Seismic anisotropy and mantle flow below subducting slabs
Seismic anisotropy and mantle flow below subducting slabs
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DOI：
10.1016/j.epsl.2017.02.023
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发表时间：
2017-05
影响因子：
5.3
通讯作者：
J. Walpole;J. Wookey;J. Kendall;T. Masters
中科院分区：
文献类型：
--
作者：
J. Walpole;J. Wookey;J. Kendall;T. Masters
俯冲作用是地幔对流和板块构造的组成部分，但对板块下地幔在俯冲过程中的作用知之甚少。一些人提出，与板块脱钩允许在次板块地幔中广泛的沟槽平行流动，尽管这在地球动力学上的可行性受到了质疑。在这里，我们使用震源侧横波分裂技术探测俯冲板块下方的各向异性，使我们能够测试岩石组构模型并约束地幔FOW的几何形状。我们的全球数据集包含6,369个高质量测量数据--横跨∼4万公里的俯冲带海沟--覆盖整个可用震源深度范围(4至687公里)--以及板片参考框架中的大范围角度。我们发现，亚板片地幔的各向异性以倾斜的横向各向同性为特征，慢对称轴垂直于板片平面。这似乎与纯粹的沟槽平行流模型不相容。另一方面，它与软流圈在俯冲过程中倾斜并被卷入的观点是一致的。海沟平行测量最常与浅层事件(震源深度50千米)有关，这表明岩石圈板块中存在一个单独的各向异性区域。这可能对应于板材弯曲打开的裂缝、裂缝和断层的形状择优取向。同时，最深的事件探测了上、下地幔，发现那里的裂解与变形的桥辉橄榄岩一致。
Subduction is integral to mantle convection and plate tectonics, yet the role of the subslab mantle in this process is poorly understood. Some propose that decoupling from the slab permits widespread trench parallel flow in the subslab mantle, although the geodynamical feasibility of this has been questioned. Here, we use the source-side shear wave splitting technique to probe anisotropy beneath subducting slabs, enabling us to test petrofabric models and constrain the geometry of mantle fow. Our global dataset contains 6369 high quality measurements–spanning∼ 40, 000 km of subduction zone trenches–over the complete range of available source depths (4 to 687 km)–and a large range of angles in the slab reference frame. We find that anisotropy in the subslab mantle is well characterised by tilted transverse isotropy with a slow-symmetry-axis pointing normal to the plane of the slab. This appears incompatible with purely trench-parallel flow models. On the other hand it is compatible with the idea that the asthenosphere is tilted and entrained during subduction. Trench parallel measurements are most commonly associated with shallow events (source depth< 50 km)–suggesting a separate region of anisotropy in the lithospheric slab. This may correspond to the shape preferred orientation of cracks, fractures, and faults opened by slab bending. Meanwhile the deepest events probe the upper lower mantle where splitting is found to be consistent with deformed bridgmanite.