Magmatic Diapirs within the Deep-water Area of Pearl River Mouth Basin（PRMB）：Insights into the Continental Breakup in the South China Sea（SCS）

The timing , distribution, and intensity of magmatism are critical to understand the process of passive margins. Previous researchers have mainly studied the geophysical characterization of magmatism, but scarcely focused on the relationship between magmatism and the final continental breakup in the SCS. In this paper, based on high-resolution regional 3D seismic survey data, 2D seismic reflection profiles and ocean drilling data, we observe many structure highs within the deep-water area of PRMB, which are caused by magma diapirism. These magmatic diapirs distribute mainly over the east of Baiyun sag , Liwan sag, uplifts surrounding the Liwan and Xingning sag, as well as the continent-ocean boundary（OCB）. The magmatic diapiers in the east of Baiyun sag are characterized by multiple stages responding to continental breakup（ 32Ma）, a ridge jump to the south（ 23.8Ma） and the Dongsha Event（ 5.5Ma）respectively, while the other magmatic diapiers response mainly to the first two tectonic events. The Baiyun and Liwan Sags are two hyper-extended rift basins that develop within the distal margin of the northern SCS, and the present minimum crustal thickness of Baiyun sag, Liwan sag, and COB are 5.5km, 8km and 5km. The IODP U1435 drilling near the OCB reveals that the basement property is continental crust and the serpentine mantle doesn't develop in this area. However, in the deep seismic reflection profile passing through this area we can identify the magmatic diapirs that developed at 32Ma. Meanwhile, we discover that the domal seismic reflection below the magmatic diapirs can extend downward to Moho, which proves that the magma material comes from mantle. The northern margin of SCS behaves completely different tectonic activities, especially magmatism, in comparison with the magma-poor and volcanic passive margins in the period of continental break-up. Based on the research above we consider that the final continental breakup in the SCS is the common result of intense lithosphere thinning and magmatism, which behaved the SCS-Type mechanism of continental breakup.