The Geological Records Of The Quaternary River Evolution Of The Qingyijiang River,West Sichuan Province,Southwest China,and Its Neotectonic Significances

The development of the river system records the process of the new tectonic movement in the related areas,on the basis of systemically arranging and analyzing the data of previous achievements and results,we analysed and summaried the gravel composition,gravel particle size and gravel direction of Mingshan-Qionglai and Danling-Simeng gravel layers related to ancient Qingyi River diversion in southwest of Longmenshan.Moreover,we discussed the characteristics of provenance region,origin and formation of the gravel layer.Combining with the latest survey and measurements to trunk stream terrace of Qingyi River,the distribution,height,structure and age of the terrace were sorted and summarized,and the characteristics of tectonic landform and drainage system are analyzed by using satellite remote sensing image and digital elevation model(DEM)data.At the same time,the chronological test of the low terrace in Qingyi River was tested and analyzed by the method of OSL dating.The age of low terrace was obtained,and the chronological data of the river terrace in Qingyi River were analyzed and compared.The paper has made the following progress:(1)It is speculated that the age of the T1 to T7 terraces of Qingyi River is about 7~9ka,40~50ka,75~85ka,129 ka,149ka,270~300ka and 740 ka.(2)The investigation and study of the paleo alluvial gravel layer and the present Qingyi River river terrace show that the Qingyi River has obvious river diversion and restructure during the development process,while the Quaternary terrace along the Qingyi River main stream is continuous and comparable,indicating that the Qingyi River channel should occur before the Quaternary or pre Pliocene.During the Quaternary period,it was mainly accompanied by regional climate and tectonic processes,which led to the formation of continuous terraces under the Qingyi River.(3)It is judged that the present Qingyi River stage should be formed in the Middle Pleistocene,that is,the present position of the Qingyi River in the Middle Pleistocene may have existed in the period of the Middle Pleistocene,and there is a dispute over the Qingyi River as holistic tilted diversion or the later river capture.(4)The phase diagram and river undercutting rate chart of Qingyi River main stream show that the terraces in the west side of Lushan are characterized by tectonic uplift terraces,there is a high order at Liziping,but the nearby contrastive terrace has not been found,and since the Middle Pleistocene the Qingyijiang River between the Lushan and Dagangding river incision rate than other downstream quickly,this is probably the seismogenic response of the Lushan seismogenic structure(blind thrust fault)during the late Quaternary.