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 (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.

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

“…Based on our observation, we speculate that either a thin layer of mid‐ocean ridge basalts covers the upwelled mantle, as in the Tyrrhenian Sea (Bonatti et al, ), or the area might have been influenced by late‐stage volcanism. Sites U1500 and U1502 are only about 200 km to the west of our survey line OBS2016‐2, but a northeast trending gravity high, interpreted as from mantle exhumation/upwelling at OBS2016‐2, does not extend to sites U1500 and U1502 (Hou et al, ). This shows a swift along‐strike variation in the final rifting features.…”

Crustal Compositional Variations From Continental to Oceanic Domain: A VP/VS Ratio Study Across the Zhongsha Block, South China Sea

“…Northeastern SCS margin (OBS2001; Zhao et al, 2010) Reed Bank (OBS973-2; Wei et al, 2015) Northern SCS margin and ZB (OBS2006-1; Wei et al, 2017) Northeastern SCS margin (OBS2016-2; Hou et al, 2019) Pearl River Delta (RF; Huang et al, 2020) Mid-northern SCS margin (OBS2006-3; Wei et al, 2011) Xisha Islands (RF; Huang et al, 2011) South China coast (RF; Huang et al, 2014) Northwestern SCS margin (RF; Zhongsha Block (OBS2017-2; This study) the lower crust of the Zhongsha Block, especially beneath the Zhongsha Atoll. This is probably associated with the convergent tectonic setting before the late Mesozoic, based on petrogenesis (Ashwal, 1993).…”

“…Comparison of V P /V S ratios in the SCS continental margin. The ranges of V P /V S ratios are compiled from forward S wave modeling results (Wei et al, 2011(Wei et al, , 2015(Wei et al, , 2017Hou et al, 2019;Zhao et al, 2010) and receiver function (RF) results (Huang et al, 2011(Huang et al, , 2014(Huang et al, , 2020Huang, Qiu, Zhang et al, 2019). The V P /V S ratio from the receiver function is the average value of the crust.…”

Numerical modeling of failed rifts in the northern South China Sea margin: Implications for continental rifting and breakup

“…LCHVA in magma-poor margins usually come from serpentinized mantle materials (Boillot et al, 1980;Chian and Louden, 1994;Van Avendonk et al, 2006), whereas those beneath magma-rich margins are interpreted as from magmatic underplating (Mjelde et al, 2005;Mutter et al, 1984;Voss and Jokat, 2007). LCHVA at different locations in the northern SCS margin may have multiple causes, including Cenozoic post-rifting magmatic underplating (Wang et al, 2006;Wei et al, 2011;Yan et al, 2001;Zhao et al, 2010), Mesozoic subduction-related magmatism (Wan et al, 2017), and mantle upwelling and serpentinization at the continent-ocean boundary (Hou et al, 2019;Wan et al, 2019).…”