| The Tethyan Orogenic Belt is one of the largest continental collisional orogens on Earth,has received widespread attention across the globe in terms of related continental collision,Asian climatic change patterns,geodynamic processes,and seismic hazards.It is suggested that a complete process of initiation,development,shrinking,consumption and convergent for orogenesis for Tethyan sea or ocean-continent transformation in time-space configuration.The collision between the Qiangtang terrane and Lhasa terrane during the Mesozoic marked the closure of the Meso-Tethys,and subsequent the India-Asia collision during the Cenozoic marked the closure of the Neo-Tethys.Therefore,the timing of the closure of Meso-Tethys and Neo-Tethys are the keys to reconstruct the evolution of the Tethyan-Himalayan Orogenic Belt.New sedimentological,petrographic,biochronologic,and detrital zircon age data from the Cretaceous-Paleogene strata in the Beijia section near Gyangze town and Sangdanlin section near Gyirong town,southern Tibet,which are situated along the southern margin of the YTSZ were reported in this study.63 species from 50 genera radiolarians were obtained from the Gyabula,Zongzhuo,and Jiachala formations,belonging to Cretaceous zonation UAZ RK1-6,and Paleogene UAZ RP1-4.The Unitary Association method is applied in this biostratigraphic study,which is based on a deterministic mathematical model designed to construct a discrete sequence of co-occurrence species.The radiolarian zonation identified in the earliest Asia-derived detritus sedimentary sequences is bound to directly tied and complied with existing absolute age models from DSDP and ODP,indicating a?61.8-61.1 Ma age.Detrital zircons from the Lower Cretaceous Gyabula Formation are dominated by Archean to Cambrian U-Pb ages,indicating a likely source from the Indian continent.In contrast,detrital zircon ages of the Eocene Jiachala formations are dominantly younger than 200 Ma,with a major peak of 76-187 Ma,which is consistent with a detrital zircon source from an intra-oceanic island arc or the Gangdese arc.Integration of the biochronologic and detrital zircon age data suggest that collision of the India continent with the Asian continent or an intra-oceanic island arc should be no later than Danian at 61.8 Ma.New discovered radiolarians and radiolarites help to reconstruct the paleoceanography and paleogeography of the Meso-Tethys and suggest that an oceanic environment developed in northern Tibet until 101 Ma.Mesozoic radiolarites development in the Meso-Tethyan oceanic basin was coeval with Alpine Tethyan basin and some continental marginal basins,indicating near continental margin and locally restricted marine conditions during Jurassic to Cretaceous.Regionally,there are similarities of radiolarian fauna in the whole Tethyan realm,support that the Meso-Tethys is still connected with the Neo-Tethys during early Cretaceous.Studies presented here supply important constraints on the timing of the collision between the Lhasa and Qiangtang terranes.In addition,we also reported 29 species from 15 genera ostracods from the Paleogene Zhepure Formation in the Gulupu section,Tuna,Yadong,southern Tibet,The formation can be further divided into a lower Limestone Member and an overlying Shale Member,which are conformable.Abundant,diverse well-preserved marine ostracods were recovered from the Shale Member.The stratigraphic distribution of the Eocene ostracod assemblages found reveals three ostracod zones,and shows strong affinity to sections previously investigated throughout the Neo-Tethyan region.The Zhepure Formation also yields diverse planktonic and benthic foraminifers,charophytes,nannofossils,and pteropods and indicates that open marine conditions existed widely during the Eocene.Based on this study of little-known Eocene ostracods,we suggest that final closure of the Tethyan seaway occurred after early Late Eocene. |
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