Constraints on lowermost mantle mineralogy and fabric beneath Siberia from seismic anisotropy
Constraints on lowermost mantle mineralogy and fabric beneath Siberia from seismic anisotropy
复制标题
地震各向异性对西伯利亚下地幔矿物学和构造的限制
DOI：
10.1016/j.epsl.2008.07.049
复制
发表时间：
2008
影响因子：
5.3
通讯作者：
J. Kendall
中科院分区：
文献类型：
--
作者：
J. Wookey;J. Kendall
地震各向异性是研究最低地幔（D”）的性质、起源和动力学的重要工具。在这里，我们通过在 ScS 阶段测量的剪切波分裂来分析西伯利亚下方的地震各向异性。数据来自两条近乎垂直的射线路径（兴都库什至加拿大北部和千岛弧至德国；震中距离均约为 80°），在核幔边界 (CMB) 上具有接近的 ScS 反射点。我们应用差分 S-ScS 分裂来最大限度地减少来自源和接收侧上地幔的污染。两条射线路径显示出不同的 ScS 分裂时间和快速剪切波方向，与推断的最下地幔大部分区域的 VTI 风格的各向异性不相容。两个方位角数据的可用性使我们有机会比以前的研究更好地理解 D'' 各向异性。例如，结果首次提供了对称面倾角的精确测量。人们提出了几种机制来解释最低地幔的各向异性，包括钙钛矿或后钙钛矿等下地幔矿物的晶格择优取向，或熔体包裹体的形状择优取向。为了推断这些机制所暗示的流动状态，我们使用已发表的变形实验中的弹性来正演剪切波分裂模型。该区域的断层扫描表明地球动力学存在南北趋势，而将后钙钛矿与[100](010)滑移系统或排列的熔体包裹体相结合的模型最自然地与这种趋势兼容。这可能表明与北太平洋过去俯冲的残余板片物质有关。
Seismic anisotropy is an important tool for studying the nature, origin and dynamics of the lowermost mantle (D″). Here we present analysis of the seismic anisotropy beneath Siberia from shear-wave splitting measured in ScS phases. Data come from two near-perpendicular raypaths (Hindu Kush to Northern Canada and the Kuril Arc to Germany; both at ~80° epicentral distance) with close ScS reflection points on the Core–Mantle boundary (CMB). We apply differential S–ScS splitting to minimise contamination from the source and receiver-side upper mantle. The two raypaths show different ScS splitting times and fast shear-wave orientations, incompatible with the VTI style of anisotropy inferred for much of the lowermost mantle. The availability of data at two azimuths give us an opportunity to better understand D″ anisotropy than in previous studies. For example, the results provide the first accurate measurement of the dip of the symmetry plane. Several mechanisms have been suggested to explain lowermost mantle anisotropy, including the lattice-preferred orientation of lower mantle minerals such as perovskite or post-perovskite, or the shape-preferred orientation of inclusions of melt. In order to infer the flow regime implied by these mechanisms we use elasticities from published deformation experiments to forward model shear-wave splitting. Tomography of the region suggests a north–south trend in the geodynamics, and a model incorporating post-perovskite with a [100](010) slip system or aligned melt inclusions are most naturally compatible with such a trend. This may suggest a connection with remnant slab material from past subduction in the north Pacific.