南海海盆残留扩张中心地壳速度结构对比及构造意义

Comparison of crustal structures in the fossil spreading center of South China Sea basins and the tectonic significance

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.

This study focuses on the magmatic activities and seamounts in the fossil spreading center of three sub-basins in the South China Sea（SCS）. The northwest sub-basin（NWSB）, southwest sub-basin（SWSB）, and east sub-basin（ESB） are of great importance for understanding the tectonic evolution and post-spreading magmatic activities of the SCS. Deep crustal structures beneath the spreading center of these basins have a significant bearing on the magmatic process. For this study, on the basis of a previous velocity model, we used two-dimensional ray-tracing methods and high-quality ocean-bottom seismometer（OBS） data collected in 2006 to image the NWSB deep P-wave velocity structures. We then performed travel-time analysis and estimated uncertainties of the final model parameters to prove that the velocity model was convincing. Finally, we compared the one-dimensional velocity structures among OBS profiles 2006-p1 in the NWSB, G8G0 in the ESB, and T1 in the SWSB. Our study of crustal structure yielded the following results：（1） the NWSB is confirmed to be oceanic crust compared with typical Atlantic oceanic crust. The crustal velocity is 5.7–7.0 km/s, whereas the crustal thickness is 6–8 km in the basin（90–130 km）. There is no high-velocity layer. The Mohorovi？i？ discontinuity（Moho） is distributed symmetrically along the spreading axis and gradually increases from the basin center to the edge. The depth of the Moho is 11–12 km in the basin（55–70 km） and changes dramatically to 16 km depth under the north margin（170–200 km） and the Zhongsha Islands（0–55 km）. The depth of the Moho in all three sub-basins increases by 1 km in the spreading center. This suggests that crustal structure in the SCS is greatly influenced by magmatic activities. The Shuangfeng seamount, with a 6.5-km-thick crust, located in the center of the spreading ridge, is a post-spreading volcanic seamount. The 6.4-km/s velocity contour beneath the Shuangfeng seamount gradually rises toward the northwest part, and the velocity at the seamount bottom is lower than 7.4 km/s. Therefore, we speculate that the magma underplating does not exist. Consistent with multichannel seismic date, we conclude that the Shuangfeng seamount was mainly formed by an intrusive process： The magma intruded along the former extinct spreading axis. The Longnan seamount in the SWSB and the Jixiang seamount in the ESB were mainly formed by an eruption process, supposedly the result of the buoyant decompression melting mechanism.（2） The detailed velocity model of the NWSB shows that the oceanic crust exhibits obvious across-axis velocity variations. The lateral variations in crustal thickness and velocity are also found in the ESB and SWSB. This shows that the previously formed oceanic crust in all three SCS sub-basins are reformed by post-spreading magma. The difference in crustal velocity of the NWSB indicates that the magma supply varied in space during or even after the seafloor spreading. The OBS 2006-p1 velocity model indicates that the velocity contours gradually rise from the southeast to northwest（140–160 km） direction. The crust layer may have experienced asymmetric basalt magma flow, and the northwest part of the basin may have received more magma than the southeast part during the process of seafloor spreading. Low crustal velocities were found in the SWSB and ESB but not in the NWSB. This indicates an inadequate supply of magma in the NWSB. Compared with the other two sub-basins, the Cenozoic tectonic activities in the NWSB were less active. The residual magma might have been consumed after the NWSB spreading. Either there are no magma chambers in the NWSB spreading center, or the magma chambers are too small to detect.