Crustal S-wave velocity structure across the northeastern South China Sea continental margin: implications for lithology and mantle exhumation

Abstract

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 (<italic>V</italic>_S) model and a <italic>V</italic>_P/<italic>V</italic>_S model for the northeastern margin by using an existing P-wave velocity (<italic>V</italic>_P) model as the starting model for 2-D kinematic S-wave forward ray tracing. The Mesozoic sedimentary sequence has lower <italic>V</italic>_P/<italic>V</italic>_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 and <italic>V</italic>_P/<italic>V</italic>_S ratios of 1.73–1.78. These values indicate a mafic composition, most likely of amphibolite facies. Also, a <italic>V</italic>_P/<italic>V</italic>_S versus <italic>V</italic>_P plot indicates a magnesium-rich gabbro facies from post-spreading mantle melting at temperatures higher than normal. A third high-velocity zone (<italic>V</italic>_P : 7.0–7.8 km/s; <italic>V</italic>_P/<italic>V</italic>_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.