| The disappearance of ancient lakes in the Ruoergai Basin,the transfixion of the Yellow River and the formation of the modern river system at the source of the Yellow River are symbolic geomorphic events in the northeastern part of the Qinghai-Tibet Plateau,which play an important role in the evolution model of retrogressive erosion and river-lake transfixion of the modern Yellow River(Pan et al.,1993,2009;Li et al.,1991;Zhang et al.,2006;Harkins et al.,2007;Craddock et al.,2010;Su,2019).The Cairima-Ningmute valley,the source of the Yellow River,is about 310 km long from the Ruoergai Basin to the Xiqing Mountain.It is rich in river terraces and sedimentary types.It is the mouth of the lake water outflow in the Ruoergai Basin,the place where the East Kunlun fault zone passes through and the favorable place for the glacier material accumulation in the Anyemaqen Mountain.based on the investigation and analysis of fluvial geomorphology and valley sediments,combined with chronological work,the latest fluvial accumulation-incision event is reconstructed,and the influence of climate change and tectonic activity on the formation of modern geomorphology of the Yellow River is discussed.The main insights gained are as follows.(1)The river valley landform has a significant spatial distribution law,from top to bottom,the river valley shape transits from the one shape of the basin area to the U shape of the Maqu river valley,and then suddenly changes into the V shape of canyon section.The corresponding sedimentary facies are mainly fluvial and lacustrine facies in the basin,gradually changing into alluvium and diluvium,moraine or glacial debris flow deposits in the wide valley section,and then developing base terrace in the canyon section.(2)Combined with the results of chronological sample test,the geomorphic evolution process of the late Quaternary in the Cairima-Ningmute reach of the Yellow River is reconstructed.During ～50-20 ka,the coarse particles such as moraine,glacial debris flow and alluvium from Anyemaqen Mountain and Xiqing Mountain were unloaded to the Maqu River valley,resulting in the blockage of the river channel and the formation of barrier dam.The upper Ruoergai Basin was in a lake state during this period,and the lower bedrock canyon area was weakly eroded,with a fluvial gravel layer accumulated on the broad terrace.During the period of ～20-12 ka,the headward erosion accelerated from bottom to top,the rift point migrated rapidly to the upper reaches,and the river terraces developed,while the Ruoergai Basin was in a shallow lake state without being cut.After ～ 12 ka,the ancient barrier dam was cut through,the Ruoergai basin was replaced by fluvial facies,the Yellow River ran through the Ruoergai basin,and the river in the basin cut down to form terraces.(3)The geomorphic evolution of the Cairima-Ningmute reach in the source of the Yellow River is affected by both tectonic movement and climate change.The East Kunlun fault zone extending along the Maqu river valley-northern margin of the Ruoergai basin controlled the development of the Yellow River rift point zone and the formation of the deep fault depression in the northern part of the basin in the Late Neogene(Sichuan Geological Survey Institute,2008;Institute of Geology,China Earthquake Administrator et al.,2022).The blocking of the Yangtze in the Maqu reach occurred at ～50-20 ka,which coincided with the advance of the Late Pleistocene glaciers in the northeastern part of the Qinghai-Tibet Plateau during MIS3 b and MIS2.The cold climate in the last glacial period led to the large-scale development of glaciers in the Anyemaqen Mountain,which constituted the main material source of the largescale alluvial fans in the Maqu reach.The latest damming of the river-the barrier-took place. |
