Some remarks on the origin of seismic anisotropy in the D” layer
Some remarks on the origin of seismic anisotropy in the D” layer
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关于D”层地震各向异性成因的几点思考
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发表时间：
1998
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通讯作者：
S. Karato
中科院分区：
文献类型：
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作者：
S. Karato
基于地震和矿物物理观测，探讨了D”层地震各向异性的物理机制。考虑了体波地震学对D”层精细结构的研究结果和矿物物理学对下地幔矿物的弹性常数、晶格优选取向以及熔穴形状优选取向的研究结果。总结了下地幔矿物，特别是（Mg,Fe）O的弹性各向异性大但与深度（压力）相关的证据，以及剪切部分熔体的倾斜形状优选方向。结果表明，部分熔体（或富铁次级相）的形状优选取向和有充分记录的滑移系统的矿物的晶格优选取向都难以与地震观测相一致。然而，高各向异性矿物（Mg,Fe）O的晶格优先取向与大多数地震观测结果一致，如果D”层条件下的主导滑动面为100，而不是在较低压力下观察到的110。在MgO（或（Mg,Fe）O）中，这种滑动平面的变化可能是由于压力引起的弹性各向异性和/或化学键性质的变化（也可能是由于高温）而发生的。固态和部分熔体的各向异性机制表明，VSH > VSV （VSV > VSH）极化各向异性意味着水平（垂直）流动。在固态机制中，环太平洋（阿拉斯加和加勒比海）下D”层中显著的VSH b> VSV意味着可能由俯冲物质与核幔边界碰撞引起的高应力水平剪切。中太平洋海底高度多变的各向异性可归因于一种复杂的三维流动引起的固体结构，这种流动可能与羽流上涌有关，但该地区的各向异性也可归因于熔体穴的形状偏好方向，其存在由非常低的平均速度表明。
Physical mechanisms of seismic anisotropy in the D” layer are examined based on seismological and mineral physics observations. The results of body-wave seismology on the fine structure of the D” layer and of mineral physics studies on the elastic constants and the lattice preferred orientation in lower mantle minerals as well as the shape preferred orientation of melt pockets are taken into account. Evidence of large but depth (pressure)-dependent elastic anisotropy of lower mantle minerals, particularly (Mg,Fe)O, and of tilted shape preferred orientation of sheared partial melts is summarized. It is shown that both shape preferred orientation of partial melts (or iron-rich secondary phases) and lattice preferred orientation of minerals with well-documented slip systems are difficult to reconcile with seismological observations. However, lattice preferred orientation of highly anisotropic mineral, (Mg,Fe)O, is consistent with most of the seismic observations if the dominant glide plane under the D” layer conditions is 100 rather than 110 as observed at lower pressures. Such a change in glide plane in MgO (or (Mg,Fe)O) is likely to occur as a result of pressure-induced change in elastic anisotropy and/or in the nature of chemical bonding (and possibly due to high temperatures). Both solid-state and partial melt mechanisms of anisotropy imply that the VSH > VSV (VSV > VSH) polarization anisotropy means horizontal (vertical) flow. In the solid-state mechanism, significant VSH > VSV in the D” layer beneath the circum-Pacific (Alaska and the Caribbean) implies horizontal shear at high stress caused presumably by the collision of subducting materials with the core-mantle boundary. Highly variable anisotropy beneath the central-Pacific can be attributed to solid-state fabrics caused by a complicated three-dimensional flow presumably related to the upwelling of plumes, but anisotropy in this region could also be attributed to the shape preferred orientation of melt pockets the presence of which is suggested by very low average velocities.