The Background And Styles Of The Mesozoic Continental Margin Rift Basin In Eastern Tethys Himalaya,South Tibet

The Mesozoic sedimentary record in the eastern the Tethyan Himalaya is crucial for understanding the tectonic evolution of Tethys.This study makes a preliminary study on the Mesozoic rift basin of the Luozha-lhunze area in eastern Tethys Himalaya,South Tibet.The rift basin is established by Upper Triassic to Lower Cretaceous stratas in the Luozha-lhunze area,experienced Late Triassic-Middle Jurassic and late Jurassic-Early Cretaceous filling cycles,southwest boundary of the basin May be located in Daglung-Luozha-Quzhuomu area.The filling process of the rift basin can be divided into two stages,The first stage is the initial continental margin rift basin,including continental slope(Nieru Fm.),shelf margin basin(Ridang Fm.)and shallow shelf(Lure Fm.)sedimentary sequences.The second stage is a continental rift basin characterized by strong volcanic activity,this stage consist of shallow sea(Zhela Fm.),coastal-delta(Weimei Fm.)and shallow sea(Sangxiu Fm.)sedimentary sequences.Provenance analysis in this study shows that Upper Triassic Nieru Fm.and Jurassic strata are characterized by the significant population of 450-650 Ma and 750-1000 Ma,strongly supporting the view of affinity to the India.However,it is worth noting that the regional Nieru Fm.exist provenance of Lhasa terrane.The chrome spinel of Weimei Fm.have low Al2O3(0.50%-20.82%),high Cr#(0.51-0.99)and Fe2+/Fe3+(2.14-41.27)ratios and variable TiO2 contents(0.00%~1.95%),these Characteristics Suggests its parent rocks may be derived from the Carboniferous-Permian strata in the Tethyan Himalaya.Through the comparative analysis of sedimentary filling sequence and provenance,this study believes that Nieru Fm.and Langjiexue Gr.are different stratigraphic units,and further proposed that in Late Triassic continued southward subduction of the Bangong-Nujiang Tethyan oceanic lithosphere beneath the Lhasa terrane led to the slab rollback,this rollback then results in back-arc rifting between the Lhasa terrane and the northern India.The rift is very narrow,so that the Lhasa terrane provides simultaneously the source for Langjiexue Gr.and Nieru Fm.However,Langjiexue Gr.received more the detrital from Lhasa,on the contrary,Niere Fm.had more affinity of India than Langjiexue Gr.In the early Jurassic,the further slab rollback lead to northward drift of the Lhasa terrane and the initial spreading of Neo-Tethys oceanic basin,at this time,Northern India is the only source of Jurassic strata in Tethys Himalaya.