Subduction-controlled mantle flow and seismic anisotropy in South America
Subduction-controlled mantle flow and seismic anisotropy in South America
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DOI：
10.1016/j.epsl.2017.04.027
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发表时间：
2017-07
影响因子：
5.3
通讯作者：
Jiashun Hu;M. Faccenda;Lijun Liu
中科院分区：
文献类型：
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作者：
Jiashun Hu;M. Faccenda;Lijun Liu
地震各向异性记录了固体地球内部过去和现在的变形。在地幔中，地震各向异性主要归因于地幔流剪切变形引起的矿物组构的晶格择优取向（LPO）。然而，不同的构造过程的贡献仍然存在争议，一个单一的地球动力学模型，同时解释所观察到的地幔结构和各种地震各向异性测量仍然缺乏。在这里，我们提出了一个模型的新生代俯冲历史在南美洲使用的地球动力学模拟约束过去的板块重建和目前的地幔地震结构。最近开发的软件包DRexS，我们进一步预测方位地震各向异性在不同的深度，并产生合成剪切波分裂（SWS）测量使用由此产生的地幔流。我们的研究结果提供了一个很好的匹配深度依赖的面波各向异性和各种陆基SWS记录。我们发现，在南美洲的地震各向异性的主要控制来自俯冲引起的地幔流，在俯冲纳斯卡板块下面的各向异性与板块运动引起的Couette流对齐，下面的覆盖南美板块遵循板引起的Poietille流。这种大规模的地幔流可以被次级板块转向，如安的列斯俯冲带下面的板块。与此相反，化石大陆各向异性和岩石圈厚度变化对地幔流调制的影响对SWS的贡献较小。南大西洋边缘下的上地幔快震异常应具有接近中性的浮力，以满足观测到的地震各向异性。
Seismic anisotropy records both the past and present deformation inside the solid Earth. In the mantle, seismic anisotropy is mainly attributed to the lattice preferred orientation (LPO) of mineral fabrics, caused by the shear deformation due to mantle flow. However, contributions from different tectonic processes remain debated, and a single geodynamic model that simultaneously explains the observed mantle structures and various seismic anisotropy measurements is still lacking. Here, we present a model for the Cenozoic subduction history in South America using a geodynamic simulation constrained by both past plate reconstructions and present mantle seismic structures. With a recently developed software package DRexS, we further predict azimuthal seismic anisotropy at different depths and generate synthetic shear wave splitting (SWS) measurements using the resulting mantle flow. Our results provide a good match to both depth-dependent surface wave anisotropy and various land-based SWS records. We find that the dominant control on seismic anisotropy in South America comes from subduction-induced mantle flow, where anisotropy below the subducting Nazca Plate aligns with plate-motion-induced Couette flow and that below the overriding South American Plate follows slab-induced Poiseuille flow. This large-scale mantle flow can be diverted by secondary slabs, such as that below the Antilles subduction zone. In contrast, the contribution to SWS from fossil continental anisotropy and from the effects due to mantle flow modulation by lithosphere thickness variation are minor. Upper-mantle fast seismic anomalies beneath the southern Atlantic margin should have close-to-neutral buoyancy in order to satisfy the observed seismic anisotropy.