扬子与华夏块体缝合带区域的Rayleigh及Love波相速度层析成像

扬子与华夏块体碰撞拼合构成华南大陆, 其间存在多条大型断裂带, 两个块体缝合于哪些断裂尚存争议, 这限制了对华南构造演化的认识.本研究使用布设于扬子和华夏块体之间的184个流动密集台站以及华南地区273个固定台站的三分量连续波形数据(2014年10月至2016年12月), 采用背景噪声层析成像方法获得研究区域内Rayleigh和Love波周期为4~25 s的二维相速度.短周期的相速度与地壳中上部的盆地和褶皱等地质构造单元吻合; 长周期的相速度在东部地区整体表现为高速异常, 可能是受较薄的地壳厚度影响, 西南部地区Rayleigh波呈现高速异常, Love波则呈现显著低速异常, 这种差异可能反映该区域地下介质存在径向各向异性.成像结果显示, 扬子与华夏块体的Rayleigh和Love波相速度存在较大的差异, 结合华南地区已有的地质与地球物理学研究成果, 本研究认为扬子与华夏块体可能在地壳浅层缝合于绍兴—江山—萍乡—永州—河池一线, 深部缝合带则可能由石台—九江—长沙延伸至永州—河池线.

Phase velocity tomography of Rayleigh and Love waves in the suture zone between the Yangtze and Cathaysia Blocks

The South China Craton consists of the amalgamated Yangtze and Cathaysia Blocks, but the NE-SW striking suture of these blocks is not well defined. Improving the definition of the boundary between these two blocks will help further characterize the regional tectonic framework and geologic history. We obtained the 2D phase velocity of Rayleigh and Love waves at periods of 4~25 s using records of seismic ambient noise observed at 457 stations in South China from 2014 to 2016. The phase velocity maps at periods of 4~12 s are consistent with geological structures like basins and folds in the upper crust. In the phase velocity maps, at periods of 16~25 s, the eastern region mainly displays high-velocity anomalies related to its thinner crustal thickness. In the southwestern region, Rayleigh waves identify high-velocity anomalies, and Love waves identify significant low-velocity anomalies. This velocity anomaly difference implies the existence of radial anisotropy in these regions. The phase velocity maps show significant differences in the Yangtze and Cathaysia Blocks. With the constraints of previous geological and geophysical studies, we infer that the Cathaysia Block subducted beneath the Yangtze Block and sutured along the line of Shaoxing-Jiangshan-Pingxiang-Yongzhou-Hechi in the shallow crust. The suture zone, now buried in the deep crust, should extend from Shitai-Jiujiang-Changsha to Yongzhou-Hechi.