Study on mid-lithospheric discontinuities in the South China block based on S-wave receiver functions

The Mid-Lithospheric Discontinuities (MLD) are an important interface recently discovered in the continental lithospheric mantle. They are closely related to the formation and tectonic evolution of the continental lithosphere. However, their origin remains unclear and has not reached a consensus. The South China Block has a very complex lithospheric structure. Its internal structure and evolution have long been major issues in geoscience. The mechanism of regional lithospheric thinning in the eastern part is still highly debated. Recent seismological studies have found evidence of MLD in the lithospheric mantle of the central and western South China Block. To further investigate the spatial distribution of MLD within the South China Block, this study uses seismic data recorded by national network stations distributed across the block. The single-station S-wave receiver function stacking technique and the k-means+[KG-*2/5]+clustering analysis method are applied to systematically detect MLD in the South China Block. The results show that MLD in the South China Block is mainly distributed in the Upper Yangtze Block. The depth ranges from 75 to 130 km. Beneath some stations in the Sichuan Basin, two possible MLD signals are observed in the mantle lithosphere. The MLD may have originally formed as the continental Lithosphere-Asthenosphere Boundary (LAB) at shallow depths. The ancient magmatic ocean LAB might have been captured or preserved. As the lithosphere cooled, it migrated to greater depths and now appears as the current LAB. In the eastern part of the South China Block, the lithosphere may have delaminated along the original MLD due to the influence of the subduction and retreat of the ancient Pacific Plate. Later, under thermal erosion and extensional forces, the lithosphere continued to thin. The MLD interface was destroyed. As a result, no MLD is observed in the eastern part of the South China Block. This study provides new insights into the spatial distribution and possible formation mechanisms of MLD in the South China Block. It also contributes to understanding the complex lithospheric evolution of the region.