Structures Of The Dongsha Movement At The Northern Margin Of The South China Sea: Implications For The Evolution Of The Passive Continental Margins

After the end of the spreading of the South China Sea (SCS), an importantstructural movement-the Dongsha Movement took place at the Northern Margin ofthe SCS. However, timing, range, tectonic attribution and dynamic mechanism of theDongsha Movement are still unclear. Based on the well explained2D/3D seismic datain the Dongsha area and its neighboring, we analysis the structures of the deformation,and define the range and the timing of the movement. The main deformation of themovement is characterized by faulting with the coeval sedimentation in the area ofuplift eroded which causes the hiatus of part of Miocene and Pliocene stratums. It alsocaused clear angular unconformity which marked the time of the Dongsha Movementon the seismic sections. The movement causes abundant secondary NW-NWWtensional and transtensional faults, the length of which is mainly several hundredmeters to several kilometers. Magmatism related to the Dongsha Movement is scarcein the study area, and little igneous activity could be found in the Neogene stratum.However, magma intruded or erupted around the ocean and continent boundary,southeast of Dongsha Rise, probably due to the weakness of the crust. Besides, thedeformation of the movement is more intensive in the eastern area than in the westernarea. The Dongsha Movement mainly affects the Dongsha Rise and the ChaoshanDepression. Besides, the eastern part of the Panyu Low-High and the southern andnortheastern part of the ZhuⅠDepression are also deformed. The movement mainlyoccurred in the late period of late Miocene and ended at the Mio/Pliocene boundary(5.5Ma) in this paper, and it occurred in the extensional environment. The DongshaMovement might be related to a collision between9-6Ma of the Luzon Arc to theEurasian Plate which was caused by the NWW movement of the Philippine Sea Platein Neogene. At the mean time, the northern margin of SCS is in a setting of extensionfor a subduction of SCS towards the Manila Trench and a cooling of oceanic crust,which caused magma underplating at the bottom of lower crust in the study area. Thismechanism resulted in the unbalanced isostasy of the area and the uplifting of the upper crust. The Dongsha Movement suggests that the passive continental marginsmay not keep stable all the time after they evolve to the mature stage. At the sametime, the movement gives more evidence to prove that the northern margin of the SCSis a non-volcanic continental margin.