Organized melt, seismic anisotropy, and plate boundary lubrication
Organized melt, seismic anisotropy, and plate boundary lubrication
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DOI：
10.1029/2010gc003296
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发表时间：
2010-12-29
影响因子：
3.5
通讯作者：
Kendall, J. -Michael
中科院分区：
文献类型：
--
作者：
Holtzman, Benjamin K.;Kendall, J. -Michael
基于在实验室和地球上的观测，我们提出了板块边界由富含熔体的剪切带网络润滑的假设。这种润滑作用会降低有效强度，并使变形集中在板块边界。这一观点来自两组观测结果：（1）实验变形地幔岩石中应力驱动的熔体分离和组织，以及（2）在三个不同板块边界（埃塞俄比亚裂谷、雷克雅内斯海脊和东太平洋海隆）观测到的地震各向异性模式。在所有三种构造背景下，各向异性的大小在板块边界内岩石圈-软流圈边界（“边缘LAB”）的可能位置处最大。上地幔的地震各向异性受优势橄榄石的晶格择优取向和熔体结构的排列控制。所观察到的各向异性图案由边缘LAB的倾角控制。在陡倾角情况下，垂直行波（如横波）中的横波分裂。例如，在一个实施例中，SKS相）对熔体的排列最为敏感，表面波应显示比Love波速度更快的Rayleigh波速度（V-SV > V-SH）。在浅倾角条件下，体波的横波分裂受橄榄石LPO控制，面波的Love波速度大于Rayleigh波速度（V-SV < V-SH）。在应变率最高的地方，由应力驱动的偏析形成的富熔体网络应该是最有效的。这些熔体润滑的剪切区将降低有效粘度相对于直接外推的粘度值来自实验室蠕变实验上均匀的样品。各向异性的地震性能的复合模型的开发测试的假设，熔体隔离沿着的LAB，将橄榄石织物与定向和隔离熔体在一系列的长度尺度。这个模型被应用于从三个例子板块边界的观测，让读者推测在其他地球动力学设置的各向异性模式的解释的影响。
Based on observations in both the laboratory and the Earth, we develop the hypothesis that plate boundaries are lubricated by networks of melt-rich shear zones. Such lubrication would serve to reduce effective strength and focus deformation at plates boundaries. This idea emerges from two sets of observations: (1) stress-driven melt segregation and organization in experimentally deformed mantle rocks and (2) seismic anisotropy patterns as observed at three divergent plate boundaries (the Ethiopian Rift, the Reykjanes Ridge, and the East Pacific Rise). In all three tectonic settings, the magnitude of anisotropy is greatest at the probable locations of the lithosphere-asthenosphere boundary within the plate boundary ("marginal LAB"). Seismic anisotropy in the upper mantle is controlled by the lattice preferred orientation (LPO) of predominant olivine and the alignment of melt structures. The observed patterns of anisotropy are controlled by the dip angle of the marginal LAB. When steeply dipping, shear wave splitting in vertically traveling waves (e. g., SKS phases) is most sensitive to the alignment of melt, and surface waves should reveal faster Rayleigh wave velocities than Love wave velocities (V-SV > V-SH). When shallowly dipping, shear wave splitting in vertically traveling body waves is controlled by olivine LPO, and surface waves show faster Love wave velocities than Rayleigh wave velocities (V-SV < V-SH). The formation of melt-rich networks by stress-driven segregation should be most effective where strain rates are highest. These melt-lubricated shear zones will reduce effective viscosity relative to the direct extrapolation of viscosity values derived from laboratory creep experiments on homogenous samples. A composite model of anisotropic seismic properties is developed to test the hypothesis that melt segregates along the LAB, incorporating olivine fabrics with oriented and segregated melt over a range of length scales. This model is applied to observations from the three example plate boundaries, leaving the reader to speculate on the implications for interpretation of anisotropy patterns at other geodynamic settings.