Serpentinization, Deformation, and Seismic Anisotropy in the Subduction Mantle Wedge
Serpentinization, Deformation, and Seismic Anisotropy in the Subduction Mantle Wedge
复制标题
DOI：
10.1029/2020gc008950
复制
发表时间：
2020-04
期刊：
影响因子：
3.7
通讯作者：
C. Horn;P. Bouilhol;P. Skemer
中科院分区：
文献类型：
--
作者：
C. Horn;P. Bouilhol;P. Skemer
叶蛇纹石是一种含水层状硅酸盐矿物，具有强烈的地震和流变各向异性。含水矿物，如叶蛇纹石，被用来解释俯冲带内的许多地质观测，包括中深地震，弧火山作用，俯冲界面的持续弱点，沟槽平行S波分裂，以及幕式震颤和滑动。为了了解含叶蛇纹石岩石的存在如何影响地震各向异性的观测，使用电子背散射衍射分析了来自巴基斯坦Kohistan古岛弧的三种糜棱岩。第四个样品，这显示晶体控制的橄榄石替代光学证据，也进行了研究，使用电子背散射衍射，以确定潜在的拓扑关系。所得数据用于模拟富含叶蛇纹石岩石的整体地震特性。糜棱岩样品表现出令人难以置信的强体各向异性（叶蛇纹石+橄榄石为10-20%）。在名义上未变形的原糜棱岩中，观察到两种拓扑关系：（1）（010）ant//（100）ol与[100]ant//[001]ol，（2）（010）ant//（100）ol与[100]ant//[010] ol。然而，由拓扑置换形成的结构的强度明显弱于在糜棱岩样品中观察到的结构的强度。由于叶蛇纹石被认为是流变性弱，我们假设，从拓扑异构反应形成的微观结构将逐步叠印的变形是本地化的区域中具有更大的百分比的蛇纹石。因此，高度剪切的蛇纹岩区域有可能强烈影响俯冲环境中的地震波速度。倾斜构造中变形叶蛇纹石的存在是对俯冲带地震波强度和分裂模式观测结果的一种解释。
Antigorite is a hydrous sheet silicate with strongly anisotropic seismic and rheological properties. Hydrous minerals such as antigorite have been invoked to explain numerous geologic observations within subduction zones including intermediate‐depth earthquakes, arc volcanism, the persistent weakness of the subduction interface, trench‐parallel S wave splitting, and episodic tremor and slip. To understand how the presence of antigorite‐bearing rocks affects observations of seismic anisotropy, three mylonites from the Kohistan palaeo‐island arc in Pakistan were analysed using electron backscatter diffraction. A fourth sample, which displayed optical evidence for crystallographically controlled replacements of olivine, was also investigated using electron backscatter diffraction to identify potential topotactic relationships. The resulting data were used to model the bulk seismic properties of antigorite‐rich rocks. The mylonitic samples exhibit incredibly strong bulk anisotropy (10–20% for the antigorite + olivine). Within the nominally undeformed protomylonite, two topotactic relationships were observed: (1) (010)ant//(100)ol with [100]ant//[001]ol and (2) (010)ant//(100)ol with [100]ant//[010]ol. However, the strength of a texture formed by topotactic replacement is markedly weaker than the strength of the textures observed in mylonitic samples. Since antigorite is thought to be rheologically weak, we hypothesise that microstructures formed from topotactic reactions will be progressively overprinted as deformation is localised in regions with greater percentages of serpentine. Regions of highly sheared serpentine, therefore, have the potential to strongly influence seismic wave speeds in subduction settings. The presence of deformed antigorite in a dipping structure is one explanation for observations of both the magnitude and splitting pattern of seismic waves in subduction zones.