Study On Deposition Palaeogeography Feature In Late Jurassic, In Southern Tibetan Plateau

The breakup of Gondwana and the unfolding or closing of Tethyan which are the most important geologic events of the geological structure evolution history in South Tibet have Caused a great change to the global palaeogeography environment. There is an important tectonic activity occurred in the Qinghai-Tibet Plateau in Late Jurassic: the Bangong-Nujiang oceanic closed, Yaruzampbo oceanic crust started to subduct northward,these tectonic movements not only changed the original pattern of the basin, but also opened a new stage of tectonic evolution. Thus, the researching for the history of Jurassic sedimentary evolution possess a great significance to understand the tectonic evolution in the Qinghai-Tibet Plateau.The filling process of sedimentary basin is controlled by source supply, subsidence and sea-level change. So sedimentary record not only includes the source of deposition and environment, but also record the tectonic movement of the sedimentary process. Therefore, the study of sedimentary basins can reveal the evolution process of basin itself, and it is also the important way to research the history of tectonic evolution.Base on the field investigations and studies, collecting and analysis previous studies, starting from the characteristics of basin-fill, throught multi-disciplinary approach, such as sedimentary basin analysis and sedimentary facies and palaeogeography etc., we have identified eight types of sedimentary environments, including continental shelf facies, delta facies, rift volcanic facies, carbonate platform facies, arc volcanic facies, slope facies, subsea apron facies, basin facies. The main sedimentary facies are continental shelf facies, delta facies, rift volcanic facies in Himalayan region. In Yaruzampbo region the main sedimentary facies are slope facies, subsea apron facies, basin facies. In Gangdise region the main sedimentary facies are plateau facies, arc volcanic facies, slope facies, subsea apron facies.According to the comparison with sedimentary facies in Late Jurassic, we hold that the paleogeography characteristics of the southern Tibet, 1) In Himalayan region, the main lithology is shallow shelf clastic rock and carbonate rock composition, but in some areas we can also found some basic volcanic rock, all of the Himalayan region have abundant fossils, so we believe that Himalayan region is the passive continental margin of the Indian plate; 2) In general, the Yaruzampbo region is bathyal-abyssl turbidite deposition, there were developed a large sets of siliceous rocks and ophiolite rocks in some areas, that the instructed Yaruzampbo area is a mature ancient Tethys oceanic in Late Jurassic; 3) In Southern Gangdise the main lithology is composited by clastic rock and carbonate rocks, some volcanic rocks of acid composition in its middle also, proved that Gangdise area was a active continental margin in Late Jurassic.Combinating the magmatic activity and the coupling relations between basin, we conclude that in the south-east of Himalayan region, there was a rift zone, it is characterized by its output of basic laban basalt. However, the north part of the Gangdise area is an active continental margin in Late Jurassic, because it have a series of arc volcanic rocks deposition. So the Yaruzampbo oceans have been starting northward subduction in the Late Jurassic at least.