Discriminating Between Causes of D? Anisotropy Using Reflections and Splitting Measurements for a Single Path
Discriminating Between Causes of D? Anisotropy Using Reflections and Splitting Measurements for a Single Path
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区分 D 的原因？
DOI：
10.1029/2018jb016993
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发表时间：
2019
期刊：
影响因子：
3.4
通讯作者：
Pisconti A
中科院分区：
文献类型：
--
作者：
Pisconti A
对深部地幔变形的认识是基于地震各向异性：地震波速和偏振沿方向的变化。测量这种方向相关性需要D“深度的方位地震覆盖--地幔底部的几百公里--这通常是恢复各向异性类型的极限。横波分裂是探测地幔各向异性的标准技术，最近，D“区的反射被用来推断方位各向异性。在这里，我们将D“反射的观测和模拟与沿给定射线路径方向的横波分裂相结合，以约束下地幔的矿物学和动力学。根据我们的模型，通过同时使用两种类型的观测，但只使用一条定向路径，可以在不同的各向异性介质之间进行明显的区分。我们通过使用摩洛哥记录的中南美洲地震来关注大西洋中部之下最低的地幔。我们发现D“反射和横波分裂参数的极性都存在复杂的方位和距离变化，这排除了该地区存在简单的各向异性--如垂直横向各向同性--的可能性。我们的首选模型包括后钙钛矿中从随机取向的桥线石到晶格择优取向组构的相变，在沿大致西南-北东向变形方向剪切的倾斜平面上发展。
Knowledge of deep mantle deformation is based on seismic anisotropy: the variation of seismic wave speed and polarization with direction. Measuring this directional dependency requires azimuthal seismic coverage at D″ depth—the bottom few hundred kilometers of the mantle—which is often a limit in retrieving the style of anisotropy. Shear wave splitting is the standard technique for probing mantle anisotropy, and recently, reflections from the D″ region have been used to infer azimuthal anisotropy. Here we combine observations and modeling of D″ reflections with shear wave splitting along a given raypath direction in order to constrain mineralogy and dynamics of the lower mantle. From our modeling, a clear distinction between different anisotropic media is possible by using both types of observations together but only one directional path. We focus on the lowermost mantle beneath the central Atlantic Ocean by using South‐Central American earthquakes recorded in Morocco. We find complex azimuthal and distance variation for both polarities of D″ reflections and shear wave splitting parameters, which rules out a simple style of anisotropy—such as vertical transverse isotropy—for the region. Our preferred model consists of a phase transition from a randomly oriented bridgmanite to lattice‐preferred orientation fabric in postperovskite, developed in a tilted plane sheared along a roughly SW‐NE deformation direction.