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

作　　者：WenAi Hou Chun-Feng Li XiaoLi Wan MingHui Zhao XueLin Qiu 

机构地区：[1]Institute of Marine Geology and Resources, Zhejiang University, Zhoushan Zhejiang 316021, China [2]Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China [3]State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China [4]South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China

出　　处：《Earth and Planetary Physics》2019年第4期314-329,共16页地球与行星物理（英文版）

基　　金：South China Sea Institute of Oceanology (SCSIO) for providing R/V Shiyan-2 to carry out this experiment,sponsored by Oceanographic Research Vessel Sharing Plan (NORC2016-08) of National Natural Science Foundation of China;funded by National Natural Science Foundation of China (Grant Nos. 41776057, 41761134051, 91858213, 41730532 and 91428039)

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

关 键 词：South China Sea CONTINENTAL margin CRUSTAL structure converted S-WAVE VP/VS ratio LITHOLOGY SERPENTINIZATION 

分 类 号：P[天文地球]