非洲南部地壳与上地幔地震波速度与径向各向异性特征及其对克拉通岩石圈中部不连续面的启示  

Seismic velocity and radial anisotropy images of the crust and upper mantle beneath the southern Africa and their implications for cratonic mid-lithosphere discontinuity

作　　者：张学臻 李伦 ZHANG XueZhen;LI Lun(Department of Earth Sciences and Engineering,Sun Yat-Sen University,Guangzhou 510275,China;Guangdong Provincial KeyLab of Geodynamics and Geohazards,Guangzhou 510275,China;Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai),Guangdong Zhuhai 519080,China)

机构地区：[1]中山大学地球科学与工程学院,广州510275 [2]广东省地球动力作用与地质灾害重点实验室,广州510275 [3]南方海洋科学与工程广东省实验室(珠海),广东珠海519080

出　　处：《地球物理学报》2023年第4期1315-1333,共19页Chinese Journal of Geophysics

基　　金：广东省引进人才创新创业团队(2017ZT07Z066);第二次青藏高原综合科学考察研究(2019QZKK0701)联合资助。

摘　　要：克拉通地区发育的岩石圈中部不连续面(Mid-Lithosphere Discontinuity,MLD)对于理解克拉通的形成与演化有着重要的意义.非洲南部的卡普瓦尔克拉通(Kaavpvaal craton)较为稳定,是研究MLD地震学特征及其成因机制的一个重点区域.本研究基于多个地震台网105个台站记录的700多个地震事件的面波波形,通过Rayleigh波和Love波成像,构建了非洲南部地壳与上地幔的三维剪切波速度与径向各向异性模型.研究结果表明,卡普瓦尔克拉通的地壳与上地幔呈现相对高速异常,其岩石圈与软流圈界面(LAB)出现在约220 km深.另外,我们在卡普瓦尔克拉通岩石圈内部约100 km深观测到一个速度突变面,可解释为MLD,并在MLD下方观测到低速层.而各向异性在上地幔的垂直方向上并未显示明显的区域性突变,似乎暗示MLD的地震各向异性特征更为复杂.结合前人的研究成果,我们推测卡普瓦尔克拉通MLD与上地幔低速层的成因可能与温度密切相关.而镁值成分异常或岩浆侵入则会局部的改变该克拉通(尤其是其北部)上地幔速度.针对MLD与上地幔低速成因的研究还需结合更多的地球物理数据和岩石实验结果.本研究为理解克拉通的演化提供重要的基础.The mid-lithospheric discontinuities(MLDs)in cratons hold the key to understanding the formation and evolution of cratons.The Kaavpvaal Craton in southern Africa,one of most stable cratons on Earth,has been regarded as a prime site to study seismic expressions and formation of MLDs.Here we constructed three-dimensional shear-wave velocity and radial anisotropy models of the crust and upper mantle in southern Africa using two-plane-wave surface wave tomography with more than 700 seismic events recorded at 105 stations in multiple seismic networks.Our results show that the crust and upper mantle under the Archean Kaavpvaal craton are characterized with relatively high velocities.The lithosphere-asthenosphere boundary(LAB)is inferred to be at~220 km depth from the shear-wave velocity model.We observed a sharp negative velocity gradient at~100 km depth under the Kaavpvaal craton that we interpret as a MLD.Additionally,the upper mantle of the Kaavpvaal craton is featured with weak anisotropy and doesn't show signs of sharp shifts in seismic anisotropy,implying that the MLD is probably linked with much more complex seismic anisotropic features,rather than layering anisotropic features.Together with previous geologic and geophysical studies,we infer that the MLD could be probably associated with temperature,whereas the abnormal Mg values or magmatic intrusion could reduce shear-wave velocity in the upper mantle of the Kaavpvaal craton in a local scale.Integrated studies from multiple types of geophysical observations and rock physics would help understand the formation of MLDs and the low-velocity layer in upper mantle of the Kaavpvaal craton.This study highlights the seismic expressions of MLD in cratons and could form an important basis for understanding the evolution of cratons.

关 键 词：克拉通 径向各向异性 剪切波速度 岩石圈中部不连续面 南非 

分 类 号：P315[天文地球—地震学]