Depositional Tectonic Deformation Characteristics And Structural Evolution Of Jurassic In Gêrzê—Rongma Area Of South Qiangtang Basin

Qiangtang Basin in Southern Tibet is located between Longmu Co-Shuanghu Suture Zone and Bangong Co-Nujiang Suture Zone, and it is a very important tectonic belt which keeps multi-period evolution information of Tethys Ocean. The geological evolution of south margin of Qiangtang Basin is closely related to the evolution of Bangong Co-Nujiang Ocean. Niangrong Co in Gêrzê area, Riganpei Co in Nima County, and Zishi Co in Rongma area have been chosen as the research ares, and these areas are close to Bangong Co-Nujiang Suture Zone. Research and discussion about the deposition and tectonic deformation behavior and tectonic evolution have been conducted in the middle area of the basin in Jurassic, and the findings are as follows:1. Bivalve fossils have been largely found in Riganpei Co area, which called Jiebuqu Formation and Shaqiaomu Formation in Jurassic by former researchers. Through identification and comparison of fossils home and abroad, the area belongs to late Triassic epoch. And after the research of its overlying-underlying contact modes and thick limestone found inside, we re-categorized the area into Sewa Formation in the lower and middle Jurassic, and Riganpei Co Formation contains collapse and slump on platform margin in late Triassic epoch, and organic bank.2. Large amount of coral fossils have been firstly found in Niangrong Co area which belongs to Zhanjin Formation in upper Carboniferous according to the former researchers. Identification of fossils suggests they are belonged to late Triassic epoch, and thus there should be no Zhanjin Formation in this area. At the same time, though research and comparison of layers, it is believed that this area belongs to Sewa Formation in middle-lower Jurassic, and coral fossils are found in the slumped and collapsed reef limestone in late Triassic epoch.3. Systematic and detailed depositional research has been conducted in Zishi Co and Riganpei Co of the Rongma area, and Niangrong Co of the Gêrzê area in south Qiangtang Basin. We analyzed and compared the depositional characteristics and sedimentary facies with other areas, and believe south Qiangtang Basin was in the slope condition in bathyal-deep-sea area, and Sewa Formation of Rongma zishi Co area had deep-water basin deposition, and turned into shallow sea with continental shelf deposit.4. We have conducted systematic research of deformation characteristics, periods and modes, and chosed one relatively well preserved area to do 1:10000 geology mapping. It is believed that two tectonic deformation had occurred caused by Yanshan Movement. The first one(D1), is the major tectonic movement and formed the complex fold mode now, and other sub-tectonic formation, and main foliation S1; the second one(D2) was the almost south- north squeezing, which caused brittle ductile deformation and Non-penetrative cleavage S2.5. Analysis of tectonic evolution in the study area indicates that the tectonic evolution of middle of Qiangtang Basin in Southern and Bangong Co-Nujiang Ocean including four stages: Quiker splitting tensile in late Triassic and early Jurassic; Subduction in middle Jurassic and early early Cretaceous; Collision orogeny in late early Cretaceous and late Cretaceous; Uplift of plateau since the Neogene.