OBS2018-H2测线反射与折射数据联合揭示南海北部陆缘洋陆转换带的深部地震结构

Deep seismic structure of the continent-ocean transition in the northern margin of the South China Sea revealed by jointly reflective and refractive data along OBS2018-H2 profile

南海北部陆缘洋陆转换带是减薄陆壳和正常洋壳之间的过渡区域,蕴藏着南海从张裂、破裂至初始海底扩张的重要信息.OBS2018-H2测线穿过了国际大洋发现计划(IODP)的钻探站位U1499和U1502,是一条平行于南海北部陆缘构造走向的深地震测线.我们利用测线上10台海底地震仪记录的走时数据,采用RayInvr正演和Tomo2d反演相结合的方法,获得了深达上地幔的速度结构;同时结合高精度多道反射地震数据以及IODP367/368/368X航次的钻探资料,将速度模型划分为西部残存的减薄地壳区(0~63 km)、中部岩浆构造转换带(63~120 km)和东部中下地壳剥露区(120~180 km)三部分;然后综合与该测线垂直的Lw7反射地震剖面,建立了南海北部陆缘张裂-破裂地质模型.研究揭示在晚始新世至早渐新世(34~30 Ma)时期,陆壳快速伸展减薄,引发了软流圈的强烈上涌,并伴生大量岩浆侵入到减薄地壳;熔融物质在~30 Ma破坏减薄陆壳并触发岩石圈破裂,快速(~28.7 Ma)过渡到火成岩地壳增生,形成了相对狭窄的洋陆转换带.该研究揭示了南海陆缘洋陆过渡带的深达上地幔的地震速度结构,以及复杂的构造-岩浆相互作用,为全球张裂-破裂过程研究提供了重要参考.

The continent-ocean transition(COT)zone in the northern South China Sea(SCS)continental margin is a transitional zone from thinned continental crust to normal oceanic crust,which records important information about how the SCS progressing through rifting,breakup to seafloor spreading.The seismic profile OBS2018-H2 is parallel to a tectonic strike of the northern continental margin.It crosses through the drilling sites U1499 and U1502 of IODP(International Ocean Discovery Program).By using the combination of RayInvr forward modeling and Tomo2d inversion software,we have obtained the seismic velocity structure as deep as the upper mantle along the OBS2018-H2 based on the travel-times data from 10 OBSs(ocean bottom seismometers)recording data.The velocity structure is divided into three parts:the western thinned continental crust region(0~63 km),the central COT region(63~120 km),the eastern exhumed middle-and lower-crust region(120~180 km)combined with high-resolution reflection data and IODP367/368/368X drilling data.The mechanism of rifting-breakup is discussed and built in the northern SCS continental margin combined with multi-channel reflective profile(Lw7).The continental crust of the northern SCS margin was stretched and thinned rapidly from late Eocene to early Oligocene(34~30 Ma),which triggered a rapid upwelling of the asthenosphere and the intrusion of magma into the crust.A large amount of molten material upwelled at~30 Ma,invaded and destroyed the thinned continental crust,then triggered the rupture of the continental crust.The continental margin rapidly transitioned into igneous crust accretion(~28.7 Ma)and formed a relatively narrow COT.This research displayed the deep crustal structure of the COT area in the northern SCS continental margin and revealed the interactions between tectonics and magmatism,which further provided important references for the rifting-breakup process in global rifted margins.