Formation and tectonic evolution of outer margin upfold zone of East China Sea Shelf

The outer margin upfold zone of the East Chine Sea Shelf is a submarine residual volcanic arc between the continent-side back-arc rift-subsidence basins (shelf basin and Okinawa Though) of the Philippine Sea Plate subduction zone.Its geophysical field features, stratigraphic composition, magmatic activity and geological structures are charec-terized by forming "zonation from west to east and blocks from north to south".The upfold zone developed from the beginning of Tertiary when it was continental margin island arc.After Miocence, because of the tension-fracture action of the Okinawa Trough behind the Ryūkyū Arc, it became the residual arc of the ancient Ryūkyū Arc.The formation mechanism was controlled by the tectonic framework of the East China Sea, and was related to the subduction of the Philippine Sea Plate and the eastward movement of the subduction zone.The formation of upfold zone of the East China Sea went through 4 stages:(1) fracture-uplift volcanic arc (E₁-E₂), (2) fold-uplift volcanic arc (E₃), (3) fold-up-lift residual arc (N₁) and (4) withered residual arc (N₂-Q).In the last stage, the upfold zone and shelf basin were depressed in whole and deposited almost 1 500 m Pliocene-Quaternary sediments of shallow-sea facies.