Seismically anisotropic magma reservoirs underlying silicic calderas
Seismically anisotropic magma reservoirs underlying silicic calderas
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DOI：
10.1130/g45104.1
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发表时间：
2018-08-01
期刊：
影响因子：
5.8
通讯作者：
Ward, Kevin M.
中科院分区：
文献类型：
--
作者：
Jiang, Chengxin;Schmandt, Brandon;Ward, Kevin M.
地震各向异性可以照亮长度尺度太细而不能在大多数地震层析成像中分辨为明显特征的构造组构或分层。径向各向异性，检测水平（V-SH）和垂直（V-SV）偏振剪切波速度之间的差异，黄石破火山口（怀俄明州，美国）和长谷破火山口（加州）下进行了研究。显著的正径向各向异性，表明V-SH > V-SV和低各向同性速度，发现在两个破火山口下，在类似的5-18公里的深度。与周围地区相比，破火山口下方的正径向各向异性（>8%）体积非常强。在传播的黄石热点的尾迹中没有类似的各向异性信号，表明在大量的岩浆活动的轨迹移动后，径向各向异性减小。我们建议，各向异性的体积代表岩床复杂的成分演化的岩浆，和岩浆的地震与地壳的对比度将在很大程度上消失结晶。不同构造背景下的岩浆储层各向异性的相似性表明，这种中地壳岩槛复合体可能是火山口形成岩浆系统的普遍特征，应考虑各向异性，以地震学方法估计熔体含量和流动性。类似的径向各向异性的情况下，在下地壳下方的破火山口表明较低的熔体分数或过渡的几何形状的岩浆通道。
Seismic anisotropy can illuminate structural fabrics or layering with length scales too fine to be resolved as distinct features in most seismic tomography. Radial anisotropy, which detects differences between horizontally (V-SH) and vertically (V-SV) polarized shear wave velocities, was investigated beneath Yellowstone caldera (Wyoming, United States) and Long Valley caldera (California). Significant positive radial anisotropy indicating V-SH > V-SV and low isotropic velocities, were found beneath both calderas at similar to 5-18 km depths. The positive radial anisotropy (>8%) volumes beneath the calderas are anomalously strong compared to the surrounding areas. The absence of a similar anisotropic signal in the wake of the propagating Yellowstone hotspot indicates that the radial anisotropy diminishes after the locus of voluminous silicic magmatism moves. We propose that the anisotropic volumes represent sill complexes of compositionally evolved magma, and the magma's seismic contrast with the crust would largely fade upon crystallization. The similarity of magma reservoir anisotropy in varied tectonic settings suggests that such mid-crustal sill complexes may be ubiquitous features of silicic caldera-forming magmatic systems, and that anisotropy should be considered to seismically estimate melt content and mobility. The absence of similar radial anisotropy in the lower crust beneath the calderas suggests lower melt fractions or a transition in the geometry of magma pathways.