Gravity modelling of crustal architecture and heterogeneity in the Nansha Block, South China Sea

The stretched structure and heterogeneity of the crust of the Nansha Block, the southern continental margin of the South China Sea (SCS), are not well understood. We used published ocean bottom seismic (OBS)/multichannel reflection seismic (MCS) profiles across the Nansha Block to establish five two-dimensional crustal structure models. Using gravity modelling with magnetic anomaly inversion, we obtained the distribution of density and local magnetic susceptibility of the crust. The models show that the distribution of density and thickness of the upper crust in the Nansha Block is uneven, and the thick upper crust is prevalent in the regions close to the continent-ocean transition (COT) showing different characteristics. The interpreted Mesozoic granite blocks and Precambrian rigid basement reflects the heterogeneity in the material composition of the SCS continental margin. Based on the thinning styles of different crustal layers, we suggest that the Nansha Block has a three-layer thinning pattern. The uppermost pre-rift layer was deformed via brittle fractures, the upper crust was sheared by discrete shear zones, and the lower crust experienced ductile deformation. The inherited pre-rift thermal regime, mechanical state, and material composition of the SCS continental margin affected the extensional structure of the crust.