Effect of lateral viscosity variations in the core-mantle boundary region on predictions of the long-wavelength geoid
Effect of lateral viscosity variations in the core-mantle boundary region on predictions of the long-wavelength geoid
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DOI：
10.1007/s11200-006-0013-0
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发表时间：
2006-04-01
影响因子：
0.9
通讯作者：
Fleitout, L.
中科院分区：
文献类型：
--
作者：
Cadek, O.;Fleitout, L.
对最低地幔的地震研究表明，核幔边界(CMB)区域在局部和全球尺度上都具有强烈的横向非均质性。这些非均质性很可能与显著的横向粘度变化有关，这种变化可能会影响长波非静力大地水准面的形状。在本文中，我们研究了这些横向粘性变化对从大地水准面推断粘性这一反问题解的影响。我们发现，CMB区存在的横向粘性变化可以显著提高预报数据与观测值的拟合度(在自由空气重力的情况下，从42%提高到70%)，而地幔粘性模型的基本特征，即粘性随深度和分层速率的增加，与最佳拟合径向对称粘性模型的情况基本相同。假定粘性在CMB区是横向相关的，在其他地区是径向相关的，我们确定了最低地幔粘性结构的大尺度特征。CMB区域的粘度模式显示，在粘度高于平均粘度的区域上方有高密度的热点。这一结果表明，最下部地幔的岩石学不均一性可能与后钙钛矿相变有关。另一种可能的解释是，CMB区域的横向粘度变化实际上对应于CMB边界力学条件的横向变化，或对应于最下部地幔不同化学成分层的大范围波动。
Seismic studies of the lowermost mantle suggest that the core-mantle boundary (CMB) region is strongly laterally heterogeneous over both local and global scales. These heterogeneities are likely to be associated with significant lateral viscosity variations that may influence the shape of the long-wavelength non-hydrostatic geoid. In the present paper we investigate the effect of these lateral viscosity variations on the solution of the inverse problem known as the inferences of viscosity from the geoid. We find that the presence of lateral viscosity variations in the CMB region can significantly improve the percentage fit of the predicted data with observations (from 42 to 70% in case of free-air gravity) while the basic characterisics of the mantle viscosity model, namely the viscosity increase with depth and the rate of layering, remain more or less the same as in the case of the best-fitting radially symmetric viscosity models. Assuming that viscosity is laterally dependent in the CMB region, and radially dependent elsewhere, we determine the largescale features of the viscosity structure in the lowermost mantle. The viscosity pattern found for the CMB region shows a high density of hotspots above the regions of higher-than-average viscosity. This result suggests an important role for petrological heterogeneities in the lowermost mantle, potentially associated with a post-perovskite phase transition. Another potential interpretation is that the lateral viscosity variations derived for the CMB region correspond in reality to lateral variations in the mechanical conditions at the CMB boundary or to large-scale undulations of a chemically distinct layer at the lowermost mantle.