Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland.
Department of Earth Sciences, University of Oxford, OX1 3AN Oxford, UK.
Nat Commun. 2017 May 15;8:15241. doi: 10.1038/ncomms15241.
Advances in our understanding of Earth's thermal evolution and the style of mantle convection rely on robust seismological constraints on lateral variations of density. The large-low-shear-wave velocity provinces (LLSVPs) atop the core-mantle boundary beneath Africa and the Pacific are the largest structures in the lower mantle, and hence severely affect the convective flow. Here, we show that anomalous splitting of Stoneley modes, a unique class of free oscillations that are perturbed primarily by velocity and density variations at the core-mantle boundary, is explained best when the overall density of the LLSVPs is lower than the surrounding mantle. The resolved density variations can be explained by the presence of post-perovskite, chemical heterogeneity or a combination of the two. Although we cannot rule out the presence of a ∼100-km-thick denser-than-average basal structure, our results support the hypothesis that LLSVPs signify large-scale mantle upwelling in two antipodal regions of the mantle.
我们对地球热演化和地幔对流方式的理解的进展依赖于对密度横向变化的稳健地震学约束。在非洲和太平洋下方地核-地幔边界之上的大型低速剪切波速度区（LLSVPs）是下地幔中最大的结构，因此严重影响了对流流动。在这里，我们表明，当 LLSVPs 的整体密度低于周围地幔时，斯通利波（Stoneley wave）的异常分裂，即一类独特的自由振荡，主要受到地核-地幔边界处速度和密度变化的干扰，解释得最好。可分辨的密度变化可以用后钙钛矿、化学非均质性或两者的组合来解释。尽管我们不能排除存在一个约 100 公里厚的比平均密度大的基底结构，但我们的结果支持这样的假设，即 LLSVPs 标志着地幔在两个对跖区的大规模上升。
