南海上地幔各向异性结构及动力学含义

Anisotropic structure and dynamic implications of the upper mantle in the South China Sea

南海处于欧亚板块、太平洋板块和印度—澳大利亚板块的交会区,是西北太平洋一系列边缘海中最大的边缘海。关于南海的打开以往研究提出了如印度板块与欧亚板块碰撞驱动挤出以及古南海俯冲拖拽等诸多模型。本文力图通过南海海盆及周边各向异性结构来约束南海演化机制。基于同济大学2012和2014年在南海中央海盆进行的两次被动源宽频带海底地震观测试验回收的10台OBS记录仪近1年的地震数据,本文采用三种不同的横波分裂方法,获取了中央海盆针对两次远震的XKS分裂结果以及南海周边20次区域地震提供的S震相分裂结果。SKS分裂结果显示,南海中央海盆下方存在快轴方向为NE-SW向的各向异性,其成因可能与海底扩张时期沿洋脊方向的地幔流以及南海海洋板块俯冲拖拽的地幔流有关。南海及其周边上地幔存在强各向异性,且不同方位观测到的各向异性不同,快轴方向与前人SKS横波分裂结果、GPS和板块运动一致,较好地对应了区域构造运动或者地幔对流模型。各向异性结果与印度—欧亚板块碰撞驱动挤出模型以及古南海俯冲板块拖拽模型预期结果一致,与理想的地幔柱上涌驱动模型不一致。由于海盆各向异性观测特别有限,各向异性结果不能证实亦不能证伪“大西洋型”海底扩张模型、弧后扩张模型和板缘破裂模型,后续还需要更多的观测结果来证实或证伪上述模型。

The South China Sea(SCS)is located at the intersection of the Eurasian,Pacific,and India-Australia plates.It is the largest marginal sea in a series of marginal seas in the Northwest Pacific Ocean.Many models have been proposed for the opening of the SCS,such as the extrusion model driven by the collision of the Indian plate and the Eurasian plate,and the slab pull model related to the subduction of the proto-SCS.This study aims to constrain the models of opening the SCS through the anisotropic structure of the central basin of the SCS and its surroundings.Based on the seismic data recorded by ten ocean bottom seismometers recovered from two passive seismic experiments conducted by Tongji University in the central basin of the SCS in 2012 and 2014,three different shear wave splitting methods are used to obtain the XKS splitting results of the central basin for two global earthquakes and the S phase splitting results provided by 20 regional earthquakes surrounding the SCS.The SKS splitting results demonstrate the presence of strong anisotropy with the NE fast direction in the central basin of the SCS,which may be related to mantle flow along the ocean ridge during seafloor expansion and the mantle flow dragged by the subduction of the proto-SCS plate.Strong anisotropy is also observed in the upper mantle surrounding SCS,and the anisotropy observed in different azimuths is different.The fast directions obtained are consistent with previous SKS-splitting results,GPS,and plate motions,and importantly correspond well to the regional tectonics or mantle convection models.The anisotropic results are consistent with the expected results of the extrusion model driven by the collision of the Indian-Eurasian plate and the slab pull of proto-SCS.The anisotropy results are inconsistent with the ideal upwelling driven model of the mantle plume.Unfortunately,due to the limited splitting observations in the central basin,the anisotropic results cannot confirm or falsify the“Atlantic-type”seafloor spreading model,the backarc spreading model,or the plate-edge rifting model.To verify the above models,further observations are needed.