华南地区上地幔波三维速度结构和动力学意义来(7)

5 结论

本文利用155个流动地震台站的连续地震波形记录，通过有限频层析成像技术，反演获得了华南地区的高分辨率P波三维速度模型.结果显示，大致以江南造山带为界，研究区南部华夏块体上地幔大部分存在一个清晰的低速异常构造，而研究区北部扬子克拉通上地幔则大部分表现为高速异常结构，且南部的低速体和北部的高速体都向下延伸到地幔转换带.在300～500 km深度范围内，这种以江南造山带为界、南部低速而北部高速的分布特征最为清晰.

在华夏块体下方地幔转换带400～500 km深度范围观测到比较强的低速异常体，指示华北地区地幔转换带中广泛存在的高速异常体没有向南延伸到华夏块体.而在华夏块体下方地幔转换带的低速异常体对应的热的地幔物质可能在(27°N，118°E)处通过410-km界面进入上部地幔，并且在上升的过程中逐渐向西和向北扩展，显示为华夏块体深部200～400 km深度的大范围低速异常，可能为华夏块体广泛分布的新生代岩浆活动提供深部来源.

而华夏块体下方，上涌流通过410-km界面在上涌的过程中向北延伸，越过江南造山带“侵入”到扬子克拉通的南部地区，造成了扬子克拉通岩石圈所对应高速异常体向南倾斜的假象.另外，117°E(郯庐断裂的南端)以东的扬子克拉通岩石圈已经被“活化”，即被来自南部的软流圈热物质替而代之.

在华夏块体下方地幔转换带内低速异常体推测可能与海南地幔柱有关.而海南地幔柱是否存在于(27°N，118°E)(通过410-km界面上涌流)处还有待于今后更大范围(包括华南地区和南海海盆)的地震台阵探测研究进行验证，尤其是P波和S波的联合反演研究.

致谢特别感谢野外小组的每一位成员的付出.本文使用Generic Mapping Tools 软件(Wessel and Smith, 1998)在生成稿件的图片.

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