Formation Process And Tectonic Mechanism Of Qingtongxia Gorge Of Yellow River In Ningxia

Qingtongxia Gorge of Yellow River(QTXG)is located in the outermost edge of the northeast margin of the Tibetan Plateau,and it is a very important structural and geomorphic node where the upper reaches of the Yellow River change from denudation to sedimentation.When the Yellow River passes through the entrance of QTXG,the Yellow River did not continue its northward course through the relatively soft Cenozoic strata at the head of the gorge,but instead turned south to follow the western boundary of Niushou Mountain and then forming an “anti-S” shaped bending,which is obviously inconsistent with the river erosion law.In this paper,based on the Yellow River terrace,alluvial-diluvial fan and ancient channel of the Yellow River in Qingtongxia and surrounding areas are identified,and the characteristics of tectonic deformation and stress field are studied and analyzed,we found out the main structural geomorphic characteristics,the evolution process of the Yellow River and structural deformation process in Qingtongxia in the late Cenozoic.We discussed the evolution process of the Yellow River in this area and the formation of the QTXG.It is of great significance to study the development history of the Yellow River and the Cenozoic tectonic evolution of the northeastern margin of the Tibetan Plateau.The main achievements and understandings are as follows:(1)Based on the field investigation,terrace study,paleocurrent characteristics and chronological study of the main structural geomorphic units in Qingtongxia and its surrounding areas.It is found that the highest Yellow River Terrace developed in the south of Niushou mountain is T11,the highest T9 terrace developed in the south of the QTXG,the highest terrace at the entrance of QTXG is T6,and the central river channel of QTXG is around and out the highest terrace is T3 that the formation age is about 85 Ka.In addition,there are late Pleistocene-Holocene alluvial fans around Niushou mountain and in the north of the QTXG,which cover the early paleochannel.(2)The paleochannel of the Yellow River around the QTXG is verified by means of detrital zircon,heavy minerals,surface features,remote sensing interpretation,shallow borehole and shallow earthquake profile.The paleochannel sediments,Ganhegou Formation,are similar to the heavy minerals and detrital zircons of the modern Yellow River in the south of Qingtongxia.The heavy minerals are mainly "hematite + garnet + zircon + epidote + quartz and alteration minerals".The peak age of zircon is mainly 200-300 Ma,400-500 Ma and 1700-2000 Ma.And the paleochannel is distributed in the east piedmont of Niushou mountain in N-S direction.In the north of the QTXG,there is also a N-S-striking ancient Yellow River channel leading to Yinchuan basin,which is abandoned at about 85 Ka.(3)Based on the analysis of the structural deformation and stress field characteristics of the main fault zones around the QTXG,the evolution process of the structural deformation sequence and stress field in this area is restored.During Eocene-Miocene,the main structural stress fields were NW-SE compression in Qingtongxia area,the development of NW-SE-striking left-lateral normal faults and E-W-striking thrust faults.During the Pliocene,the tectonic stress field changed into NE-SW compression,and the development of NW-SE-striking right-lateral thrust and right-lateral normal fault.Since the late Pleistocene,N-S-striking normal faults and NW-SE-striking right-lateral strike slip faults have dominated the fault activity,and the structural stress field has changed to E-W extension.(4)Based on the study of paleochannel sedimentation,terrace,neotectonic chronology,structural deformation and stress field characteristics,the structural mechanism and evolution process of QTXG are preliminarily established.Before the late Pleistocene,the Yellow River mainly developed in N-S-striking at the Western piedmont of Niushou mountain and flowed into Yinchuan basin.In the late Pleistocene,Liumugao fault,Baima fault and Niushoushan piedmont fault are mainly NW-SE right-lateral strike slip normal fault activities.At the same time,the Liumugao fault and Niushoushan piedmont fault are left-stepped,forming a local compressive stress environment in Qingtongxia area,further contributing to the South diversion of the Yellow River ancient channel and cutting through the Ordovician bedrock along the early valley system,and forming the present “anti-s” shaped Grand Canyon.Later,the Yellow River flowed into Yinchuan basin along QTXG.Therefore,the formation of this special geomorphic feature of QTXG is closely related to the structural deformation and uplift of the northeast margin of the Tibetan Plateau.The establishment of this structural geomorphic process provides a scientific basis for the study of the structural and geomorphic evolution of the northeast margin of the Tibetan Plateau.