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 

作者机构：Institute of Marine Geology and Resources Zhejiang University Zhoushan Zhejiang 316021 China Laboratory for Marine Mineral Resources Qingdao National Laboratory for Marine Science and Technology Qingdao 266237 China State Key Laboratory of Marine Geology Tongji University Shanghai 200092 China South China Sea Institute of Oceanology Chinese Academy of Sciences Guangzhou 510301 China 

出 版 物：《Earth and Planetary Physics》 (地球与行星物理（英文版）)

年 卷 期：2019年第3卷第4期

页      面：314-329页

核心收录：

学科分类：07[理学] 0708[理学-地球物理学] 

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) 

主　　题：South China Sea continental margin crustal structure converted S-wave VP/VS ratio lithology serpentinization 

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.