| The Hengduan Mountains is located in Southwest China, which consists of East Xizang, West Sichuan and Northwest Yunnan. The Hengduan Mountains having formed in the Cenozoic, is famous for its deep canyons and unique geomorphology. Based on tectonic division, the studied region is situated in the copula of South China block and close South China continental block group (Ⅰ) and the Gandwana and close Gondwana continental block group (Ⅱ). In this region, tectonic movement is very active, and many Cenozoic fault basins are formed during the tectonic movement. The formation, development, and distribution of basins are strongly influenced by tectonic forces. In the Hengduan Mountains region, the mountains are parallel and are predominantly north-south alignment. Rivers distributing among mountains are rich in water source. Moreover, the new tectonic movement is active in this region. Therefore the Cenozoic sedimentary basins in this region are an ideal place to study the rock properties, features of stratigraphy and depositional systems, palaeogeography as well as palaeoevironment of the Eastern edge in Qinghai-Tibet Plateau.The Cenozoic stata and plant fossils in four basins were studied,i.e.,the Mangkang Basin,Yongping basin, Jianchuan basin and lanping basin.Based on stratigraphy theory and in combined with stratigraphy, geochemistry and paleontology, petrology, geochemistry, palaeogeography geography and palaeoenviroment, together with other multi-disciplinary comprehensive research, the changes of Cenozoic palaeogeography and palaeoenvironment in this region were discussed.Basins in this study are mainly controlled by Jinshajiang-redriver deep fault, Lancangjiang deep fault, Bijiang fault, Deqin-Weixi-Qiaohou fault, Lijiang-Jianchuan fault and Chenghai fault.Cenozoic basins in the Hengduan Mountains mainly consist of Paleogene (Palaeocene series, Eocene series) and Neogene strata(Miocene series, Pliocene series). Paleogene stratas distribute in Lijiang-Jianchuan Basin. Neogene stratas are found in Changdu-Lanping-Simao Basin. Volcanic rocks exist in Jianchuan Basin and Mangkang Basin.The Jianchuan Formation is dominated by intermediate rocks and consists of eight volcanic-eruptive cycles, Whereas the Lawula Formation mainly consists of trachyte-dacite,and comprises two cycles of volcanic eruptions.The geochemical features of the volcanic rocks in the two basins indicate that they have the same tectonic background and tectonic dynamics. In both the JianChuan and Lawula formations,the volcanic rocks are not only showing the characteristics of intraplate basalts, but also having some characteristics of arc volcanic rocks; At the same time, they may have the elements of the upper mantle, the shell source materials dragged into the the mantle wedge along the subduction zone, and the mixed metasomatic products of ocean sediments, i.e. the recycling of the continental crust materials into the mantle.Based on our investigations on the volcanic magma activitives both in Mangkang and Jianchuan area and the magmation activity in Jinshajiang-redriver structural belt, we conducted that during the first stage of movement of the Qinghai-Tibet plateau at the Late Eocene, when the Neo-tethys were not closed completely, the volcanic island arc was formed close to one side of the mainland; During the Eurasian plate subducting under the Indian plate, a series of strike-slip faults were formed on the volcanic island arc of the continental margin,which resulted in a magnanimous E and ES tectonic extrusions on the Changdu-Lanping-Simao block.It also evovled a series of pulling-apartures and fault basins along the fault zone of Jinsha river-Red river, and formed a series of volcanic eruptions and hypabyssal intrusive actions in peripheral basins. What’s more, the chronology analysis also shows that both the JianChuan Formation volcanic rocks (36.34±0.27Ma) and Lawula Formation volcanic rocks (35.19±1.3Ma or36.44±0.29Ma) were formed in the late Eocene, and they produced magmatic rocks at the same period.The Cenozoic sediments are widely distributed in the studying area, but the classification and correlation of these sediments are inconsonant. According to our synthetic lithostratigraphically,biostratigraphically and chronostratigraphically studies of the Cenozoic in this area, the age of the JianChuan Formation in JianChuan region and Lawula Formation in MangKang region is belonging to the Eocene.Our data also shows that the Oligocene sediments are absent in Hengduan Mountains.Cenozoic sedimentary basins in the study area,have various types of sedimentary facies, mainly including braided river delta, lacustrine, alluvial fan, and braided river facies. The sedimentary systems have been discussed in detail in this study. Vertical variation of the sedimentary facies of the Sanying Formation in Yongping is mainly including the fluvial delta facies at early sedimentary stage, and lake facies at the late sedimentary stage. The Sanying Formation in Lanping can be roughly divided into two sedimentary cycles.Each sedimentary cycle represents one process from subsidence expansion to filling shrink; The vertical variation in sedimentary facies of the Sanying Formation in Lanping is mainly lake facies and delta facies at the early sedimentary stage, lake facies at the middle sedimentary stage, and braided river facies at the late sedimentary stage. The volcanism of JianChuan Formation comprises of eight cycles, including a major eruptive and few small eruptive facies, therefore, a large number of pyroclastics were formed. The middle-upper sedimentary facies of Lawula Formation in Mangkang predominented by the lacustrine facies, but conspicuously changed in the vertical direction from the volcanic eruption facies to lacustrine facies, volcanic eruption facies, braided river facies, volcanic eruption facies, braided river facies, and lacustrine facies. The volcanism of the lower Lawula Formation is comprised of two cycles, e.g.major eruptive and overflow facies. Through field investigation, there are a large number of fossils in Sanying Formation and Lawula Formation. The palaeoclimatic analysis shows that the climate in Hengduan Mountains changed from hot arid to cool mois condition. |
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