Review of seismic velocity anomalies in the lower mantle
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摘要: 下地幔体积占地球总体积50%以上，对地球的演化具有重要的影响. 早期研究认为下地幔的组分比较均一，但1970年代以来，地震层析成像揭示了地球的深部速度结构，发现下地幔存在很多复杂的波速异常区. 进入21世纪以后，台阵数据的积累和计算机技术的进步使我们能够进一步约束这些下地幔波速异常区的空间范围和波速结构，由于这些异常结构通常与俯冲板片和地幔柱等有紧密的联系，了解这些波速异常体的精细结构对于古板块的重建和地幔动力学有重要的意义. 本文重点总结了近30年以来利用地震数据研究下地幔异常体的方法和结果，详细地描述了不同类型的波速异常区在全球范围内的分布情况及其特征，并逐一分析了不同类型波速异常构造体的成因. 下地幔LLSVP主要有两个，分别是非洲LLSVP和太平洋LLSVP，它们在横向上可扩展至数千千米，垂直方向上从核幔边界的高度超过1000 km. 现在观测结果发现LLSVP边界处的速度突变较大，主流的观点认为含有成分异常的热化学作用形成了LLSVP. ULVZ位于下地幔底部，其横向扩展大部分小于1000 km，但部分ULVZ的范围可以超过1000 km，高度仅为十几到几十千米，相应的S波速度异常可达−40%～−20%. 一般认为ULVZ的形成与地幔底部的部分熔融有关，而成分异常促进了部分熔融的发生. D"速度不连续面在很多地区也非常明显，特别是存在俯冲板块的地区，可能由于俯冲板块降低了核幔边界处的温度，使得pPv能够稳定存在，从而形成了稳定的D"速度不连续面. 除了波速异常之外，下地幔波速的各向异性也是值得关注的重点之一，因为各向异性直接反映了地幔流动状态. 现有研究结果表明，下地幔这几种主要的波速异常结构（LLSVP、ULVZ、 D"间断面、各向异性）在空间分布上密切相关，这些异常体可能与俯冲板片有关，其周围地幔温度和化学成分的差异可能是产生这些异常体的重要因素. 最后，本文还总结了中国地区的下地幔波速结构的结果，展望了我国未来在地幔波速异常和各向异性研究的方向.Abstract: The lower mantle accounts for more than 50% of the total volume of the Earth, and influences the evolution of the Earth. Early studies showed that the lower mantle is relatively homogeneous, but since the 1970s, seismic tomography revealed the velocity structure of the deep Earth and found many velocity anomalies in the lower mantle. The accumulation of new array data and the progress of computer technology in the present century have enabled us to further constrain the seismic structures of these anomalous zones. Since these structures could be closely related to subduction plates and mantle plumes, it is of great significance to understand the fine structure of these wave velocity anomalies to study the reconstruction of ancient plates and the mantle dynamics. This study focuses on summarizing the methods and results in revealing the lower mantle anomalies using seismic data in the past 30 years, especially the distribution and characteristics of different types of velocity anomalous zones worldwide. African and Pacific LLSVP are the two most important LLSVPs in the lower mantle. Their lateral and vertical extents span approximately a thousand and more than a thousand kilometers, respectively. The present study found sharp velocity contrasts along the boundary of the LLSVP, which argues for chemical anomalies in the LLSVP. The ULVZ is located at the bottom of the lower mantle, with extremely low S-wave velocities that are 20%~40% smaller than the average. The lateral extent of the ULVZ is generally smaller than a thousand kilometers, but some ULVZ exceed that extent; the heights range from ten kilometers to dozens of kilometers. Partial melting, particularly related to chemical anomalies, is thought to be the most plausible explanation for ULVZ. D" discontinuities were also observed in many regions, especially where high-velocity subducted slabs were found. The slabs reduce the temperature near the core-mantle boundary, which yields stable pPv and D" discontinuities. In addition, the seismic anisotropy of the lower mantle is also a focus because it reflects the flow conditions of the mantle. The existing results show that several wave velocity anomaly structures (LLSVP, ULVZ, D", anisotropy) in the lower mantle may be closely related and connected to the subducting slabs in space, and the difference in mantle temperature and chemical composition may be an important factor that produces these anomalies. Finally, this study also summarizes the results of the velocity structure of the lower mantle in China and looks forward to the future research direction of the velocity anomaly and anisotropy of the lower mantle in China.
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Key words:
- lower mantle /
- seismic structure /
- anisotropy /
- LLSVP /
- ULVZ /
- D" layer /
- slab
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图 1 地幔内主要构造活动示意图（修改自Kopper et al., 2021）
Figure 1. A cartoon showing tectonics in the mantle (modified from Kopper et al., 2021)
图 2 地震层析成像揭示的太平洋和非洲LLSVP的结构（修改自French and Romanowicz, 2015），红色和蓝色分别表示低速和高速异常
Figure 2. Structures of the Pacific and African LLSVP revealed by seismic tomography (modified from French and Romanowicz, 2015). The red and blue colors denote low- and high-velocity anomalies, respectively
图 3 研究ULVZ使用的主要震相及其射线路径（修改自Yu and Garnero, 2018）. 虚线表示S波，实线表示P波
Figure 3. Popular phases and their corresponding ray paths that are used to study the ULVZ structures (modified from Yu and Garnero, 2018). Dashed and solid lines denote S and P waves, respectively
图 4 全球的ULVZ区域分布图（修改自Yu and Garnero, 2018），底图为2800 km深的S波速度异常（图1; 修改自 French and Romanowicz, 2015）. （a）确定ULVZ存在的区域. （b）ULVZ可能存在的区域. （c）ULVZ不存在的区域
Figure 4. Distribution of ULVZ (modified from Yu and Garnero, 2018). Background colors show the S wave velocity anomalies (Fig. 1; modified from French and Romanowicz, 2015). (a) ULVZ is confirmed. (b) ULVZ possibly exists. (c) ULVZ does not exist
图 5 夏威夷地区下方大型ULVZ结构示意图（修改自Jenkins et al., 2021）
Figure 5. ULVZ structures under Hawaii (modified from Jenkins et al., 2021)
图 6 ULVZ和D"间断面在产生衍生震相时的对比. （a）经过ULVZ的ScS及其前后驱震相CMB处的射线路径，d表示ULVZ的厚度. （b） S、Scd和ScS在地球内的射线路径. （c）利用修改的PREM模型获得的远震S、Scd、ScS理论地震图（修改自Fan et al., 2021）
Figure 6. Raypaths generated by ULVZ and D" discontinuity. (a) ScS and its pre- and post-phases related to the ULVZ. (b) Ray paths of S, Scd, and ScS in the Earth. (c) Synthetic waveforms of S, Scd, and ScS generated by a modified PREM model (modified from Fan et al., 2021)
图 9 后钙钛矿（pPv）、布里奇曼石（Pv）和铁橄榄石（Fp）在下地幔环境下发生简单剪切（a）、纯剪切（b）和拉伸变形（c）产生的定向排列的S波各向异性（修改自Creasy et al., 2021）. 红色表示弱各向异性，蓝色表示强各向异性，各向异性最大值（max）标注在每个子图顶端. 后钙钛矿可能存在三种主要组构，从左到右分别是pPv1 (010) [100]、pPv2 (001)[100]和pPv3 {011}[0–11]+(010)[100]
Figure 9. S wave anisotropy generated by pPv, Pv, and Fp under simple shear (a), pure shear (b), and extensional deformations (c) in the lower mantle (modified from Creasy et al., 2021). The red and blue colors denote weak and strong anisotropy, respectively; maximum values are labeled above each subfigure. pPv has three possible fabrics, which are pPv1 (010) [100], pPv2 (001) [100], and pPv3 {011}[0–11] + (010) [100], from left to right
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