Mesozoic Geological Evolution Of The Middle Bangonghu-Nujiang Suture

Bangonghu-Nujiang Suture is an important tectonic units in qinghai-tibet plateau. It is located in the middle of gangdisi plateau and qiangtang plateau of northern qinghai-tibet plateau. From late Paleozoic to Mesozoic, global plate motion is relative activity, the formation and cracking of Pangaea occured during this important period. Bangonghu-Nujiang suture zone is also a very important tectonic units in evolution process of tethys. In recently20years, Many scholars have done a lot of research on Bangonghu-Nujiang suture zone, and also have made many important results.But there are still many significant differences results as follows:first, the nature of the suture zone. Which type of ocean basin does it represent? Mature ocean basin or island arc basin?Second, Evolution mode. What is the evolution of Bangonghu-Nujiang Ocean basin? Ever,did it have underwent the stage of large-scale of Ocean basin expansion and subduction? Third, the direction of the subduction of the ocean basin and closing time.When did Bangonghu-Nujiang Ocean basin start subductioning? And which direction did it subduct? In the plate tectonic theory basis, this paper used the research methods of petrology、stratigraphy、sedimentology etc, research and discuss the Mesozoic sedimentary evolution and evolution model of Bangonghu-Nujiang Suture, and think:1. Bangonghu-Nujiang Ocean may opened in the Permian, early to middle Triassic the main basin expand,to late Triassic, it dived to the nort at area of jiangcuo-pengcuo-suoxian.To early Jurassic, Ocean basin had closed,and formed a island arc with expansion. It is a typical of the groove-arc-basin system. From the middle Jurassic to late Jurassic, at the area of dongqiao eronggou depositted radiolarians siliceous rocks. Late Jurassic,gangdisi plate extrused to the north causing qiangtang plate uplift and eroding. Due to the expansion of the yaluzangbujiang Ocean basin,the seawater went into this area and depositted the Upper Jurassic-shamuluo group Angle unconformity on top of Super mafic rocks and Biological reef. It represents that the Bangonghu-Nujiang ocean basin had closed in researched area. In early Cretaceous seawater exited research area from the east to west, and the south of qiantang plate uplift and depositted mountain rivers deposition. Late cretaceous Marine deposits history is over to the mountains rivers deposition.To middle late cretaceous, gangdisi plate hit with qiangtang plate.2.The middle of Bangonghu-Nujiang Ocean underwent a more complete evolution mode of Wilson cycle. Namely, the Permian rift infancy-tiny ocean ocean stage; The early-middleTriassic-the Triassic for ocean adulthood-ocean basin expansion stage; The late Triassic-early Jurassic for ocean recession-ocean basin subducted cut stage; Middle-late Jurassic residual period-the residual ocean basin stage; the cretaceous period-plate collision left mark orogenic stage.3. Upgrowth of a biological reef layer represents bangong lake-nujiang closing time limit.In the east of area,the main ocean basin had closed befor Middle Jurassic.And, in the west of area, Back-arc spreading basins had closed befor Late Jurassic. During middle Jurassic Suoxian biological reef have taken shape in residual ocean basin,and during late Jurassic dongqiao biological reef have taken shape in Arc residual ocean basin.4.In the area, the east and west of main ocean basin of Bangonghu-Nujiang Ocean might open and close at the same time.5. Bangonghu-Nujiang ocean basin show that "open scissors"mode in the open process.For the middle and west area locate in hinterland of tibet plateau,them depositted for nearly dozens of thousands meters of the sedimentary cover layer from Ordovician to Carboniferous.They belong to stable sedimentary.The east area is a activity areas which is clamped by zhongzan plate、changdu plate and Yangtze microplate as well as langcangjiang and jinshjiang suture zone.lt deposited activity type deposition which are composed of contaminated rock、metamorphic rock、 volcanic rock and flysch formation etc. from Ordovician to Middle Devonian. Bangonghu-Nujiang ocean basin opened and Crack trapped activities show in activities and weak place of the earthcrust,then it shows in stable and Large thickness area.So it shows that "open scissors"mode during the open process.