CRETACEOUS DENUDATION OF THE ILI BASIN AS REVEALED BY FISSION-TRACK THERMOCHRONOLOGY

The fission-track dating method has been widely used to constrain the exhumation of mountains and tectono-thermal evolution of basins.This paper mainly presents the results of apatite fission-track dating and discusses the Mesozoic uplift-denudation event in the Ili basin.Fission-track dating revealed that the fission-track ages of three sandstones from drilling wells at the south margin of the basin are 112.6±9,79.3±11.4 and 94.0±7.4 Ma respectively,and that the corresponding fission-track lengths are 13.45±0.17,13.15±0.16 and 13.17±0.17μm respectively with a single peak distribution pattern.Further temperature-time modeling of apatite was conducted using the apatite fission-track solution program after the model of Ketcham,based on measurements of the fission-track lengths and pooled ages.The results show that one important uplift-denudation and cooling event took place in the Ili basin at 115-95 Ma.This cooling event is fairly consistent with the absence of Upper Jurassic-Lower Cretaceous strata in the basin,suggesting the uplift and subsequent denudation of the Ili basin during this period of time.The coexistence of the single apatite fission-track ages older and younger than the sedimentation age indicates that the apatite fission track was partially annealed after sedimentation as response to the temperature increase resulting from the burial.Supposing that the geothermal gradient is 30℃/km and that the temperatures of the partial annealing zone of apatite fission track vary from 80℃ to 120℃,it can be estimated that the samples were buried at ～2.6-4 km depth and that at least at least 1.8-km-thick material has been denuded with a denudation ratio of 0.09mm/a during the Mid-Late Cretaceous(115～95Ma).Correlation and analysis of regional data suggest that the Mid-Late Cretaceous uplift and denudation event was widespread in the whole Tianshan Range and even the whole orogenic belt in northern Xinjiang.