跨南海西南次海盆OBS、多道地震与重力联合调查

A joint investigation using OBS,multi-channel seismic and gravity data across the southwestern sub-basin of the South China Sea

对跨南海西南次海盆及两侧陆缘的一条1050km长的、包括海底地震(OBS)、长排列多道地震和重磁在内的综合地球物理探测剖面(CFT)进行了构造成像和研究。在多道地震成像基础上建立了CFT剖面初始速度模型,进而通过初至波层析成像方法反演了CFT剖面的速度结构模型,在重力异常资料的约束下建立了CFT剖面的综合地壳结构模型。讨论了沿CFT剖面出现的下地壳高速体、龙门海山的低密度物质等地质问题。结果表明,下地壳高速层在北部陆坡、西南海盆和南部南沙地块均有分布,厚度在0~4km之间,可能与陆缘下地壳物质和地幔物质熔融混合,以及深海盆海底扩张期间构造拉伸导致地幔蛇纹岩化有关。

A 1050-km long comprehensive geophysical profile(CFT) was acquired across the conjugate margins of the southwestern sub-basin of the South China Sea,which includes 49 OBSs,6-or 8-km long streamer,gravimeter,and magnetometer.Various refined processing procedures were applied to the aforementioned geophysical data;and a joint reflection and refraction seismic travel time inversion was performed to derive a 2-D velocity model of the crustal structure and upper mantle.Based on this new tomographic model and shipboard gravity data,a comprehensive crustal structure model was created.Finally,some interesting issues including High Velocity Layer(HVL) in the base of crust,an anomalous low density seamount(long-men seamount) along the CFT profile are discussed in this paper.HVL are widely distributed under the northern slope,southwestern sub-basin and Nansha block along the CFT profile,with the velocity varying from 7.0 to 7.5 km·s-1,and thickness between 0 and 4 km.HVL in the marginal lower crust might be derived from melting and mixing of lower crust material and mantle material,and HVL in the oceanic crust might have originated from serpentinization by tectonically dominated seafloor spreading.