华南壳幔结构与动力学的宽频地震观测研究

了解华南各岩石圈块体壳幔结构和各向异性方面的差异是揭示华南深部构造演化的基础。本文利用布设于华南的两条宽频地震测线观测数据,采用多种地震学方法对华南的地壳上地幔结构和各向异性进行了研究。接收函数结果表明,华南地区地壳厚度和岩石圈厚度都较薄,地壳厚度自东南沿海向西北内陆增厚,扬子克拉通的泊松比(波速比)低于华夏块体,表明扬子克拉通地壳较华夏块体更偏长英质。约北纬29°以北的扬子克拉通地幔转换带厚度明显增厚,可能是由地幔转换带底部停滞的冷的古太平洋板片或中生代克拉通碰撞残留造成的。层析成像结果显示华南上地幔具有很强的横向差异性,上地幔中的强烈低速异常体可能对应了晚中生代发生广泛岩浆作用时的岩浆房和岩浆通道。台湾下方的上地幔存在南北横向差异明显的高速异常,分别对应台湾南部向东俯冲的欧亚板块及台湾北部向北俯冲的菲律宾海板块。俯冲的欧亚板块在台湾南部是连续的,而在台湾中北部,由于与菲律宾海板块的相互作用,俯冲的欧亚板块被折断。剪切波分裂结果显示,以江绍断裂为界,华夏块体与扬子克拉通的岩石圈地幔各向异性存在明显的横向变化,表明两者的构造演化过程有显著差异。

Crust and upper mantle structure and geodynamics beneath Southeast China revealed by broadband seismic observation data

Determing the crust and upper mantle structures and the anisotropies of the lithospheric blocks in the South China block(SCB)is the basis to reveal their deep tectonic evolution. Two broadband seismic observation profiles were deployed in the SCB and several seismic methods were employed to study the crust and upper mantle structure and the anisotropy of the SCB. Receiver function results show that both the crust and the lithosphere of the SCB are thin and the crust is thickening from the southeast coastal area to the northwest inland. Poisson's ratios(V_P/V_S) of the Yangtze Craton(YC)are lower than those of the Cathaysia block(CB), which indicates that the crust of the YC is more felsic than that of the CB. The YC's mantle transition zone(MTZ)is obviously thickened to the north of about 29°N, which may be due to the stagnant cold ancient Pacific plate or the remains of the Mesozoic collision of cratonic units. Tomographic images show strong heterogeneities beneath the SCB. The prominent low V_P anomalies in the upper mantle may reflect the remnant magma chambers and channels of the Late Mesozoic magmatism. The high V_P anomalies in the upper mantle beneath the south and the north of Taiwan represent the subducted Eurasian plate(EUA) and Philippine Sea plate(PHS), respectively. The subducted EUA is continuously visible beneath the South Taiwan but break-off beneath the central and North Taiwan which may be caused by the interaction between the EUA and the PHS under central and North Taiwan. Shear-wave splitting results show significant fast-velocity direction variations across the Jiangshao fault between the YC and the CB, which implies their different tectonic evolutions.