九嶷山及邻区地壳结构噪声成像及其对华南地区的构造演化启示

Crustal shear wave velocity structure near the Jiuyishan area from seismic ambient noise tomography:Implications for tectonic evolution in South China

目前研究一般认为华南块体是由扬子块体和华夏块体在新元古代拼合形成,并同时形成位于扬子块体东南边缘的江南造山带.但是由于华南地区构造历史复杂,对于扬子块体与华夏块体的分界及构造属性仍存在较大争议.为了研究华南块体的地下速度结构及构造属性,我们利用块体交界处的九嶷山及其附近的流动和固定台网的地震波数据,采用地震背景噪声互相关方法反演研究区域2~40 s瑞利波群速度和相速度分布,并进一步得出了该区域地壳的精细三维S波速度结构.反演成像结果显示,扬子块体与华夏块体的地壳及上地幔的结构特征差异显著.10~20 km的S波速度分布图显示呈线性的、连续分布低速异常,可能为扬子块体与华夏块体的具体分界位置.结合华南地区地球化学研究结果和构造历史,该低速异常可能代表了来自上地壳的变质沉积岩,即沉积岩受到上地幔物质上涌或底侵作用的加热变质形成.成像结果对了解华南地区的构造演化历史提供了地震学约束.

The South China Block is composed of Yangtze Block to the northwest and the Cathaysia Block to the southeast.It is amalgamated during the Early Neoproterozoic.Due to the complicated tectonic history of this region,the boundary of the Yangtze Block and the Cathaysia Block at the south part is not conclusive,and the tectonic process need further exploration.Here in this study,we use Rayleigh wave dispersion curves form ambient noise cross correlation to investigate the velocity structure and the tectonic evolution of the South China Block.Three-dimensional shear wave velocity structure of the crust and uppermost mantle beneath this region are inverted.Results reveal different structures in crust and uppermost mantle between the Yangtze Block and Cathaysia Block.At 10~20 km in the middle crust,there exists obvious linear and continuous low-velocity zones in northwest-southeast direction.It may represent the boundary of the Yangtze Block and the Cathaysia Block.Combining results from geochemistry isotopes and the tectonic evolution history of South China,these low-velocity anomalies may represent metasedimentary rocks from supracrustal components that may have been subducted and later were heated by upwelling or intrusion melts.The results provide detailed seismic constraints on both the structure and the tectonic evolution in South China.