Formation age and its tectonic geomorphological significance of Qingyiji-ang River terraces in the southern of Longmenshan, Sichuan

The earth's surface survey found that, across the south part of the Longmen Mountain, Qingyijiang River has incised can-yon along the upstream section, and terrace development is more complete along the downstream section. The chronological test of low terrace in Qingyijiang River was carried out by the method of OSL dating, and the formation age of low terrace was obtained. In combination with the systematic arrangement, comparison and analysis of the previous data and an analysis of the existing chronologi-cal data of the Qingyijiang River terrace, the authors found that the T1-T7 terrace in the middle and lower reaches of the Qingyi River was mainly formed at about 7.3～8.0ka, 40～50ka, 75～85ka, 129ka, 149ka, 270～300ka, 740ka. According to further inference from the results of the surface investigation, it is found that the present Qingyijiang River main channel already existed in the Middle Pleistocene and earlier. Therefore, the formation of the present Qingyijiang River resulted from the broken block tilting diversion or late river capture, which needs further research. In addition, the estimation of the age results of the geomorphic surface of the lower terrace of the Qingyijiang River shows that the crustal uplift rates of the Late Quaternary in different sections of the Qingyijiang River Valley in the south of Longmen Mountain are different. Since the Middle Pleistocene, the river incision rate of Lushan to Dagang-ding section has been significantly larger, which can probably be attributed to the geomorphic response of the Lushan seismogenic tec-tonics in the late Quaternary period.