Seismic Imaging Of Complex Structures And Its Application Research In South China Sea

The complexities of regional structures in the South China Sea increases the difficulty of seismic imaging and interpretation.In this paper,Pre-stack seismic imaging(pre-stack time migration and pre-stack depth migration)has achieved good results in seismic imaging in Xisha Archipelago Domain,northern Manila subduction zone and Northwest subbasin area.The results and conclusions are as following:(i)In terms of data processing,Increasing the velocity analysis points is used to obtain more accurate velocity in the steep-dipping area,such as seamounts;Kirchhoff pre-stack time migration method is used in Xisha sea area and has obtained good imaging results.Pre-stack depth migration technique is used to solve the problem of scattering and diffraction of seismic waves caused by lateral velocity variation,and has been applied well in the subduction zone and the northwestern sub-basin.(ii)In the Xisha uplift area,this study revealed that the accelerated thermal subsidence after magmatic activity is the main reason for the submergence of the reefs through analyzing the post-rift magmatic activity and the drowning processes of 14 isolated carbonate platform.The isolated carbonate platform in the northwestern of the Xisha uplift began to submerge early in the late Miocene,and was related to the rapid abnormal subsidence after the South China Sea stopped spreading.In the early Pliocene,large-scale magmatic intrusions and volcanic eruptions occurred in the Xisha uplift,The most intense magmatic activities were located in the northwestern sub-basin of the South China Sea and the Xisha Eastern Depression,After then,the subsequent thermal subsidence started the drowning process of reefs in the northeastern and western regions of the Xisha uplift.During the early Quaternary,large-scale magmatism also occurred in the Xisha uplift,and the most intense magmatic activities were located in the southern region of the Xisha uplift.The subsequent thermal subsidence resulted in a new rapid tectonic subsidence,which caused the reefs to drown in the southern and southeastern regions of the Xisha uplift.(iii)Through analyzing the depth migration sections and velocity model,this study analyzed the detailed structure of the accretionary wedge in the northern Manila subduction zone.The Manila accretion wedge is divided into four parts,the original sedimentary segment,the fold deformation segment,the thrust nappe segement and the back thrust fault segment,representing the different stages of the evolution of the accretion wedge,respectively.There are progressive evolutionary relationships in the interior.According to the internal structure and evolution characteristics of the accretionary wedge,combined with the depth velocity model,it can be inferred that there is an "fore arc baseplate" formed by residual plate of the Philippine Sea in the lower part of the accretionary wedge,and the "fore arc baseplate" is the key structure in controlling the formation and evolution of the Manila accretionary wedge.It is closely related to the sharp drop of the detachment surface and the development of the internal out-of-sequence-thrust in the accretion wedge,and the evolution process of the Manila accretionary wedge is also analyzed.The "fore arc baseplate" is integrated with the Luzon Arc,it is continuously squeezed and obducted over the South China Sea plate under the promotion of the Philippine Sea Plate.The leading edge is the front of scraping the surface material of the South China Sea crust,and squeezing the front formation,which is the stress resource of the thrust of the thrust and fold deformation segement to the trench.The intrusion of the "fore arc baseplate" into the bottom of thrust nappe segment also resulted in the development of the out-of-order thrust fault,and continues to merge the scraped sedimentary material and the thrust nappe layers into the back thrust nappe segment,which resulted in the continuous uplifting of the back thrust nappe segment of the upper slope,thus making the out-of-order thrust fault an important geomorphic boundary dividing the upper and lower slopes.(iv)In the northwest sub-basin,this study analyzed continent-ocean transition zone and magmatic activity of the northwest sub-basin using depth migration results of three typical seismic profiles.Results show that more rifting volcanoes were identified on the northwestern sub-basin slope area,and an velocity anomaly is found in the upper crust below the continental shelf fault basin.It is inferred that it is caused by the invasion of deep matter induced by the fissure in the basin.The magmatic activities in the post-rift period of the northwestern sub-basin are equally rich,and are distributed in continental shelf,continental slope,and ocean basin.Among them,the Miocene magmatic activity is mainly confined to the interior of the northwestern basin.The Pliocene and Quaternary volcanoes are scattered everywhere,and are often inherited in the early Miocene igneous rocks.Compared with the Cenozoic magmatism in the northern part of the South China Sea and combining with the latest achievements in the study of the mantle plume model in Hainan,it is believed that the rifting and post-rift magmatic activities in the northwestern sub-basin are both upwelling from deep mantle and below the continental margin of the northern South China Sea.The magma reservoir formed in the upper mantle has the same source as the igneous rocks in the Leiqiong area and the Dongsha uplift,but belongs to different branches.The magmatism in the northwestern basin sub-basin and the magmatic activities in the Xisha Trough,Xisha and Zhongsha areas during the rifting period may belong to the contemporaneous magmatic activity.The magma may come from the mantle plume branch in the northern part of the Xisha Trough,and is induced by the early rift and the thinning of the crust.