【摘要】 西北次海盆的深部地壳结构蕴含着南海北部陆缘拉张过程的重要信息.广角反射/折射测线(OBS2006-2)长386km,是目前唯一的一条沿NEE向穿过西沙地块、并平行于西北次海盆扩张脊的深地震测线.通过射线追踪与走时模拟方法(RAYINVR),获得了OBS2006-2测线下方的速度结构.结果表明:西沙地块的沉积层厚度约为1～2km,而西北次海盆的沉积层厚度大约为2～3km;Moho界面从西沙地块的27km逐步抬升到西北次海盆的12km,Moho界面下方的速度为7.8～8.0km/s;未发现壳内高速层和低速层.在西沙地块和西北次海盆的过渡区,有着较大量的岩浆活动信息,推测与西北次海盆的初始扩张有关.OBS2006-2测线中114.5°E以西的地区为减薄的陆壳,而114.5°E以东的地区为洋壳,莫霍面在陆壳与洋壳的结合处剧烈抬升,地壳厚度明显减薄.西北次海盆的扩张脊下方可能有残余岩浆的存在.更多还原

【Abstract】 Deep crustal structure in the northwest sub-basin contains the key information of the rifting process of the northern continental margin of the South China Sea(SCS).A 386km-long seismic profile(OBS2006-2)with wide-angle reflective and refractive ocean bottom seismometer(OBS)data was acquired across Xisha block and the Northwest sub-basin(NW sub-basin) of the SCS in NEE direction,which is the only profile hitherto parallel to the expanding ridge of the NW sub-basin.The crustal structure along OBS2006-2has been obtained by using ray-tracing simulation method(RAYINVR),which yields information on the velocities,tectonics,and geophysical properties.Our velocity structure indicates that sediment thickness increases from Xisha block(about 1-2km)to NW sub-basin(about 2-3km).Moho interface changes from typical continent margin of South China(～27km)to oceanic crust(～12km).The velocity at the top of upper mantle varies from 7.8to 8.0km/s.No high velocity layer and low velocity layer have been identified.Voluminous magmatism in transition zone between Xisha block and NW sub-basin,is possibly related to the initial extending of the NW sub-basin.The part of model in the domain of 220-370km would be of oceanic crust while the western part in the domain of 0-220km may be of thinned continental crust.Residual magma may exist under the speading ridge of the NW sub-basin deduced by the anomaly of Pn seismic phase.更多还原