Thermochronology Of The Jianghan Basin And Peripheral Aeras And Its Implications For The Formation Of The Yangtze Three Gorges

The Yangtze River is one of the longest rivers in Eurasia,almost across the whole East Asia.Its formation and evolution is always linked to the climate and geomorphic changes of East Asia.However,the evolution of the Yangtze River is the most controversial issue in geological topic,especially on the formation time of the Yangtze Three Gorges and the First Bend.The Yangtze Three Gorges is considered to be the last step in the evolution of the Yangtze River,and its formation represents the establishment of the modern Yangtze River system.At present,there are three main views on the formation of the Yangtze River: 45-40 Ma,36.5-23 Ma and 3.2-0.75 Ma respectively.The main reason for the controversy is related to the limited methods and incomplete sediments.Located in immediate downstream of the Three Gorges,the Jianghan Basin contain complete Cenozoic sediments which would record the formation information of the Three Gorges.Based on the source-sink system,the complete Cenozoic sediments from the Jianghan Basin and the bedrock along the Yangtze River in the Three Gorges are expected to detect the formation time of the Three Gorges.Apatite(U-Th)/ He and apatite fission track dating of bedrocks along Yangtze River are used to constraint the incision of the Three Gorges.Detrital apatite fission track dating,detrital zircon U-Pb dating and heavy minerals are used to analyze source-sink system.Combined incision of the Three Gorges and provenance analysis,the formation of the Three Gorges is constrainted.Based on apatite fission track and apatite(U-Th)/ He data along the Yangtze River,thermal history suggest there is an enhanced cooling events during 23-32 Ma in the Zigui Basin,but absent in the Huangling massif(Three Gorges).Combined with provenance analysis of the Jianghan Basin,the Oligocene cooling event is related to the incision of the Three Gorges.The Cenozoic sediments of Jianghan Basin show that Paleocene-Lower Oligocene Shashi Formation,Xingouzui Formation,Jingsha Formation,Qianjiang Formation and Jinghezhen Formation contain siltstone,mudstone and gypsum,which are lacustrine environment.The heavy mineral analysis of clastic rocks in the same period shows that the basin received the main source from the adjacent orogens.The upper OligoceneMiocene Guanghuasi Formation are composed of conglomerate and sandstone,which is fluvial environment,thus representing the sediments of the Yangtze River when the Three Gorges formed.This is proved by detrital apatite fission track and detrital zircon U-Pb.Detrital apatite fission track dating indicate that the eastern Jianhan Basin was supplied by the Dabie and Jiangnan orogens during the Shashi,Xingouzui,Jingsha,Qianjiang and Jinghezhen stage,and the western basin was related to Huangling and Xiangexi fold-thrust belts.However,during the late Oligocene-Miocene Guanghuasi stage,Huangling massif is the main sources of the eastern basin.The change of source reflects that the Three Gorges was incised by the Yangtze River.The zircon U-Pb age distribution of Jingsha,Qianjiang and Jinghezhen Formations are undistinguished from Cretaceous sediments of Jianghan Basin,This indicates that the source-sink system of Jianghan Basin is relatively stable during Cretaceous-early Oligocene.However,the occurance of the Cenozoic zircons which are the unique zircons of the Tibetan Plateau indicates that the upper reaches of the Yangtze River have supplied detritus to the Jianghan Basin at least during Guanghuasi stage.Based on the 23-32 Ma Oligocene cooling events in the Three Gorges area,the sedimentary environment changes during the Guanghuasi stage,the strong incision of the Three Gorges and the Cenozoic zircon from the Tibetan Plateau,the formation of the Three Gorges is proposed to be formed in the early Guanghuasi stage(late Oligocene to early Miocene,? 26-23 Ma).