Scientists propose a new model of two-stage rupture of the lithosphere in the South China Sea

Recently, Zhang Jiangyang, associate researcher of the South China Sea Institute of Oceanology, Chinese Academy of Sciences, Zhao Minghui and Sun Zhen, together with a number of scientists, funded by the National Natural Science Foundation of China and the U team project of the Guangdong Special Support Program, proposed a new model of “two-step” in the evolution of the continental margin of the South China Sea after the thinning of the lithospheric mantle and then the rupture of the earth’s crust. Related research papers were published in Structural Physics.

During the process of the elongation and thinning of the continental lithosphere to the final rupture to form the ocean basin, differences in the rheological properties of the lithosphere may lead to differences in the thinning-fracture sequence of the crust and lithospheric mantle, leading to differences in tectonic processes and magmatic action, and the formation of continental margins with different characteristics. The South China Sea is Asia’s largest marginal sea, and its deep tectonic and sedimentary features record the entire process of the lithosphere from stretch to fracture, making it an ideal place to study the transition from continents to oceans.

Two-stage rupture pattern diagram of the lithosphere of the South China Sea. Photo courtesy of the research team

The researchers conducted seafloor wide-angle seismic detection experiments perpendicular to the northern continental margin of the South China Sea and through the drilling area of the Integrated Ocean Drilling Program. It was found that from the slope to the ocean basin, the thickness of the earth’s crust was thinned from ~20km to about 6km. In addition, under the Yunli bulge at the far end of the continental margin and under the ocean-land conversion zone, nearly 10km thick and 3~4km thick lower crust high-speed layers were found, respectively. Due to the absence of the Yunli raised strata and the forced folds in the Liwan sag mainly occurred 38 million to 32 million years ago, and the magma eruptions revealed by nearby depression drilling occurred 43 million years, the researchers believe that the lower crust high-speed layer is caused by the intrusion of the homoto-fissure magma floor. According to the empirical relationship between the degree of lithospheric thinning and magma thickness, the high-speed layer of the lower crust with a thickness of 10 km should be caused by the strong thinning or even rupture of the lithosphere under the Yunli bulge.

Based on this, the researchers proposed a two-stage rupture model of the lithosphere in the South China Sea: the first stage is the thinning rupture of the lithospheric mantle that occurred about 43 million to 38 million years, when magma was not strong before this time node, and the extension of the earth’s crust was characterized by large dissociation faults. After about 43 million years, the strong thinning of the mantle promoted large-scale bottom invasion of magma. The second stage was a crustal rupture that occurred about 31 million to 29 million years below the present-day ocean-land transition zone, and the crust rupture formally formed the South Ocean Basin.

The life cycle of the earth’s plate is an international frontier scientific problem, and it is also one of the eight strategic goals of future ocean drilling, and the extension and rupture of the continental lithosphere and the birth of the ocean basin are important stages of the plate life cycle, which brings matter and energy from the earth’s interior to the surface, promoting the evolution of the earth system itself and shallow life. This study innovatively proposes a two-stage model of continental margin rupture in the South China Sea, which is not only of great significance for understanding the tectonic and magmatic processes of the South China Sea, but also provides a new perspective for understanding the diversity of the earth’s evolution. (Source: China Science News Zhu Hanbin)

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