Alpine Fault‐Related Microstructures and Anisotropy of the Mantle Beneath the Southern Alps, New Zealand
Alpine Fault‐Related Microstructures and Anisotropy of the Mantle Beneath the Southern Alps, New Zealand
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DOI：
10.1029/2022jb024950
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发表时间：
2022-11
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影响因子：
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通讯作者：
Yilun Shao;D. Prior;J. Scott;S. Kidder;M. Negrini
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作者：
Yilun Shao;D. Prior;J. Scott;S. Kidder;M. Negrini
来自新西兰南阿尔卑斯山的地幔捕虏体提供了与澳大利亚-太平洋板块边界相关的地幔地震各向异性起源的见解。阿尔卑斯断层横向距离100 km以内的捕虏体大多为粗粒捕虏体，少数为细粒原糜棱岩。原糜棱岩含有与同一捕虏体中的粗碎斑岩不同的晶体学择优取向（CPO）的细颗粒连接网络，表明原糜棱岩和粗颗粒样品记录了不同的变形运动学。原糜棱岩中细粒的CPO具有单斜对称性，其2重旋转轴垂直于包含橄榄石[010]和斜方辉石[100]极大值的平面，表明原糜棱岩变形涉及显著的简单剪切。一些粗颗粒样品包含不连接的透镜和细颗粒层，其具有与粗颗粒相同的CPO。显微结构表明，这些细颗粒是由亚颗粒旋转重结晶形成的，原糜棱岩可能代表了这种显微结构的向上应变进程，其中细颗粒的连通性使它们能够局部剪切并形成新的阿尔卑斯断层CPO。这些样品告诉我们25 Ma时的地幔状态，也就是板块边界的早期历史。如果这套样品代表了现今阿尔卑斯断层下的地幔，那么我们可以将岩石圈地幔中复杂的地震各向异性模式解释为包含与阿尔卑斯断层变形相关的狭窄剪切带并列的旋转较老的CPO的块体的代表。
Mantle xenoliths from the Southern Alps, New Zealand, provide insight into the origin of mantle seismic anisotropy related to the Australian‐Pacific plate boundary. Most xenoliths from within 100 km lateral distance of the Alpine Fault are coarse grained, but a small number are finer grained protomylonites. The protomylonites contain connected networks of fine grains with a different crystallographic preferred orientation (CPO) to coarse porphyroclasts in the same xenolith, suggesting that protomylonites and coarse‐grained samples record different deformation kinematics. The CPOs of fine grains in protomylonites have monoclinic symmetry, with the 2‐fold rotation axis normal to a plane that contains olivine [010] and orthopyroxene [100] maxima, suggesting that the protomylonite deformation involved significant simple shear. Some coarse‐grained samples contain unconnected lenses and layers of fine grains with the same CPO as the coarse grains. Microstructures suggest that these fine grains formed by subgrain rotation recrystallization and that protomylonites may represent an up‐strain progression of this microstructure, where the connectivity of fine grains has allowed them to localize shear and develop a new Alpine Fault CPO. The samples tell us about the state of the mantle at 25 Ma, in the early history of the plate boundary. If this suite of samples is representative of the mantle beneath the Alpine Fault in the present day, then we can interpret the complex seismic anisotropy patterns in the lithospheric mantle as representative of blocks containing variably rotated older CPOs juxtaposed by narrow shear zones associated with Alpine Fault deformation.