A Study On Apatite Fission Track Thermochronology Of Eastern Qilian Mountain

The process, time rate, and mechanism are key issues in understanding uplift and extension of the Tibetan Plateau, and attract consistent general concern in the geo-science field in recent decades. Margining the northeast Tibetan Plateau, the Qilian Mountain suffers dramatic tectonic deformation since Neogene, deduced from researches of ubiquitous thrust faults, active folds, and tectonic uplift in situ. Moreover, owing to its sensitive reflection upon the uplift, it provides an outstanding field laboratory for studies of plateau uplift and therefore receives much more concern recently. Nevertheless, the investigations on eastern Qilian Mountain is still insufficient comparing to the western Qilian Mountain, and in some extent, lack of data in this area impedes the full understanding of the effects of uplift of the Tibetan Plateau on surround regions.Thus, an enhancement of investigation on eastern Qilian Mountain is urgently required. Apatite fission track (AFT) dating is one of the most important thermochronology method springing up recently, and because of its sensitivity to temperature variation and lower closure temperature (100±20℃), AFT serves as a powerful approach in studies of tectonic activities,surface processes and mountain evolution.Here, we analyzed a suite of apatite fission track data from local ordovician granite samples in eastern Qilian Mountain, modelled their thermal history evolution, calculated the long-term surface denudation rates, and then discussed the regional tectonics and the effects of topography on fission track ages and long-term surface denudation rates.Our investigation draws some conclusions as follows:1.Apparent ages of 13 apatite fission track samples from eastern Qilian Mountain range from late Cretaceous to Paleocene(-56-86Ma) and concentrate upon 70Ma to 75Ma, far younger than the age of strata. The horizontal confined tracks are relatively short, in the order of～12.5μm, the distributions of track length are broad and display single skewed peak, which suggest a strong annealing with the samples while going through the upper part of partial annealing zone.2.The modeling result of thermal evolution history with fission track data indicates three main cooling stages in eastern Qilian Mountain since late Cretaceous:①～75 Ma to 18 Ma, a relative stable state, with an low erosion rate of～0.01mm/a.②～18 Ma to～8 Ma, a stage of relatively rapid exhumation, and the erosion rates is-0.07 mm/a.③～8Ma to present, a dramatic high exhumation stage, which characterized a multiple sub-stage, and the average erosion rates is-0.24 mm/a.3.The thermal evolution history of eastern Qilian Mountain illustrates the outward growth of the Tibetan Plateau prompting the gradual uplift of Qilian Mountain from northwest to southeast along the Qilian active faults system since middle-late Miocene, and the latest 8 Ma is the major stage of uplift of Qilian Mountain.4. Topography may have significant influence on apatite fission track ages, especially to the intense sampling dating as this paper involves. The age inversion may come from the discrepant annealing induced by feedback between topography and rock uplift-surface denudation process. Long-term surface denudation rates are in first order controlled by tectonic uplift and could reflect the geomorphical evolution under the influence of tectonics.