LATE QUATERNARY DEPOSITION AND INCISION SEQUENCES OF THE GOLMUD RIVER AND THEIR ENVIRONMENTAL IMPLICATION
Chen Yixin~① Li Yingkui~② Zhang Yue~① Zhang Mei~①Zhang Jingchun~① Yi Chaolu~③ Liu Gengnian~①(①College of Urban and Environmental Sciences,Peking University,Beijing 100871;②Department of Geography,University of Tennessee,Knoxville,TN 37996,USA;③Key Laboratory of Tibetan Environment Changes and Land Surface Processes,Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing 100085)
Golmud River is a large interior river that originates from the Kunlun Mountains and drains into the Qaidam Basin on Northern Tibetan Plateau.Golmud River valley has experienced intensively tectonic movements and drastic climate changes since the Last Glacial,which deeply influenced the geomorphologic evolution of the river.In addition,well-developed fluvial terraces and sediments are preserved in the river valley.Therefore,Golmud River is an ideal place to examine the climate changes and tectonic movements on the Northern Tibetan Plateau,as well as their roles in influencing fluvial landform evolution.This research is mainly through field investigation and sediments analysis.The chronology of the terrace sequences is established using optically stimulated luminescence(OSL)and radiocarbon dating methods.Detailed work was carried out in Sanchahe(35°52.567' ～35°52.992'N,94°21.590' ～ 94°21.990'E;3728 ～ 3793m a.s.1.),Nachitai (35°52.951' ～ 35°54.229'N,94°32.904' ～ 94°37.291'E; 3510-3584m a.s.1.),and Xiaogangou (36°08.173'N,94°48.645'E;3198m a.s.1.)sections,and 24 OSL samples and 2 radiocarbon samples were collected. Four terraces were identified in the Golmud River valley,T4 expands continuously and raises 26 ～ 62 m above the riverbed.It is a filled terrace consisting of Sanchahe Formation.The Sanchahe Formation,typical braided river sediments,started to deposit no later than 82ka,and ended in 16ka when the river began to cut down and formed T4.T3,T2 and T1 are all strath terraces with 0.5 ～ 5.Om-thick layers of fluvial sediments covering the Sanchahe Formation.These terraces are not continuously distributed,but could be found at Nachitai and Sanchahe sections. The OSL ages of these three terraces are 13ka,llka and 5ka correspondingly. Based on these results,two landform evolution stages since Last Glacial are identified:Sanchahe deposition stage (82 ～ 16ka)and Golumd incision stage (since 16ka).During the deposition stage,the Golmud River valley began to subside,the climate was cold and humid,and the glaciations were intensive and persistent.All of the above factors accelerated the deposition of the Sanchahe Formation.The average deposition rate was 0.94mm/a,and the highest one was 13.3 mm/a during 37.8 ～ 36.Oka (MIS 3),which reveals an intensive exhumation period during MIS 3.The Golmud incision stage began the Last Glacial Maximum.It can be further divided into four substages which correspond to the incision stage of the four terraces T4(16 ～ 13ka),T3(13 ～ 11ka),T2(11 ～5ka)and T1 (since 5ka).During the incision stage,major faults became active in this area,and the surface uplift was significant;in addition,deglaciation also accelerated the tectonic uplift,which led to the incision of the Golmud River.The incision rates increased from T4 ～ T3 (16 ～ 13ka,9.33 ～ 3.23mm/a)to T3 ～ T2 (13 ～ 11ka,12.0 ～ 5.5 mm/a)and then decreased since T2 (since 11ka,1.00 ～ 0.33 mm/a,which indicates that the isostatic adjustment induced by glacial retreating was an important impact factor on regional tectonic uplift.Besides the temporal difference,the uplift rates also varied in different sections,the uplift rate was the largest one in Sanchahe section(62m,3.88mm/a),while the rate was mild in Nachitai section(26m,1.61mm/a).This deformation of the terraces suggests that the Yeniugou fault at Sanchahe section became active since the Last Glacial.The Yeniugou fault is a thrust fault,and its uplift rate started to rise since 16 ～ 13 ka,peaked during 13 ～11ka and then decreased after 11 ka. Since the Last Glacial,both tectonic movements and climate changes influenced the geomorphologic evolution of the Golmud River.The two factors usually co-vary and show an obvious interactive effect.
【Fund】： 国家自然科学基金面上项目(批准号:40571014和40971002);; 国家重点基础研究发展规划项kt(973项目)(批准号:2005CB422001)资助
【CateGory Index】： P343.1
【CateGory Index】： P343.1