Crustal S-wave Velocity Structure Across The Northeastern South China Sea Continental Margin

The northern margin of the South China Sea(SCS),developed from continental rifting and break up,is usually thought as a non-volcanic margin.However,post-spreading volcanism is massive and lower crust high-velocity anomalies are widespread,which complicate the nature of the margin here.To better understand crustal seismic velocities,lithology and geophysical properties,we present an S-wave velocity(Vs)model based on ocean bottom seismic(OBS)and multiple channel reflection seismic(MCS)data along a geophysical profile 2016-2,using 2-D kinematic forward ray tracing.Together with the calculated Vp/Vs ratios and their implications on lithology,my conclusions are as follows:1.The crust is divided into eight layers,with Vs changing from 0.34 km/s near the seabed to 4.16 km/s in the lower crust.The Moho fluctuates but deepens from 12-10 km in the deep oceanic basin to 27-15 km in depth in the continental margin.2.The Mesozoic sedimentary layer at the Chaoshan Depression has a maximum thickness of 5 km and Vs of 2.4-3.1 km/s,and the layer pinches out southwards to the continent-ocean boundary(COB).The Vp/Vs ratio decreases from 2.8 to 1.74 with depth through the shallow sedimentary layers,due to increased compaction and consolidation of the rocks.3.Two isolated high-velocity zones are found in the lower crust of the continental slope,showing S-wave velocities of 4.0-4.2 km/s and Vp/Ns ratios of 1.73-1.78.These values indicate a mafic composition,most likely of amphibolite facies.Vp/Vs versus Vp plot also indicates a magnesium-rich gabbro facies from post-spreading mantle melting at temperatures higher than normal.4.A third high velocity zone(Vp:7.0-7.8 km/s;Vp/Vs:1.85-1.96)of 70-km wide and 4-km thick in the continent-ocean transition zone is likely ascribed to serpentinization of upwelled upper mantle.Both seismic velocity structures and gravity anomalies indicate that mantle upwelling/serpentinization could be the most severe in the northeasternmost COB of the SCS.Empirical relationships between seismic velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 43%in the lower crust to 37%into the mantle.