Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions
Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions
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DOI：
10.1029/2019jb018560
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发表时间：
2020-02
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通讯作者：
T. Rajaonarison;D. S. Stamps;S. Fishwick;S. Brune;A. Glerum;J. Hu
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作者：
T. Rajaonarison;D. S. Stamps;S. Fishwick;S. Brune;A. Glerum;J. Hu
在过去的几十年里，方位向地震各向异性测量被广泛地用于研究岩石圈过去和现在的变形以及表征地幔中的对流。在大陆地区之下，由于测量的深度限制差以及缺乏区域尺度的地球动力学模型，区分各向异性的浅源和深源仍然很困难。在这里，我们限制了马达加斯加地下地震各向异性的来源，在那里，复杂的模式不能用绝对板块运动、全球地幔流动或地质等单一过程来解释。我们验证了这样的假设，即边缘驱动对流(EDC)或来自地幔风场与岩石圈地形相互作用的地幔流动是马达加斯加下方各向异性的主要来源。因此，我们用区域尺度的三维计算模式模拟了两套地幔对流模式。然后，我们计算了沿地幔流动模型路径发展的晶格择优取向，并用它们来计算合成分裂参数。预测的地震各向异性与观测的地震各向异性的比较表明，在马达加斯加北部和南部，EDC模型很好地符合，但地幔风场只在马达加斯加北部很好地符合。这一结果表明，测量到的各向异性的主要控制可能来自EDC，但不能排除马达加斯加南部狭窄剪切带中局部化石各向异性的作用。我们的结果表明，马达加斯加北部和南部的软流圈以位错蠕变为主。在大陆岩石圈其他区域之下的软流圈上部，位错蠕变流变性可能占主导地位。
Over the past few decades, azimuthal seismic anisotropy measurements have been widely used proxy to study past and present‐day deformation of the lithosphere and to characterize convection in the mantle. Beneath continental regions, distinguishing between shallow and deep sources of anisotropy remains difficult due to poor depth constraints of measurements and a lack of regional‐scale geodynamic modeling. Here, we constrain the sources of seismic anisotropy beneath Madagascar where a complex pattern cannot be explained by a single process such as absolute plate motion, global mantle flow, or geology. We test the hypotheses that either Edge‐Driven Convection (EDC) or mantle flow derived from mantle wind interactions with lithospheric topography is the dominant source of anisotropy beneath Madagascar. We, therefore, simulate two sets of mantle convection models using regional‐scale 3‐D computational modeling. We then calculate Lattice Preferred Orientation that develops along pathlines of the mantle flow models and use them to calculate synthetic splitting parameters. Comparison of predicted with observed seismic anisotropy shows a good fit in northern and southern Madagascar for the EDC model, but the mantle wind case only fits well in northern Madagascar. This result suggests the dominant control of the measured anisotropy may be from EDC, but the role of localized fossil anisotropy in narrow shear zones cannot be ruled out in southern Madagascar. Our results suggest that the asthenosphere beneath northern and southern Madagascar is dominated by dislocation creep. Dislocation creep rheology may be dominant in the upper asthenosphere beneath other regions of continental lithosphere.