Cenozoic Magmatism In The Northern Continental Margin Of The South China Sea And Its Implication For The Tectonic Evolution Of The Rifted Margin

Extensive magmatic activity of the South China Sea(SCS) and its adjacent regions recorded important information about the formation and evolution of the SCS, and its associated dynamic mechanism. Therefore, the research of igneous rock of the SCS is important for the theory and reality to understand the tectonic of the continental margin, seafloor spreading, interaction between crust and mantle and resource effect.Based on the latest high resolution multi-channel seismic data in combination with other geophysical data and drilling well data and regional geological document, this study was carried out mainly on the seismic characteristics of igneous rock in the northern margin of the SCS and its indicative function for identifying the timing of magmatism; the geometric occurrence, formation time and distribution of sill complexes, and its mechanism; the type of the northern margin of the SCS and the influence of Hainan plume for post-rift magmatism. Innovative results had been achieved as follows. 1) set up the testable standard for magmatism interpretation. This study has identified two types of igneous rock, intrusion and extrusion, and their seismic characteristics were analyzed. Intrusion is consisted of piercement and implicit- piercement reflecting different diapiric energy. While extrusion is divided further into flat-topped and conial-topped seamount in the light of the appearance. In addition, the scale and quantity of all kind igneous rocks have been to gather statistics in the northern SCS. In the absence of drilling well data, the seismic characteristics are used to estimate the timing of magmatism. In the context of resolvable-scale of seismic data, the occurrence of strata, the contact relationship between the igneous rock and surrounding rock, strata thickness and the occurrence of igneous rock are taken into account, moreover, three criterions are summarized. They are the contact relationship, the sedimentary thickness and the seismic reflection. Thereby, igneous rock which formed during the seafloor spreading is identified. Further, these standards are applied to recognize three periods of Cenozoic magmatism in the northern margin of the SCS. 2) take obviously seismic anomalies with abrupt termination boundary as the seismic recognition criteria of sill complex. Most of sill complexes are saucer-shaped, they widely distributed by individual or combined with each other in the Lower Miocene strata of the western of Baiyun sag. We speculated that the formation of these sill complexes is 15.5Ma. It suggested that after the seafloor spreading, residual magma utilized syn-rift faults as magma channel, the deep and large sill complex fed the shallow and small one. The emplacement mechanism is along horizontal discontinuity, sills are fed radially from the inner sill, and form new sill. 3) the Ocean-Continent Transition(OCT) of northern SCS manifests high-angle listric faults which terminated at the surface of fault-depression transition; the volcanic rocks distribution apparently concentrated at the seaward; the crust thickness sharply decreased in the OCT; there is no widely influence by mantle plume during the rifting. All these provide more firm evidence as the northern margin of the SCS as a magma-poor margin. Extensive magmatism occurred since the Late Miocene and took off the heat from mantle, which made the deep material cooler and heavier, and increased the basin tectonic subsidence and the accommodation, finally led to abnormal thermal subsidence in the northwestern margin. 4) The influence of the NWW subduction of the Paleo-Pacific Plate since Early Jurassic and /or the collision of the Indo-china block with Eurasia at Early Paleogene are the key factors of the evolution of the magmatism from the Late Yanshanian to rifting to post-rifting. As a dominating factor, mantle plume influenced the formation time and scale of magmatism after the cessation of the seafloor spreading. Because of the special characteristics such as smaller size, lower temperature and slower upwelling rate retarded the magmatism.