Cretaceous in age. Development of significant late Cenozoic relief in the Chinese Altai is permissible if it was accompanied by minimal erosional exhumation (i.e., <2-3 km).