Crustal Radial Anisotropy and Linkage to Geodynamic Processes: A Study Based on Seismic Ambient Noise in Southern Madagascar
Crustal Radial Anisotropy and Linkage to Geodynamic Processes: A Study Based on Seismic Ambient Noise in Southern Madagascar
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DOI：
10.1029/2017jb015273
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发表时间：
2018-06
期刊：
影响因子：
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通讯作者：
J. Dreiling;F. Tilmann;X. Yuan;J. Giese;E. Rindraharisaona;G. Rümpker;M. Wysession
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文献类型：
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作者：
J. Dreiling;F. Tilmann;X. Yuan;J. Giese;E. Rindraharisaona;G. Rümpker;M. Wysession
我们从垂直和水平极化横波(VSV和VSH)的速度之差确定了马达加斯加南部地壳的径向各向异性，该速度差是由地震环境噪声相关确定的瑞利和乐夫波频散得到的。马达加斯加南部东部的前寒武纪拼合单元和西部的显生界Morondava盆地是由不同的地球动力学过程形成的。在泛非造山带冈瓦那拼接过程中，结晶基底发生了强烈的变形和不同程度的变质作用，而Morondava盆地则是在非洲和马达加斯加分离后形成的。这种不同的发展反映在径向各向异性图案的一阶差异上。在前寒武纪领域，正各向异性(VSVVSH)夹在中间。上地壳各向异性可能反映了太古宙及其邻近的叠瓦式推覆体内部的浅倾分层，而下地壳各向异性可能代表了泛非造山带后造山或同造山坍塌期间的地壳流动化石。负各向异性层可能保存了泛非晚期垂直方向的大型剪切带。在莫伦达瓦盆地内，∼最上部5公里处的负各向异性可能是由陡峭的正断层、节理和岩浆岩脉侵入所产生的。深部沉积物和下伏地壳基底具有正各向异性特征。这与沉积中的水平层理和盆地形成过程中伸展作用在基底中形成的组构排列一致。
We determined radial anisotropy in the crust of southern Madagascar from the differences between the speeds of vertically and horizontally polarized shear waves (VSV and VSH), which we derived from Rayleigh and Love wave dispersion determined from seismic ambient noise correlations. The amalgamated Precambrian units in the east and the Phanerozoic Morondava basin in the west of southern Madagascar were shaped by different geodynamic processes. The crystalline basement was strongly deformed and metamorphosed to varying degrees during the assembly of Gondwana in the Pan‐African Orogeny, whereas the Morondava basin was completed with the separation of Africa and Madagascar. The different developments are reflected in first‐order differences in the radial anisotropy patterns. In the Precambrian domains, positive anisotropy (VSVVSH) sandwiched in between. The upper crustal anisotropy may reflect shallowly dipping layering within the Archean and adjacent imbricated nappe stacks, whereas the lower crustal anisotropy likely represents fossilized crustal flow during the postorogenic or synorogenic collapse of the Pan‐African Orogen. The negative anisotropy layer may have preserved vertically oriented large shear zones of late Pan‐African age. Within the Morondava basin, negative anisotropy in the uppermost ∼5 km could have been generated by steep normal faults, jointing, and magmatic dike intrusions. The deeper sediments and underlying crustal basement are characterized by positive anisotropy. This is consistent with horizontal bedding in the sediments and with fabric alignment in the basement created by extension during the basin formation.