Recently, Huang Haibo, associate researcher at the Deep Geophysics Department of the Key Laboratory of Fringe Sea and Ocean Geology (OMG), the Chinese Academy of Sciences, and researcher Qiu Xuelin, a researcher at the French National Institute of Marine Development (Ifremer), and Frauke Klingelhoefer, a researcher at the French National Institute of Marine Development (OBS) detection and deep structure simulation, revealed the characteristics of the segmented deformation of the land margin crust in the northwest of the South China Sea and the control role of the ductile layer in the shell on continental extension and rupture processes. Relevant research results were published in Tectonics ("George Tectonics").
The lithosphere in the South China Sea has been transformed by subduction, land block tiling and magma, and its lithological composition, rheology and thermal structure have lateral unevenness. There are a series of dead rift valleys and micro-land blocks distributed in the northwestern part of the South China Sea. The crust structures below different tectonic units vary greatly, and the deformation process in the shell and the transition mechanism of the oceanic continent are still unclear.
Researchers picked up reliable in-shell reflection and refractive seismic phases from the OBS seismic profile, and obtained the longitudinal wave velocity structure through the Xisha plot, the Zhong/Xisha trough and the Zhongsha South Sea Basin through the walk-time simulation (Figures 1 and 2). The results show that the upper crust of the Zhongsha trough is significantly thinned, and there is a 20km-wide window for the upper crust in the center of the trough. At the same time, the seismic wave velocity of the lower crust increases, forming a velocity section at its top boundary. There is a medium crust tough layer about 5km thick between the upper and lower crusts, with uniform seismic wave velocity. The north base of Zhongsha Sea Trough is controlled by a low-angle fault, and the south base and high-precision topography both show a linear structure parallel to the direction of the rift extension, indicating the local crust dismantling process. The Zhongsha South Sea Basin is a high magnetic anomaly zone with a thickness of only 6km. Below the Moho surface is the serpentinized upper mantle, and the seismic wave velocity is 7.7km/s. The Zhongsha South China Sea Basin has an ultra-thinned land crust property, and has early magmatic activity, with similar velocity structural characteristics to other continental conversion zones in the northwestern South China Sea.
The premature rift valleys distributed in the land margin of the South China Sea or the deep-sea basins with super-thinned land crusts have not finally evolved into mature ocean crusts, which is related to the dynamical transition during the land margin of the South China Sea (Figure 3). The cracking of the land margin in the South China Sea stagnated when it spreads from east to west. Under the expansion of the land margin in the northwestern South China Sea, the Xisha trough and Zhongsha South China Sea Basin were formed in the near east-west direction under the expansion of the land margin during this period. As the cracking effect continues to be promoted, the crustal structure is subjected to strong local transformation, resulting in transformation, stretching, deformation or detachment structure, which is consistent with the phenomenon of upper crust detachment and middle and lower crust exposure found in the Zhongsha Trough. By comparing the deep structure detection results and plate reconstructions of the northwestern and southern continental margins of the South China Sea, it is speculated that the northeastern extension of the Zhongsha Trough and the Lile West Trough both occurred before the seabed expansion in the southwest subbasin.
The data used in this study were collected and obtained through the National Natural Science Foundation of China's Geophysical Shared Voyage Project on the South China Sea Institute of Oceanography "Experiment 2". The research work has been funded by major special projects introduced by the National Key R&D Plan, the National Natural Science Foundation, and the Guangdong Laboratory of Southern Marine Science and Engineering (Guangzhou) talent team.
Figure 1. OBS seismic line used in this study (OBS2013-1). The solid yellow line is a magnetic strip, the purple line is the residual expansion ridge, and the white line is the ocean-land boundary
Figure 2. The seismic wave velocity structure below the OBS2013-1 measurement line. a. Longitudinal wave velocity profile; b. Variation abnormal profile
Figure 3. Land margin evolution and crust extension deformation process in the northwest of the South China Sea. XST: Xisha Trough; ZST: Zhongsha Trough; LYXT: Limu Xihai Trough; ZB: Zhongsha Plot; RB: Limu Plot; ZNF: Zhongnan-Limu break; Blue and white arrows indicate the early and late seabed expansion direction
Source: Nanhai Institute of Oceanography, Chinese Academy of Sciences
Source: Voice of the Chinese Academy of Sciences
Source: Voice of the Chinese Academy of Sciences
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