Seismic Velocity Structure of the Magnetic Quiet Zone and Continent-Ocean Boundary in the Northeastern South China Sea

In 2016, we carried out a coincidental multichannel reflection seismic and wide-angle reflection/refraction seismic experiment along a 320-km-long profile across the northeastern margin of the South China Sea. Based on these new data, we studied velocity structure of the transitional continental crust using both forward 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. Using forward magnetic modeling, we confirm that the presence of this thick Mesozoic layer with low magnetic susceptibility induces the magnetic quiet zone and show that the lower crust high-velocity materials have limited effect on magnetic anomalies. This Mesozoic layer pinches out seaward near the continent-ocean boundary (COB). 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 postspreading magmatism caused these high-velocity zones. A third high-velocity zone, showing a distinctly large velocity gradient, is revealed near the COB. This indicates upper mantle upwelling and/or exhumation right at the COB, despite recent unsuccess in directly sampling these deep materials in the South China Sea.