Drainage Evolution Of The Hanjiang River In The Qinling Mountains

River capture is of great significance to landform evolution and hominine migration.In the north-south transition zone in China,the Hanjing River,originating from the Qinling-Daba Mountains,shows a special drainage system pattern,which is quite different from the Jialing River in the west.There is a viewpoint that Jialing River captured Hanjiang River,but this is still controversial.Because the evolution of drainage pattern is usually slow and continuous.It is necessary to combinate sediment provenance and fluvial geomorphy for drainage evolution analysis.In addition,the intermontane basins of Qinling Mountains are the important regions for paleolithic archaeological research in China.Reconstructing the drainage evolution processes in the Qinling Mountains may provide important materials and evidences for exploring the migration routes of hominins between North and South China.The study area is the Hanjiang River catchment in the Qinling Mountains.The evolution processes in this area are discussed by calculating the geomorphic indexes(11),9), and ? of the main rivers,and combining with the provenance analysis of paleocurrent directions,heavy mineral compositions and detrital zircon U-Pb age distributions.In terms of geomorphic evidences,the Hypsometric Integrals()reflect that the landform gradually changes from the old stage to the youth stage,from the Hanzhong Basin to the Shiquan-Ankang Basin.The valley shapes and width-to-height ratios((11))indicate two possible rapid incisions by the river captures.In the Shiquan-Ankang basin,wide valleys did not match with discharge,and a series of wind gaps developed.The ?values show that the watershed was moving to the side of the Yuehe River,and the Yuehe River was gradually shrinking.According to the sedimentary records,paleocurrent directions show that the east-west drainage pattern had been formed when terrace T5 was formed in the Hanzhong Basin.But the higher elevation fluvial sediments show different provenance characteristics.In the southwest of the Hanzhong Basin,the zircon U-Pb age distributions of CY is complex,covering all major age peaks compositions of the rocks around Hanzhong Basin(100~199Ma,200~250Ma,250~400Ma,400~505Ma,700~900Ma and>1000Ma).In the central Hanzhong Basin,the provenance of the sample on the south side of the modern Hanjiang River,indicates that it was derived from granitoids in the northern basin(200~250Ma,250~400Ma)and the heavy mineral compositions is obviously different.In Shiquan-Ankang Basin,sediments from high-altitude river terraces show the dominant indosinian age peak(200–250 Ma),which was mainly derived from granitoids in the northern Ningshan region.However,the modern fluvial sediments only show a single peak of Jinning period(700~900Ma)from the Yunxi Group and Yaoling Group..In conclusion,we suggest that the Hanjiang River gradually captured the Jialing River from east to west,accompanied by the evolution of the ancient Yangtze River:(1)the ancient Hanjiang River cut through the regional compression ridge,capturing the Hanzhong Basin river system(the ancient Jialing River drainage)from east to west,and(2)the southern tributary captured the trunk with the uplift of the divide in the Shiquan-Ankang Basin,forming the modern drainage pattern in the upper Hanjiang River.The activities of the regional strike-slip fault,and the associated compression uplift played a key role in the river capture,the drainage evolution,and the related landforms in the Shiquan-Ankang basin.In addition,it is shown that the evolution of the upper tributary basins lagged behind the response of the trunk channel.The interconnected wide valleys caused by river capture may have provided convenient geomorphic conditions for human migration into the Qinling-Daba Mountains along the river valleys.