Study On Apatite Fission-Track Thermo-chronology In Northern Qilian Mountain

Expansion and uplift of the Tibetan Plateau has received numerous studies and becomes one of hot debates in the field of earth science. Especially the uplift history of the Qilian Mountain which is lying along the northeastern margin of the Tibetan Plateau, is one of the keys to understand the uplift and expansion of the Tibetan Plateau. Cenozoic tectonic studies have shown that, strong tectonic deformation occurs in the whole Qilian area. But until now, the study of the uplift history of the Qilian Mountain is still very weak, and lack of detailed thermochronology research on the mountain range. Apatite Fission Track (AFT) is a more developed low temperature thermochronology method, and can effectively reflect the cooling history of the upper crust. In enriching to the current researcher, which is mostly focusing on sedimentology and active tectonic deformation, AFT analysis would greatly promote the study on the uplift history of the Qilian Mountain.11 granite samples were collected from Paleozoic granite intrusion at the Northern Qilian Mountain (Qilian Shan), and apatite crystals are extracted for fission-track analysis. Analysis results give that the AFT ages of the samples distribute from 31 Ma to 119Ma, and most are assembled in 50-90Ma. The fission track ages of the samples are extremely younger than their crystallizing ages in Paleozoic. The average length of the apatite tracks in the middle part of the Qilian Shan is 11.1μm, witch shows that the apatite fission tracks have undergone obvious annealing. The average length of the apatite tracks in the eastern part is about 12μm, also indicate an annealing, but is relatively weaker than that in the middle.Relied on the modeling result of the thermal history, the samples have undergone two different uplift-cooling processes from apparent age to now. In the middle Qilian Shan, it shows a slow cooling process from Late Cretaceous to late Miocene, and the average surface erosion rate is 0.005mm/a, From about 7Ma B.P., the cooling rate was obviously fastened, and have an average erosion rate of 0.647 mm/a. In the eastern Qilian Shan, also there is a slow cooling process from the Late Cretaceous to late Miocene, with an average surface erosion rate of 0.018mm/a, and since about 9Ma ago, the cooling rate was obviously increased, in an average erosion rate of 0.212mm/a.The cooling history of Granite in the Qilian Shan is probably document the uplift-erosion history of the mountain range. From late Cretaceous to late Miocene, the Qilian Shan experimented a slow erosion process, indicating a low relief in the mountain range, and beginning at 7-9Ma ago, the mountain range sustained a intense erosion,which may suggest the Qilian Shan began to uplift in a fast rate and increase the mountain relief. From the spatial distributions of AFT ages and cooling histories along the northern Qilian Mountain, we could find the northern Qilian Shan may experience a same uplift history since late Cretaceous.