Understanding the deformation microstructures and seismic properties of blueschists is important for interpreting the seismic anisotropy at the top of subducting oceanic crust. The deformation microstructures and seismic properties of epidote blueschist, epidote-lawsonite blueschist, and lawsonite blueschist from Alpine Corsica, France, were studied using electron backscatter diffraction mapping and transmission electron microscopy. The crystal-preferred orientations (CPOs) of glaucophane show a strong maximum of the [001] axes aligned sub-parallel to the lineation. The maximum of the (010) poles of epidote is aligned sub-parallel to the lineation. The [001] axes of lawsonite are strongly aligned sub-normal to the foliation. Intracrystalline misorientation, dislocation, and occasional fragmentation of glaucophane indicate that glaucophane CPO was developed mainly by dislocation creep. The intracrystalline deformation features of epidote, such as subgrain boundaries and dislocations, indicate that the CPO of the epidote was formed by dislocation creep. In contrast, our data indicate that lawsonite CPO was developed by a combination of rigid-body rotation and dislocation creep with a minor effect of twinning. The P-wave anisotropy (AVp) and maximum S-wave anisotropy (max.AVs) of epidote blueschists (AVp = 12.3–16.9% and max.AVs = 6.53–11.75%) are higher than those of lawsonite blueschists. The seismic velocities of the blueschists are lower than those of mantle peridotites. The coexistence of epidote and lawsonite in the blueschist results in variations in seismic anisotropy and P-wave velocity in the whole rock, depending on the modal content of epidote and lawsonite. A high content ratio of epidote to lawsonite in blueschist produces strong P- and S-wave anisotropies of blueschist when the glaucophane content is more than ∼50%. The P-wave velocity of blueschist decreases proportionally with increasing epidote content. Our results indicate that the seismic properties of deformed blueschists contribute to the seismic anisotropy and the low-velocity layer in subducting oceanic crusts in subduction zones.

中文翻译：

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法国科西嘉高山蓝片岩的变形微观结构及其对地震各向异性和俯冲洋壳低速层的影响

了解蓝片岩的变形微观结构和地震特性对于解释俯冲洋壳顶部的地震各向异性具有重要意义。利用电子背散射衍射图和透射电子显微镜研究了法国科西嘉岛高山绿帘石蓝片岩、绿帘石-硬钠长石蓝片岩和硬钠石蓝片岩的变形微观结构和地震特性。蓝绿闪石的晶体择优取向 (CPO) 显示出与线状排列近平行的 [001] 轴的强最大值。绿帘石 (010) 极的最大值与线纹近平行排列。硬铝石的[001]轴与叶理的次法线强烈对齐。蓝闪石的晶内取向错误、位错和偶尔的碎裂表明蓝闪石CPO主要是通过位错蠕变形成的。绿帘石的晶内变形特征，如亚晶界和位错，表明绿帘石的CPO是由位错蠕变形成的。相比之下，我们的数据表明，硬钠石 CPO 是通过刚体旋转和位错蠕变的组合而形成的，孪生效应较小。绿帘石蓝片岩的 P 波各向异性 (AVp) 和最大 S 波各向异性 (max.AVs) (AVp = 12.3–16.9% 和 max.AVs = 6.53–11.75%) 高于硬钠铝石蓝片岩。蓝片岩的地震速度低于地幔橄榄岩的地震速度。蓝片岩中绿帘石和粗钠长石的共存导致整个岩石的地震各向异性和纵波速度发生变化，具体取决于绿帘石和粗钠长石的模态含量。当蓝闪石含量超过～50%时，蓝片岩中绿帘石与硬钠石的含量比较高，导致蓝片岩具有较强的纵波和横波各向异性。蓝片岩的纵波速度随着绿帘石含量的增加成比例地降低。我们的结果表明，变形蓝片岩的地震特性有助于俯冲带中俯冲洋壳的地震各向异性和低速层。