全球下地幔波速异常结构综述

Review of seismic velocity anomalies in the lower mantle

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.

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.