Seismic velocity structures of the magnetic quiet zone and continent-ocean boundary in the northeastern South China Sea
Xiaoli Wan1, Wenai Hou2, Chun-Feng Li2,3*, Minghui Zhao4, Enyuan He4, Siqing Liu4, Xuelin Qiu4, Yu Lu1, Nan Chen1
1. State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China.
2. Institute of Marine Geology and Resources, Zhejiang University, Zhoushan 316021, China.
3. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China.
4. South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China.
* Corresponding author: Chun-Feng Li, cfli[at]zju.edu.cn
Abstract
In 2016, we carried out a coincidental multi-channel reflection seismic and wide-angle reflection/refraction seismic experiment along a 320-km long profile across the northeastern margin of the South China Sea (SCS). Based on these new data, we studied velocity structure of the transitional continental crust using both ray-tracing based seismic phase modeling and travel-time tomography. The velocity model shows that Mesozoic sediments have velocities of 4.3-5.3 km/s and are about 3-6 km thick in the magnetically quiet Chaoshan Depression and the attenuated transitional continental crust. This confirms that it is this thick layer of weakly magnetized Mesozoic sediments that causes the magnetic quiet zone. This Mesozoic layer pinches out seawards near the continent-ocean boundary. Two apparently isolated high-velocity anomalies are revealed in the lower crust of the continental slope. The first has velocities of 7.0-7.5 km/s and a maximum thickness of 8 km, and the second has velocities of 7.0-7.3 km/s and a maximum thickness of 3 km. We find that the discontinuity of the high-velocity zones is not due to the lack of some seismic phases or lose of a seismometer. We suggest that post-spreading magmatism caused these high-velocity zones. A third high-velocity zone, showing a distinctly large velocity gradient, is revealed near the continent-ocean boundary (COB). This indicates upper mantle upwelling and/or exhumation right at the COB, despite recent unsuccess in directly sampling these deep materials in the SCS.
Keywords: South China Sea, Seismic velocity structures,
Topic: Coastal Degradation and Coastal Hazard