McNamara Allen K, Zhong Shijie
Department of Physics, University of Colorado, Boulder, Colorado 80309-0390, USA.
Nature. 2005 Oct 20;437(7062):1136-9. doi: 10.1038/nature04066.
Large low-velocity seismic anomalies have been detected in the Earth's lower mantle beneath Africa and the Pacific Ocean that are not easily explained by temperature variations alone. The African anomaly has been interpreted to be a northwest-southeast-trending structure with a sharp-edged linear, ridge-like morphology. The Pacific anomaly, on the other hand, appears to be more rounded in shape. Mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes. It has not been shown, however, that such models can lead to thermochemical structures that satisfy the geometrical constraints, as inferred from seismological observations. Here we present numerical models of thermochemical convection in a three-dimensional spherical geometry using plate velocities inferred for the past 119 million years. We show that Earth's subduction history can lead to thermochemical structures similar in shape to the observed large, lower-mantle velocity anomalies. We find that subduction history tends to focus dense material into a ridge-like pile beneath Africa and a relatively more-rounded pile under the Pacific Ocean, consistent with seismic observations.
在非洲和太平洋下方的地球下地幔中检测到了大型低速地震异常，仅靠温度变化很难解释这些异常。非洲的异常被解释为一个呈西北 - 东南走向的结构，具有边缘清晰的线性、脊状形态。另一方面，太平洋的异常似乎形状更圆润。具有非均质成分的地幔模型已将这些结构与致密的热化学堆或超级地幔柱联系起来。然而，尚未证明这样的模型能够产生满足从地震学观测推断出的几何约束的热化学结构。在这里，我们使用过去1.19亿年推断出的板块速度，展示了三维球形几何中热化学对流的数值模型。我们表明，地球的俯冲历史可以导致形成与观测到的大型下地幔速度异常形状相似的热化学结构。我们发现，俯冲历史倾向于将致密物质集中到非洲下方的一个脊状堆以及太平洋下方一个相对更圆润的堆中，这与地震观测结果一致。
