A potential post-perovskite province in D? beneath the Eastern Pacific: evidence from new analysis of discrepant SKS-SKKS shear-wave splitting
A potential post-perovskite province in D? beneath the Eastern Pacific: evidence from new analysis of discrepant SKS-SKKS shear-wave splitting
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D 中潜在的后钙钛矿省？
DOI：
10.1093/gji/ggaa114
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发表时间：
2020
影响因子：
2.8
通讯作者：
Kendall M
中科院分区：
文献类型：
--
作者：
Kendall M
地震各向异性观测在最底地幔- d″-是丰富的。众所周知，地震各向异性是作为对地幔中塑性流动的响应而发展起来的，限制最低地幔各向异性使我们能够更好地理解地幔动力学。测量横贯最底地幔的体波相的剪切波分裂是约束这种各向异性的有力工具。从最下层地幔各向异性中分离信号需要使用多个剪切波相位，如asSKSandSKKS。这些相位也可以用来约束D″的方位各向异性：sksandskks3的射线路径在上地幔中几乎重合，但在核幔边界处明显发散。在每个相位测量到的剪切波分裂中的任何显著差异都可以归因于D″的各向异性。我们在东太平洋下面的420sks - skkseven站点对样本D″的数据集中寻找剪切波分裂的统计显著差异。为了确保结果的鲁棒性，我们开发了一种新的多参数方法，该方法将测量剪切波分裂的特征值最小化方法与现有的分裂强度方法相结合。这种结合的方法可以更容易地自动化差异剪切波分裂分析。使用这种方法，我们确定了30sks - skk7站对的差异。这些主要位于260°-290°的方位范围内。根据我们的结果，我们解释了东太平洋下D″的方位各向异性区域，其特征是nullskks分裂和finskks的平均延迟时间。这些测量证实并扩展了先前在该区域使用sks - skksands - scphases进行的观察。我们对这种各向异性的首选解释是后钙钛矿的晶格优选取向。造成这种各向异性的一种合理的变形机制是来自法拉龙板块的俯冲物质在核心-地幔边界的撞击。
Observations of seismic anisotropy in the lowermost mantle—D″—are abundant. As seismic anisotropy is known to develop as a response to plastic flow in the mantle, constraining lowermost mantle anisotropy allows us to better understand mantle dynamics. Measuring shear-wave splitting in body wave phases which traverse the lowermost mantle is a powerful tool to constrain this anisotropy. Isolating a signal from lowermost mantle anisotropy requires the use of multiple shear-wave phases, such asSKSandSKKS. These phases can also be used to constrain azimuthal anisotropy in D″: the ray paths ofSKSandSKKSare nearly coincident in the upper mantle but diverge significantly at the core–mantle boundary. Any significant discrepancy in the shear-wave splitting measured for each phase can be ascribed to anisotropy in D″. We search for statistically significant discrepancies in shear-wave splitting measured for a data set of 420SKS–SKKSevent–station pairs that sample D″ beneath the Eastern Pacific. To ensure robust results, we develop a new multiparameter approach which combines a measure derived from the eigenvalue minimization approach for measuring shear-wave splitting with an existing splitting intensity method. This combined approach allows for easier automation of discrepant shear-wave splitting analysis. Using this approach we identify 30SKS–SKKSevent–station pairs as discrepant. These predominantly sit along a backazimuth range of 260°–290°. From our results we interpret a region of azimuthal anisotropy in D″ beneath the Eastern Pacific, characterized by nullSKSsplitting, and mean delay time ofinSKKS. These measurements corroborate and expand upon previous observations made usingSKS–SKKSandS–ScSphases in this region. Our preferred explanation for this anisotropy is the lattice-preferred orientation of post-perovskite. A plausible mechanism for the deformation causing this anisotropy is the impingement of subducted material from the Farallon slab at the core–mantle boundary.