Localized Anisotropy in the Mantle Transition Zone Due to Flow Through Slab Gaps

Measurement of anisotropy advances our understanding of mantle dynamics by linking remote seismic observations to local deformation state through constraints from mineral physics. The Pacific Northwest records the largest depth‐integrated anisotropic signals across the western United States but the depths contributing to the total signal are unclear. We used the amplitudes of orthogonally polarized P‐to‐S converted phases from the mantle transition zone boundaries to identify anisotropy within the ∼400–700 km deep layer. Significant anisotropy is found near slab gaps imaged by prior tomography. Focusing of mantle flow through slab gaps may lead to locally elevated stress that enhances lattice preferred orientation of anisotropic minerals within the transition zone, such as wadsleyite.

各向异性的测量通过矿物物理的约束将远程地震观测与局部变形状态联系起来，从而增进了我们对地幔动力学的理解。美国太平洋西北地区记录了美国西部最大的深度积分各向异性信号，但对总信号有贡献的深度尚不清楚。我们利用来自地幔过渡带边界的正交偏振P波到S波转换震相的振幅来识别约400 - 700千米深的地层内的各向异性。在先前层析成像所显示的板块间隙附近发现了显著的各向异性。地幔流通过板块间隙的汇聚可能导致局部应力升高，从而增强过渡带内各向异性矿物（如瓦兹利石）的晶格择优取向。