Jurassic integrative stratigraphy,biotas,and paleogeographical evolution of the Qinghai-Tibetan Plateau and its surrounding areas

The Qinghai-Tibetan Plateau experienced a unique geological evolution during the Jurassic,driven by the termination of the Palaeotethys and the reduction of the Neotethys.The Indian Plate separated from the northern margin of Gondwana and drifted northward from the Southern Hemisphere.Given that the timing of strata serves as the basis for reconstructing geological history,the present work aimed to develop a new multiple stratigraphic and chronologic framework for the Jurassic strata of the Qinghai-Tibetan Plateau region via a synthesis of the material on lithostratigraphy,palaeontology,iso-radiometric dating,magnetostratigraphy,and other techniques with an emphasis on recent progress and findings.The new framework included the Jurassic System from the four major subdivisions of the plateau:the Baryan Har,Qiangtang,Lhasa-Gandise,and Southern Xizang(Himalaya).Ultimately,a more complete,refined biostratigraphic sequence was proposed,comprising the most common fossils in the plateau and those that are stratigraphically significant for the Jurassic stratigraphy,including ammonites,bivalves,brachiopods,foraminifera,radiolarians,and dinoflagellate cysts for the marine strata,and pollen and spores,and charophytes for the terrestrial sediments.This biostratigraphic framework was correlated with the Jurassic international standard zonation of the Geological Time Scale 2020 via standard or representative species or genera of ammonites.Based on this framework,we constructed a lateral correlation of the Jurassic strata between different basins of the plateau.The palaeontologic correlation in the present work shows that the Lhasa-Gandise Block had a closer relationship with the Qiangtang Block than with the Southern Xizang Himalaya during the Jurassic Period.Meanwhile,the Lhasa-Gandise Block and Qiangtang Block shared similar marine fauna features of the north marginal East Tethys.This contrasts the opinion suggesting that the Yarlung Zangbo Tethys was a small back-arc basin.A combination of stratigraphical,palaeontological,and sedimentological analyses implies that the Bangong Co-Nujiang Tethys may have begun rifting in the Late Triassic,evolving to the birth at the late Early Jurassic with the formation of ocean crust.However,this resulted in failure after it grew into the climax at the end of the Middle Jurassic when the Qiangtang Block began subducting under the Lhasa-Gandise Block.In the Early Cretaceous,the two blocks finally merged.