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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。
Seismic waves show significant anisotropy (the velocity of seismic waves varies with azimuth) when they pass through the bottom of the mantle (ie, the D “layer). [1-5] The formation mechanism of anisotropy in layer D” Is caused by the lattice-oriented arrangement of the crystals due to the deformation of the main phase-change mineral MgSiO3-post-perovskite (ppv) of the D “layer. If this explanation is correct, then each The anisotropy to infer ppv crystal sliding mechanism is not sufficient, because the ppv crystal is orthorhombic. Based on the structural features of the remnants of the subducted oceanic plate remaining at the core-mantle boundary in the northern and central parts of the Americas [6], we selected shallow and deep earthquakes through the D ” We have separately processed over 700 shear waveforms of the D "layers across the three study areas, each processing simultaneously data from two different orientations The study found that the previously assumed horizontal isotropy of the vertical axis of symmetry (the wave velocity does not change with azimuth) [2, 3, 7] mechanism is not possible and more sophisticated mechanisms need to be introduced. We compared MgSiO3-ppv deformation mechanisms with observations and found that the shear and observed data at (001) coincide with expected results of shear deformation at the core-mantle boundary of the subduction zone. Using the new model of the mantle flow and the improved method of experimental observations of the ppv slip system, we can image the deformation features of the core-mantle boundary and map the dynamic processes (eg, the formation of mantle plumes) in the D layer to Other parts of the mantle are linked.