Melt-induced seismic anisotropy and magma assisted rifting in Ethiopia: Evidence from surface waves
Melt-induced seismic anisotropy and magma assisted rifting in Ethiopia: Evidence from surface waves
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DOI：
10.1029/2010gc003036
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发表时间：
2010-06-08
影响因子：
3.5
通讯作者：
Stuart, G. W.
中科院分区：
文献类型：
--
作者：
Bastow, I. D.;Pilidou, S.;Stuart, G. W.
埃塞俄比亚的东非裂谷在世界范围内是独一无二的，因为它抓住了从大陆裂谷到海底扩张过渡的最后阶段。最近的一项研究表明，岩浆侵入在大陆分裂的最后阶段发挥了重要作用，但它被纳入延伸板块的机制仍然不明确：广角地震数据和重力分析等补充地球物理工具对地下熔融侵入体的几何形状并不敏感。在过渡性主埃塞俄比亚裂谷（MER）下的近垂直SKS相位的剪切波分裂的研究提供了强有力的和一致的证据，裂谷平行的快速各向异性方向。然而，很难区分取向熔体袋（OMP）和晶格择优取向（LPO）的各向异性的基础上SKS研究单独的原因。对于LPO和OMP诱导的各向异性，水平传播的Love（S-H）和Rayleigh（S-V）波的速度随方位角以类似的方式变化，但它们的相对变化对于每种机制都是不同的。利用这种诊断研究传播的表面波从一套方位角横跨MER。各向异性大致垂直于板块的绝对运动方向，因此排除了由于非洲板块缓慢移动而导致的各向异性。相反，三种机制的各向异性下的MER的行为：周期性薄层的地震快，慢的材料在最上面的类似10公里，OMP之间的类似20-75公里的深度，和橄榄石LPO在上地幔下面。最好的解释结果的模型中，低纵横比熔融包裹体（岩墙和静脉）被侵入到一个扩展的板块在后期破裂。来自埃塞俄比亚的观测加入了来自世界各地裂谷和被动边缘的越来越多的证据，这些证据表明岩浆侵入在没有明显地壳减薄的情况下在适应扩张方面发挥着重要作用。
The East African rift in Ethiopia is unique worldwide because it captures the final stages of transition from continental rifting to seafloor spreading. A recent study there has shown that magma intrusion plays an important role during the final stages of continental breakup, but the mechanism by which it is incorporated into the extending plate remains ambiguous: wide-angle seismic data and complementary geophysical tools such as gravity analysis are not strongly sensitive to the geometry of subsurface melt intrusions. Studies of shear wave splitting in near-vertical SKS phases beneath the transitional Main Ethiopian Rift (MER) provide strong and consistent evidence for a rift-parallel fast anisotropic direction. However, it is difficult to discriminate between oriented melt pocket (OMP) and lattice preferred orientation (LPO) causes of anisotropy based on SKS study alone. The speeds of horizontally propagating Love (S-H) and Rayleigh (S-V) waves vary in similar fashions with azimuth for LPO- and OMP-induced anisotropy, but their relative change is distinctive for each mechanism. This diagnostic is exploited by studying the propagation of surface waves from a suite of azimuths across the MER. Anisotropy is roughly perpendicular to the absolute plate motion direction, thus ruling out anisotropy due to the slowly moving African Plate. Instead, three mechanisms for anisotropy act beneath the MER: periodic thin layering of seismically fast and slow material in the uppermost similar to 10 km, OMP between similar to 20-75 km depth, and olivine LPO in the upper mantle beneath. The results are explained best by a model in which low aspect ratio melt inclusions (dykes and veins) are being intruded into an extending plate during late stage breakup. The observations from Ethiopia join a growing body of evidence from rifts and passive margins worldwide that shows magma intrusion plays an important role in accommodating extension without marked crustal thinning.