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南海西北、西南和东部3个次海盆停止扩张之后都存在岩浆活动,扩张中心有多个海山.为了研究海山和扩张中心的地壳结构特征,本文使用穿越南海西北次海盆扩张中心的广角地震剖面(测线OBS 2006-p1),针对海盆区域,通过P波二维地震射线追踪正演,获得了西北次海盆垂直扩张方向的深部速度结构,讨论了西北次海盆及其扩张中心深部地壳结构特征;并结合南海东部次海盆和西南次海盆横穿扩张中心的海底地震仪(ocean bottom seismograph,OBS)剖面,对不同海盆的地壳结构进行了对比研究,获得了扩张后期岩浆活动对洋壳改造的新认识:(1)发现西北次海盆地壳具有明显的洋壳性质,莫霍(Moho)面埋深为11~12 km(从海平面起算),地壳厚度为6~8km,速度为5.7~7.0 km/s.扩张中心轴部双峰海山为西北次海盆扩张之后岩浆活动的产物;(2)西北次海盆、东部次海盆、西南次海盆东北部沿垂直扩张脊方向地壳速度结构均有明显的横向变化,在扩张中心处都表现为洋壳厚度增大约1 km,莫霍面埋深增大,表明后期岩浆活动对地壳结构有较大的改造作用.
In the northwestern, southwestern and eastern South China Sea, there are magmatic activities after the expansion of the three sub-basins, and there are several seamounts in the center of expansion.In order to study the crustal structure characteristics of seamounts and expansion centers, a wide-angle seismic profile ( Line OBS 2006-p1). For the basin area, the deep velocity structure in the vertical expansion direction of the Northwest sub-sea basin is obtained through the P-wave two-dimensional seismic ray tracing forward modeling. The deep crustal structural features of the Northwest sub-sea basin and its expansion center are discussed. Based on the ocean bottom seismograph (OBS) section of the eastern South China Sea sub-sea basin and the southwestern sub-sea basin, the crustal structure of different sea basins was compared and the new magma activity in the extension of the oceanic crust was reconstructed Recognition: (1) It is found that the shell of the Northwest sub-sea basin has obvious oceanic crustal properties. The Moho depth is 11-12 km (from the sea level), the crust thickness is 6-8 km, and the velocity is 5.7-7.0 km / s. The bimodal seamount in the axis of dilatancy is the product of magmatism after the expansion of the Northwest sub-basin; (2) the northwest sub-basin, the eastern sub-basin, The direction of the crustal velocity structure has obvious lateral variation. At the center of expansion, the thickness of the oceanic crust increases by about 1 km and the depth of the Moho increases. This indicates that the magmatic activity in the late stage had a greater transformation effect on the crustal structure.