Katayama Ikuo, Hirauchi Ken-ichi, Michibayashi Katsuyoshi, Ando Jun-ichi
Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
Nature. 2009 Oct 22;461(7267):1114-7. doi: 10.1038/nature08513.
Seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the Earth's interior, as it primarily reflects the deformation-induced preferred orientation of anisotropic crystals. Although seismic anisotropy in the upper mantle is generally attributed to the crystal-preferred orientation of olivine, the strong trench-parallel anisotropy (delay time of one to two seconds) observed in several subduction systems is difficult to explain in terms of olivine anisotropy, even if the entire mantle wedge were to act as an anisotropic source. Here we show that the crystal-preferred orientation of serpentine, the main hydrous mineral in the upper mantle, can produce the strong trench-parallel seismic anisotropy observed in subduction systems. High-pressure deformation experiments reveal that the serpentine c-axis tends to rotate to an orientation normal to the shear plane during deformation; consequently, seismic velocity propagating normal to the shear plane (plate interface) is much slower than that in other directions. The seismic anisotropy estimated for deformed serpentine aggregates is an order of magnitude greater than that for olivine, and therefore the alignment of serpentine in the hydrated mantle wedge results in a strong trench-parallel seismic anisotropy in the case of a steeply subducting slab. This hypothesis is also consistent with the presence of a hydrous phase in the mantle wedge, as inferred from anomalously low seismic-wave velocities.
地震各向异性是探测地球内部变形几何形态和样式的有力工具，因为它主要反映了由变形引起的各向异性晶体的择优取向。虽然上地幔中的地震各向异性一般归因于橄榄石的晶体择优取向，但在几个俯冲系统中观测到的强烈的平行海沟各向异性（延迟时间为1到2秒），即便整个地幔楔都作为一个各向异性源，也难以用橄榄石各向异性来解释。在此我们表明，上地幔中主要的含水矿物蛇纹石的晶体择优取向，能够产生在俯冲系统中观测到的强烈的平行海沟地震各向异性。高压变形实验显示，蛇纹石的c轴在变形过程中倾向于旋转到垂直于剪切面的方向；因此，垂直于剪切面（板块界面）传播的地震波速度比在其他方向上的速度要慢得多。对变形后的蛇纹石集合体估算的地震各向异性比橄榄石的地震各向异性大一个数量级，所以在俯冲板块倾角很大的情况下，水合地幔楔中蛇纹石的排列导致了强烈的平行海沟地震各向异性。这一假说也与地幔楔中存在含水相的情况相一致，这是由异常低的地震波速度推断出来的。
