Seismic anisotropy of the lithosphere and asthenosphere beneath southern Madagascar from teleseismic shear wave splitting analysis and waveform modeling
Seismic anisotropy of the lithosphere and asthenosphere beneath southern Madagascar from teleseismic shear wave splitting analysis and waveform modeling
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DOI：
10.1002/2016jb013020
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发表时间：
2016-09
期刊：
影响因子：
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通讯作者：
M. Reiss;G. Rümpker;F. Tilmann;X. Yuan;J. Giese;E. Rindraharisaona
中科院分区：
文献类型：
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作者：
M. Reiss;G. Rümpker;F. Tilmann;X. Yuan;J. Giese;E. Rindraharisaona
马达加斯加在冈瓦纳超大陆的组装和分裂中占据关键位置。它已被用于许多地质研究，以重建其在冈瓦纳大陆的原始位置，并推导出板块运动学。迄今为止，马达加斯加的地震观测很少。使用一个临时的，密集的地震剖面在马达加斯加南部，我们提出了第一次发表的研究地震各向异性剪切波分裂分析的Escherichia阶段。获得的分裂参数显示出整个剖面上快速偏振方向和延迟时间的显著小尺度变化，快速偏振从剖面中心的NW旋转到东部和西部的NE。延迟时间范围在0.4和1.5 s之间。在每个站的波形的联合反演被应用以导出假设的单层分裂参数。我们使用有限差分，全波形建模来测试关于地震各向异性的起源和程度的几个假设。我们的观测结果可以用软流层各向异性与地壳各向异性块相结合来解释，软流层各向异性具有50°的快速偏振方向，近似平行于绝对板块运动方向。从全球地幔流模型或全球各向异性面波层析成像推断的地震各向异性预测与观测结果不一致。分裂参数的小尺度变化需要显著的地壳各向异性。考虑到马达加斯加的复杂地质，我们将快轴方向的变化解释为地壳中约150 km宽的韧性变形区，这是泛非造山运动期间岩石圈物质强烈改造的结果。这种化石各向异性模式的基础是由板块运动引起的软流圈各向异性。
Madagascar occupies a key position in the assembly and breakup of the supercontinent Gondwana. It has been used in numerous geological studies to reconstruct its original position within Gondwana and to derive plate kinematics. Seismological observations in Madagascar to date have been sparse. Using a temporary, dense seismic profile across southern Madagascar, we present the first published study of seismic anisotropy from shear wave splitting analyses of teleseismic phases. The splitting parameters obtained show significant small‐scale variation of fast polarization directions and delay times across the profile, with fast polarization rotating from NW in the center to NE in the east and west of the profile. The delay times range between 0.4 and 1.5 s. A joint inversion of waveforms at each station is applied to derive hypothetical one‐layer splitting parameters. We use finite‐difference, full‐waveform modeling to test several hypotheses about the origin and extent of seismic anisotropy. Our observations can be explained by asthenospheric anisotropy with a fast polarization direction of 50°, approximately parallel to the absolute plate motion direction, in combination with blocks of crustal anisotropy. Predictions of seismic anisotropy as inferred from global mantle flow models or global anisotropic surface wave tomography are not in agreement with the observations. Small‐scale variations of splitting parameters require significant crustal anisotropy. Considering the complex geology of Madagascar, we interpret the change in fast‐axis directions as a ~150 km wide zone of ductile deformation in the crust as a result of the intense reworking of lithospheric material during the Pan‐African orogeny. This fossil anisotropic pattern is underlain by asthenospheric anisotropy induced by plate motion.