| The India-Eurasia plates collision during the Cenozoic formed the remarkable Tibetan plateau,which strongly influenced the global and regional climate change,the origin of modern Asian monsoon and aridification in central Asian.The uplifting progression and mechanism have been an important research highlights for many geologists.Quantitative paleoelevation reconstructing of the plateau can precisely reveal the uplift history and provide qunantitative constraint for the plateau uplift mechanism and evolution.Previous studies have shown that different parts of the Tibetan plateau has different uplift histories.The Tibetan plateau has underwent a gradual-stage progression.However,as the paleoelevation reconstructing has been contradictory by the different methods(stable isopote,fossils,sedimentology,structure etc),we need compare the existing paleoelevation datas obtained by different methods,and combined them with the paleogeographic background and paleoenvironment.The upper Oligocene-lower Miocene Dazhuka Formation(Fm.)was deposited unconformity above the Yarlung-Zangbo suture zone extending 1500 Km from west to east,was located in the core area of the India-Eurasia plates collision.The Dazhuka Fm.not only has richly recorded the uplift and denudation history of the Gangdse arc,Lhasa terrane and Tethys Himalay on the both sides of the suture zone,but also revealed the growth process of the southern Tibetan plateau during the late Oligocene-early Miocene.In this paper,we have measured seven upper Oligocene-lower Miocene Dazhuka Fm.sections along the Yarlung-zangbo river valley.According to the magneostratigraphy,volcanic rock and detrital zircon U-Pb dating,we have established the precise chronostratigraphy framework of the Dazhuka Fm.Based on the age control,the sedimentology and provenance of seven Dazhuka Fm,we formed the sedimentary-tectonic model of the Dazhuka Fm.Combining with the paleoelevation and paleoenvironment obtained by the plant fossils study of the Dazhuka Fm.,we discussed the paleoenvironment and uplift history of the southern Tibetan plateau during the late Oligocene-early Miocene.The main achievements are as follows:1.Based on the detailed analysis of lithofacies,lithofacies assemblages and sedimentary structure of the seven measured Dazhuka Fm.section,combining with the statistical analysis of paleocurrent and provenance,we suggest that the Dazhuka Fm.was deposited in an alluvial-fan and fluvial system,dominated with conglomerate and sandstone,with lesser volumes of siltstone and tuff.2.Our magnetostratigraphy study of the Dazhuka Fm.in the Menshi section(K2)yielded that the age of this section spanned from 25.1-21.8 Ma,and the age of the plant fossils occurred at 23.3 Ma.According to the yongest detrital zircon U-Pb age control and previous chronologic studies of the Dazhuka Fm.,we prefer the main depositing age of the Dazhuka Fm.were late Oligocene-early Miocene,trending youth from west to east.3.We distinguished 14 leaf morphotypes amongst 90 plant megafossils,occurred at 23.3 Ma,that were collected from the mensi Dazhuka Fm.section(K2)in the Kailas basin.The plant fossil assemblage comprises a needle of pine(Pinaceae),leaf fragments of TyphaL.,and 12 morphotypes of nonmonocot angiosperms including Populus cf.glandulifera Heer,Populus cf.balsamoides G?ppert,Quercus section Heterobalanus sp.,Alnus sp.,Betula sp.,Carpinus sp.,Leguminosae sp.and five types of dicotyledon leaves the affinities of which are not clearly unknown.Poplar and legume are most abundant in the fossil assemblage.Nearly half of the fossil leaves(41/90)belong to Populus,and 18 fossil leaves are referred to Leguminosae.Betulacea appear most diverse in fossil assemblage,including three genera.Extant Populus,Quercus section Heterobalanus,Alnus,Betula and Carpinus are all deciduous trees or shrubs,suggesting that a low diverse,poplar and legume dominated,deciduous broad-leaf vegetation with a few pines in the Kailas Basin,southern Tibet,indicating a temperate and humid environment during the latest Oligocene.By compiling the environmental niches of the living genera families that are related to these fossils,we inferred that the fossil plant assemblage is mostly like occurred at elevation of 1500–2900 m around 23.3 Ma in the Kailas Basin.Our paleoelevation obtained in the Kailas basin shows a low elevation in the western part of the Gangdse mountains during late Oligocene,combining with the high elevation obtained in the eastern part of the Gangdse mountains(Lin zhou)during Eocene,we inferred that the Gangdse-Yarlung-zangbo area was low in the west and high in the east during Eocene-Oligocene.4.According to the statistical analysis of paleocurrent directions,the paleocurrent of Dazhuka Fm.is generally from the east to west.The lower part of the Dazhuka Fm.was mainly southward and westward;the middle to upper part of Dazhuka Fm.was generally westward,but also shows a two-way characteristic of south and north,representing a aixal westward paleo-Yarlung-Zangbo River along the Yarlung-Zangbo suture zone during the late Oligocene-early Miocene,contrary to the present Yarlung-Zangbo river.5.According to the statistical analysis of gravel composition and detrital zircon age spectrum,the provenance of the Dzhuka Fm.in the lower part mainly came from the Gangdse arc,Lhasa terrane,and in the upper part of the Dazhuka Fm.from both sides of the south and north.Based on the provenance analysis,we inferred that the Gangdse in the north was the main erosion source area during the early depositing of the Dazhuka Fm.,and the Himalayan then was in a low elevation at that time.Later,the Himalayan in the south began to uplift and became as the main source of erosion area.6.According to the study of sedimentary facies,provenance,paleoelevation and paleoenvironment of the Dazhuka Fm.,we have established the sedimentary-tectonic model of the upper Oligocene-lower Miocene Dazhuka Fm.,suggesting that the Dazhuka Fm.deposits was controlled by east to west collison of the Indian-Euraisan plate and the continuous northward subduction of the Indian plate. |
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