利用地震各向异性方法研究地幔底部的形变特征
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摘要
地震波在穿过地幔底部(即D″层)时表现出很明显的各向异性特征(地震波速度随着方位角变化而变化)[1-5]。D″层各向异性的形成机制一般认为是由D″层的主要相变矿物MgSiO3-post-perovskite(ppv),由于形变导致晶体的晶格定向排列引起的。如果这种解释机制正确的话,那么先前从单个方向测量D″层的各向异性来推断ppv晶体的滑动机制就是不充分的,因为ppv晶体是正交晶系。我们根据美洲北部和中部地区在核-幔边界处仍残留着俯冲海洋板块的残片[6]的构造特点,挑选了穿过D″层的浅震和深震,来增加射线在D″层的方位覆盖,处理得到美洲北部和中部D″层的各向异性。我们分别处理了穿过3个研究区的D″层的700多个剪切波波形数据,每一次同时处理来自两个不同方位的震相,研究发现,先前假设的垂直对称轴的横向各向同性(波速不随方位的变化而变化)[2-3,7]机制是不可能的,需要引入更复杂的机制。我们将MgSiO3-ppv多种形变机制和观测结果进行对比,发现在(001)面上的剪切与观测到的数据和俯冲带的核-幔边界处剪切形变的预期结果相吻合。利用新的地幔流动模型及改善后的ppv滑移系的实验观测确定方法,我们可以对核-幔边界的形变特征进行成像,将D″层中的动力过程(比如,地幔柱的形成)与地幔的其他部位联系起来。
引文
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