The northeastern margin of the South China Sea (SCS), developed from continental rifting and breakup, is usually thought of as a non‐volcanic margin. However, post‐spreading volcanism is massive and lower crustal 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 (V S) model and a V P/V S model for the northeastern margin by using an existing P‐wave velocity (V P) model as the starting model for 2‐D kinematic S‐wave forward ray tracing. The Mesozoic sedimentary sequence has lower V P/V S ratios than the Cenozoic sequence; in between is a main interface of P‐S conversion. Two isolated high‐velocity zones (HVZ) are found in the lower crust of the continental slope, showing S‐wave velocities of 4.0–4.2 km/s andV P/V S ratios of 1.73–1.78. These values indicate a mafic composition, most likely of amphibolite facies. Also, aV P/V S versus V P plot indicates a magnesium‐rich gabbro facies from post‐spreading mantle melting at temperatures higher than normal. A third high‐velocity zone (V P : 7.0–7.8 km/s;V P/V S: 1.85–1.96), 70‐km wide and 4‐km thick in the continent‐ocean transition zone, is most likely to be a consequence of serpentinization of upwelled upper mantle. Seismic velocity structures and also gravity anomalies indicate that mantle upwelling/ serpentinization could be the most severe in the northeasternmost continent‐ocean boundary 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.

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