Cenozoic Tectonic Evolution Of The South China Sea And A Numerical Simulation Study On Its Formation

The marginal sea is a special geological structure unit, which developed in the transition zone between continental and oceanic lithosphere, with unique formation and evolution mechanism, and is a very perfect site to study the geodynamics. The aim to study marginal sea is to explore the rules of the formation and evolution and the resources, further to develop some important scientific issues, such as the mechanism of the continental marginal rift and accretion, the interaction between the continent and ocean, and substance exchange and energy transport, and to enrich the theories of geodynamics to push the related preceding fields.The South China Sea(SCS) is the biggest marginal basin of the West Pacific, with an area of about 300 × 104km2, located in the junction of Indo-Australia plate, Eurasian plate and Pacific plate in the Cenozoic. Underwent the restriction of the Tethyan and the Proto-Pacific structure fields and complicated evolution process in its geological history, formed multiplicate geological structures, such as rift, nappe structure, oceanic basin, et al., and multiplicate geological units, such as continental crust, transitional continental crust and oceanic crust. There were developed multiplicate geological unit and related mineral resources. It is considered to be "the best natural laboratory in the Earth" by numerous domestic and overseas geologists and geophysicists.The formers have done numerous researches on the formation and evolution of the SCS, and put forward some representative scenario models, such as, back-arc spreading model, collision-extrusion-escape model, and upwelling mantle (plume) model. But there are some limitations because their models only focus on one factor that influence on the SCS. In this study, the SCS is put into the conditions which interacted with the tethyan tectonic field and the Pacific tectonic field, we obtain the evolution stage of regional stress field of the SCS. Then the authors build some reasonable geological models on the base of analysing synthetically the previous geological studies on the SCS. The SCS is put into the conditions which interacted with the Indo-Australia plate, Eurasian plate and Pacific plate. Some reasonable boundary conditions and mechanical properties are set in these models, then these models are run using the Finite Difference Method(FDM). In this work, the plan-view models were constructed to simulate the India-Eurasia collision, its resultant intra-plate deformation and lateral motion along the Red River Fault, to address the question whether the collision can generate sufficient eastward tectonic extrusion, lateral shearing or approximately N-S directed extension, the slip movement of the Red River Fault during the collision. The authors then use 2D cross section models tosimulate the influence of deep asthenosphere upwelling on lithospheric deformation. The authors conclude the region stress field of the SCS and its peripheral regionsexperiences five different evolutive stages under the controls of the Pacific tectonic field and the tethyan tectonic field, then a temporal-spatial evolution mode of the SCS in Cenozoic is put forward.The results of plan-view models show that the India-Eurasia collision can result in extensive east-southeastward tectonic extrusion, approximately N-S directed extension in the SCS region. During the early stage of the India-Eurasian plate collision, the modelled Red River Fault first experienced huge left-lateral shearing and then changed to right-lateral shearing in the late collision stage. The style of shearing motion along the Red River Fault is a function of the distance between the fault and the India-Eurasia collision frontier, which decreases with time and controls relative extrusion movement between the South China Block and Indochina Block.The results of cross-section models further demonstrate that such horizontal extension can only generate limited thinning of the continental lithosphere in the SCS region. In contrast, asthenosphere upwelling is much more efficient in generating lithospheric upper mantle thinning but still inefficient for crust thinning. It is the combination of mechanical extension and asthenosphere upwelling that proves to be the most efficient way to thin the entire lithosphere, and that represents the most likely driving mechanism for the opening and spreading of the SCS.The authors also conclude the opening of the SCS was controlled by the India-Eurasian collision, subduction of the Pacific plate to the Eurasian plate and the mantle flow upwelling.