Deep crustal structure and tectonic implications of the Nansha Trough from wide-angle reflection and refraction seismic data

Abstract

Deep crustal structure of the Nansha Trough (NT) plays a crucial role in understanding its crustal nature and magmatic activity. This study used the data from a wide-angle seismic profile (NDL1), which passes through the Jinghong seamount (JHSM) and Yinqing seamount (YQSM) along the trough. By using 2D forward ray-tracing and joint refraction and reflection travel-time inversion methods, we obtained the velocity structure of the NT. The model results show a thinned continental crust with a thickness of ∼9–18 km in the east of the trough, which is variable drastically below the seamounts. The upper crust slightly varies at a thickness of ∼5 km except at the seamounts, while the lower crust shows lateral variations in thickness and velocity, accompanied by high velocities below the JHSM due to the magmatic addition. Velocity models of the NDL1 also reveal crustal differences between the JHSM and YQSM. Combined with the magnetic anomaly and multichannel seismic data, we infer that the seamounts formed from multiple stages of magmatism after the spreading of the South China Sea and the NT are still active. High velocities in the lower crust result from the mantle upwelling and decompression melting of flexural responses of the NT, which may provide a genesis clue for the formation of the submarine seamounts.