The discrimination of Mesozoic sequence and its tectonic attribute in the southeastern South China Sea

Discriminating the distribution of the Mesozoic strata on the southeastern South China Sea (SCS) continental margin is of great importance for understanding the SCS evolution as well as the oil and gas potential. Due to limited data constraints and varied stratigraphic system divisions, the distribution and tectonic features of the Mesozoic sequences in the southeastern SCS have remained controversial. Assisted by drilling and dredging data, the authors comprehensively identified the Mesozoic sequence using a combination of well-seismic comparison, seismic reflection characteristics, internal velocity analysis, contact relationship between igneous edifices and strata, and structural deformation characteristics. The results show that the Mesozoic sequence is characterized by parallel, continuous, and low-frequency reflections, with internal velocity of 3400-4200 m/s. As the burial depth or/and the degree of metamorphism increases, the internal velocity increases to 4500-5500 m/s. The seismic reflections of the Mesozoic strata in the depth is fuzzy, and most of them have no obvious bottom reflection. Despite the fact that the characteristics of seismic reflection are clear in a specific area, it is still difficult to make a comparative study and conduct a whole region seismic explanation because the range of the Mesozoic strata is limited. Among them, the Mesozoic strata in the Northwest, Southwest Palawan basin, Liyue and Andubei basin show low-angle tilting or nearly horizontal characteristics. By contrast, the Mesozoic strata in the northern part of Jiuzhang basin located in the southwest of Liyue have a velocity of 3500-4500 m/s, which is higher than the overlying Cenozoic velocity and coincides with the result constrained by drilling layer. In addition, the Mesozoic strata tilt or deflect at a high angle, which may be related to the emplacement of magmatic activity. Compared with the distribution of Mesozoic volcanic arc and identified igneous bodies, the authors infer that the first style Mesozoic strata are scattered in the forearc basin near volcanic arc and thus show a relatively weak tectonic activity, whereas the second style strata are distributed in the inter-arc basin and have a strong magmatic activity.