| The uplift of Tibetan Plateau is an important geological event in the earth history.Its age,mechanism,and environmental effects have always been passionately discussed for several years.For researchers,Cenozoic terrestrial sediments provide a good perspective for further understanding the growth of Tibetan Plateau.These sediments are often continuous and well-exposed in the fault-controlled basins,which not only record paleoclimate information,but also reflect basin dynamics and tectonic activities of orogenic belt.Since the Late Cretaceous,a series of NW-SE-trending sedimentary basins have developed in the southeast Tibet,with a thickness of thousands of meters.As one of the fault-controlled basins,Gonjo basin is located to the north of the eastern Himalaya syntaxis,adjacent to the Jinshajiang and Bangong-Nujiang sutures,where there is a transition zone between southeast Tibetan Plateau and Yunnan Plateau.Terrestrial red beds with a thickness of 2000-3000 m are well preserved in Gonjo basin,consisting of reddish-brown sandstone,siltstone,mudstone and conglomerate,which can be divided into Gonjo and Ranmugou Formations.However,previous studies of Gonjo basin are controversial,especially the age of Gonjo sediments,which has not reached a consensus yet.For instance,palm fossils indicate the depositional age was from Eocene to Oligocene whereas sporopollens show a broader age that ranges from Paleocene to Miocene;Some researchers proposed a53-43 Ma depositional age by detrital zircon dating;Some used magnetostratigraphy to constrain the age to be 69-41.5 Ma although the isotopic age was not directly from the sampling section.These contradictory results have hindered researchers from understanding basin evolution and its potential response to regional tectonic-geomorphological process.Therefore,based on detailed field investigations,by using high-resolution magnetostratigraphy and U-Pb zircon dating,our study chose GD(Gongzhong-Dongtuo)and GR(Gonjo-Ranmu)sections from central Gonjo basin to establish a reliable chronostratigraphic framework.Moreover,systematic sedimentological work,anisotropy of magnetic susceptibility experiment,and sediment accumulation rates calculation were carried out.Under the guidance of basin-mountain coupling theory,this study aims to explore the tectono-sedimentary history of Gonjo basin and its potential response to regional tectonic and climatic change.The preliminary research contents and conclusions are as following:(1)The GD and GR sections are composed of two upward-fining sequences,corresponding to Gonjo and Ranmugou Formation,respectively.Sedimentological analysis indicates that Gonjo Formation can be subdivided into two units: the lower unit is a braided river deposit with local alluvial fan deposit;the upper unit is a floodplain deposit with mud cracks,calcareous nodules and paleosols.Ranmugou Formation can also be subdivided into two units: the lower unit is a braided river deposit,with scour structure/lag deposit,channel bar deposit and small-scale floodplain deposit;the upper unit is a lacustrine deposit,consisting of reddish-brown siltstone/mudstone interbedded with greenish mudstone,with gypsums and marls.(2)Three axis thermal demagnetization experiments show two types of thermal demagnetization curves.Greenish clay has relative higher percentage of low coercivity component,which shows an unblocking temperature of 575-585 °C,indicative of the presence of magnetite.The other one is reddish-brown sandstone/siltstone/mudstone,which contains more high and medium coercivity components with unblocking temperature of ?680?,indicating hematite is the main magnetic mineral.Thermal demagnetization experiments show most samples have high-stability characteristic remanent magnetization(Ch RM)component and demagnetize rapidly until 620°C,indicating hematite is the main magnetic mineral in most Gonjo sediments.(3)In this study,a 30 cm thick volcaniclastic layer was found at the top of GR section.The zircon U-Pb dating is 50.40±0.56 Ma,which help establish a reliable magnetostratigraphy.As a result,the age of central Gonjo basin is constrained to be69-50 Ma.Gonjo Formation lasted from 69 to 63.5 Ma,and the age of Ranmugou Formation was 63.5-50 Ma.Sedimentation began in the the central Gonjo basin(?69Ma),but finally ended in the northern and southern regions(?43 Ma).(4)Anisotropy of magnetic susceptibility(AMS)indicates that sediments from both GD and GR sections suffered ENE-WSW compression.However,our study suggests this reflects a post-depositional deformation at ?50 Ma or later,which is a direct response to the crustal shortening and volcanic activities in the Eocene,corresponding to the enhanced uplift of southeast Tibetan Plateau.(5)The evolution of Gonjo basin can be divided into five phases according to this study:Phase one(?69 Ma): The southeast Tibet was in a NW-SE-trending transtensional environment,and the regional climate was arid before the formation of Gonjo basin.Aeolian sandstone was scattered deposited in the central and southern Gonjo region.Phase two(69-63.5 Ma): With the development of strike-slip faults in the southeast Tibet,the central part of Gonjo region subsided and gradually formed a pull-apart basin.Almost the same time,the climate became semi-arid,and a braided river formed in the basin.Phase three(63.5-54 Ma): With the development of strike-slip pull-apart basin,sedimentation expanded from central to northern part of Gonjo region.Meanwhile,braided river deposits still dominated the Gonjo basin under a semi-arid climate.However,sediment accumulation rates decreased drastically with low sediment supply,which might be due to the weak weathering.Phase four(54-50 Ma): The strike-slip pull-apart basin was completely formed during this time,with increasing accommodation space and sediment supply.As a result,mega-thick beds of laminated siltstone and mudstone were widely deposited in the central,northern and southern Gonjo region,with relatively high sediment accumulation rates.In addition,Asian monsoon appeared in the southeast Tibet,which led Gonjo basin to be more humid.Phase five(<50 Ma): Due to the crustal shortening and uplift of southeast Tibetan Plateau,Gonjo basin experienced enhanced deformation in the Eocene,which caused basin inversion and thus ended sedimentation. |
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