Abstract:

The South China block, located in the east of the Eurasian plate, mainly consists of the Yangtze block and the Cathaysia block. The South China block is bounded by the eastern margin of the Qinghai-Tibet Plateau in the west, the Qinling-Dabie orogenic belt in the north, and its eastern boundary extends from the southeast coast to the north, through the Taiwan Strait, and then along the Ryukyu Island arc to the west direction. The neotectonic movement of the South China block is intense. It is not only the continental margin with the most active crustal growth and continental accretion, but also the tectonic belt with the most intense core-mantle mass transfer and the coupling zone of the inner layers of the Earth. Therefore, the crust-mantle velocity structure of the South China block and its formation and evolution have always been a hot topic in earth science research.

In this paper, we collected continuous vertical component broadband seismic data between January 1, 2010 and December 31, 2012 from the regional networks of 609 stations and used ambient noise tomography method to inverse the three-dimensional S-wave velocity structure of South China block and its adjacent area. Firstly, the seismograms are cut into daily segments and decimated at a sampling rate of 1Hz. After the removal of the mean, trend, and instrument response, a 3~150s band-pass filter is applied. In order to reduce the effect of earthquakes and instrumental irregularities on cross-correlations, we normalized the seismograms with a time-frequency normalization method. Then, we computed daily cross-correlations for each station pairs and stacked all of them by using normalized linear stacking method to obtain cross-correlation functions. Next, the phase velocity dispersion curves of Rayleigh surface wave were extracted by frequency-time analysis method. Finally, the three-dimensional S-wave velocity structure of the study area was obtained by using nonlinear Bayesian Monte Carlo inversion method.

The results show that the S-wave velocity distribution has a good correlation with surface geological and tectonic features, and could clearly reveal the lateral velocity variation in the crustal. The shallow S-wave velocity in basin and graben area presents low velocity anomaly due to the influence of sedimentary layer. The high velocity anomaly exists in the middle and lower crust of Jianghan Basin and Sichuan Basin, indicating that the middle and lower crust of these basins are cold and hard. Due to the phenomenon of arching existing in the upper mantle of Sichuan Basin, the S-wave velocity of the crust and mantle is relatively high in the upper mantle, meanwhile, the S-wave velocity in the center of the basin is higher than that in the edge. Although both the Yangtze block and Cathaysia block are located in the South China block, their upper mantle S-wave velocity structures are quite different due to their different evolutionary processes. The high S-wave velocity of the Yangtze block indicates the internal structure of the block is relatively stable, while the low S-wave velocity of the Cathaysia block indicates the strong magmatic activity during its evolution. The crust-mantle S-wave velocities in the west of the southwest boundary of the South China block show low velocity anomalies, which may indicate the existence of asthenosphere in the middle and lower crust of the eastern margin of the Qinghai-Tibet Plateau. The S-wave velocity structures of the eastern and western parts of the Qinling-Dabie orogenic belt are quite different, and the crustal thickness transition zone is the boundary of the S-wave velocity structure, which is high in the east and low in the west. The crust-mantle S-wave velocity of Ordos block is relatively high, indicating that the inner structure of ordos block is relatively stable. However, the S-wave low velocity anomaly in the upper mantle at the southwest corner of the Ordos Basin may indicate that the heat flow of the upper mantle of the North China Craton has begun to “invade” the Ordos lithosphere.

Key words: ambient noise, phase velocity, dispersion curve, S-wave velocity, crust and upper mantle velocity structure

摘要：

文中使用华南地块及其邻区609个宽频地震仪记录的2010年1月-2012年12月共36个月垂直分量(Z分量)的连续噪声数据, 通过波形互相关和叠加计算得到各台站对间的经验格林函数, 采用时频分析方法(FTAN)提取台站对之间的瑞利波相速度频散曲线, 并使用非线性贝叶斯蒙特卡罗方法反演获得华南地块及其邻区的三维S波速度结构。结果显示, S波速度的分布特征与地表地质和构造特征表现出较好的相关性, 能清晰地揭示出地壳内部的横向速度变化。盆地和地堑地区由于受沉积层的影响, 浅层的S波速度表现为低速异常。 江汉盆地和四川盆地的中下地壳存在高速异常, 表明盆地的中、 下地壳较冷、 硬。四川盆地内部由于存在上地幔上拱现象, 其壳-幔S波速度整体相对较高, 且盆地内部中心区域的S波速度高于边缘区域。位于华南地块内部的扬子地块和华夏地块由于演化过程有所不同, 上地幔的S波速度结构存在较大差异。扬子地块的S波速度相对较高, 说明其块体内部结构相对稳定, 而华夏地块的S波呈现低速异常, 预示着在其演化过程中存在强烈的岩浆活动。位于华南地块西南边界以西区域的壳-幔S波速度呈低速异常, 可能预示着青藏高原东缘中下地壳存在软流层。秦岭-大别造山带东、 西2段的S波速度结构存在较大差异, 以地壳厚度过渡带为界呈现东高西低的分布特征。鄂尔多斯块体的壳-幔S波速度相对较高, 说明其块体内部结构相对稳定, 但其西南角上地幔中的S波呈现低速异常, 可能表明华北克拉通上地幔热流已经开始对鄂尔多斯的岩石圈进行“侵入”改造。

关键词: 背景噪声, 相速度, 频散曲线, S波速度, 壳-幔速度结构