Subduction dynamics and structural controls on shear wave splitting along the South American convergent margin
Subduction dynamics and structural controls on shear wave splitting along the South American convergent margin
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DOI：
10.1016/j.jsames.2020.102824
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发表时间：
2020-12
影响因子：
1.8
通讯作者：
C. Lynner;S. Beck
中科院分区：
文献类型：
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作者：
C. Lynner;S. Beck
横波分裂的观测为各种构造环境下的地幔变形模式提供了约束，包括那些具有复杂动力学的环境，如俯冲带。南美洲俯冲系统的特征是目前存在的最长的横向连续俯冲板块。因此，它是一个极好的天然实验室，可以研究沿数千公里俯冲带的各向异性变化。我们在智利海岸的59个站点进行剪切波分裂分析，这些站点横跨2500多公里的俯冲纳斯卡板块。该数据集专门针对采样最少量加厚的南美地壳和地幔楔物质的台站，这些物质更容易将分裂归因于板块下地幔。我们观察到，从玻利维亚中部奥罗斜到智利南部的分裂过程发生了明显的转变，这表明俯冲板块下的地幔变形从奥罗斜下的辐合流向更南的海沟平行流转变。这支持了之前的地幔动力学概念模型，在该区域，玻利维亚奥罗斜下方存在一个滞止点，导致板块运动驱动的流动与海沟平行的南北逃逸流。我们的结果以及之前的测量结果在很大程度上与最近的一项数值模拟研究一致，该研究提出了类似的大规模变形模式以及由于板结构（如撕裂）引起的局部偏差。然而，模型预测与我们的观测结果之间的一致性并不是普遍存在的。我们看到有几个区域的测量分裂与预测有很大的偏差。这些领域值得进一步研究，并将有助于改进未来的地球动力学建模工作。
Observations of shear wave splitting have provided constraints on patterns of deformation in the mantle in a variety of tectonic settings, including those with complex dynamics such as subduction zones. The South American subduction system is characterized by the longest laterally continuous subducting slab present today. As such, it represents an excellent natural laboratory to study variability in anisotropy along thousands of kilometers of a subduction zone. We perform shear wave splitting analyses at 59 stations along the Chilean coast spanning over 2500 km of the subducting Nazca slab. This dataset specifically targets stations that sample minimal amounts of thickened South American crust and mantle wedge material making it easier to attribute splitting to the sub-slab mantle. We observe a stark transition in splitting from the central Bolivian Orocline to southern Chile indicative of a transition in mantle deformation beneath the subducting slab from convergent flow beneath the Orocline to trench parallel flow farther south. This supports previous conceptual models of mantle dynamics in the region where a stagnation point exists beneath the Bolivian Orocline resulting in plate motion driven flow with trench parallel escape flow to both the north and south. Our results as well as previous measurements largely agree with a recent numerical modeling study that has proposed similar large-scale deformation patterns as well as local deviations due to slab structures like tears. The agreement between model predictions and our observations, however, is not universal along strike. We see several regions where measured splitting deviates significantly from predictions. These areas warrant further study and will aid in refining future geodynamic modeling efforts.