Detection of upper mantle flow associated with the African Superplume
Detection of upper mantle flow associated with the African Superplume
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DOI：
10.1016/j.epsl.2004.05.026
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发表时间：
2004-08-15
影响因子：
5.3
通讯作者：
Silver, PG
中科院分区：
文献类型：
--
作者：
Behn, MD;Conrad, CP;Silver, PG
非洲中下地幔的大陆尺度低地震速度异常是全球层析模型的一个重要特征。假设低速度与温暖、密度较低的物质有关，非洲地震异常被归因于来自下地幔的长期热上升流。如此大规模的上升流也会影响上地幔区域水平流场。为了验证这一模型，我们比较了从剪切波分裂测量中推断出的地震各向异性与从地震层析成像中推断出的包含地幔密度结构的瞬时流动计算。我们计算了非洲周边13个海洋岛屿台站的分裂参数。从岛屿台站分离的测量对于解释软流圈流引起的各向异性是理想的，因为它们没有厚厚的覆盖岩石圈，这也可能导致观测到的各向异性。我们通过比较分裂测量结果和化石扩散方向来测试岩石圈可能的贡献。我们发现，尽管化石岩石圈结构与距离脊轴< 500 km的观测站观测到的快速极化方向非常吻合，但它们与距离脊轴更远的观测站的数据拟合度较差。因此，我们得出结论，在远离脊轴的地方，观测到的各向异性主要是软流层流。为了测试地幔上升流的活跃成分，我们考虑了几种模型，这些模型对软流层底部的速度有不同的假设：它是(1)静止在无净旋转（NNR）或热点参考框架中移动的板块下方，(2)由地球表面的板块运动驱动，或(3)由地震层析成像或俯冲历史推断的板块运动和地幔密度非均匀性的组合驱动。我们发现最适合的流场是由起源于南部非洲下地幔的大规模上升流的板块运动和密度非均质性产生的，并表现为软流圈底部的径向流模式。该模型与观测到的各向异性的拟合程度明显优于地幔流是由俯冲的被动响应驱动的模型。所得的亚软流圈流场估计速度为0-3 cm/年，软流圈黏度约为3.10(19)Pa(.)s，与区域各向异性、大地水准面高度和动力地形最为一致。(C) 2004 Elsevier b.v.版权所有
A continental-scale, low seismic velocity anomaly in the mid to lower mantle beneath Africa is a robust feature of global tomographic models. Assuming the low velocities are associated with warm, less dense material, the African seismic anomaly has been ascribed to a long-lived thermal upwelling from the lower mantle. Such a large-scale upwelling should also affect the regional horizontal flow field in the upper mantle. To test this model, we compare seismic anisotropy inferred from shear-wave splitting measurements with instantaneous flow calculations that incorporate mantle density structure inferred from seismic tomography. We calculate splitting parameters at 13 ocean island stations surrounding Africa. Splitting measurements from island stations are ideal for interpreting anisotropy induced by asthenospheric flow because they lack a thick overlying lithosphere that may also contribute to the observed anisotropy. We tested for a possible lithospheric contribution by comparing the splitting measurements with the fossil spreading directions. We find that although the fossil lithospheric fabric closely matches the observed fast polarization directions at stations < 500 km from a ridge axis, they are a poor fit to the data at stations located farther off-axis. Thus, we conclude that far from a ridge axis, the observed anisotropy is dominated by asthenospheric flow. To test for an active component of mantle upwelling, we considered several models with varying assumptions about the velocity at the base of the asthenosphere: that it is (1) stationary below plates moving in the no-net-rotation (NNR) or hotspot reference frames, (2) driven by plate motions at the Earth's surface, or (3) driven by a combination of plate-motion and mantle density heterogeneity inferred from either seismic tomography or the history of subduction. We find that the best-fitting flow field is generated by plate motions and density heterogeneity associated with large-scale upwelling originating in the lower mantle beneath southern Africa and is manifest as a radial pattern of flow at the base of the asthenosphere. This model provides a significantly better fit to the observed anisotropy than a model in which mantle flow is driven through a passive response to subduction. The resulting sub-asthenospheric flow field is estimated to have velocities of 0-3 cm/year, and an asthenospheric viscosity of similar to 3.10(19) Pa(.)s is found to be most consistent with the regional anisotropy, geoid height, and dynamic topography. (C) 2004 Elsevier B.V All rights reserved.