The Earth's long‐ and intermediate‐wavelength geoid anomalies are surface expressions of mantle convection and are sensitive to mantle viscosity. While previous studies of the geoid provide important constraints on the mantle radial viscosity variations, the mantle buoyancy in these studies, as derived from either seismic tomography or slab density models, may suffer significant uncertainties. In this study, we formulate 3‐D spherical mantle convection models with plate motion history since the Cretaceous that generate dynamically self‐consistent mantle thermal and buoyancy structures, and for the first time, use the dynamically generated slab structures and the observed geoid to place important constraints on the mantle viscosity. We found that non‐uniform weak plate margins and strong plate interiors are critical in reproducing the observed geoid and surface plate motion, especially the net lithosphere rotation (i.e., degree‐1 toroidal plate motion). In the best‐fit model, which leads to correlation of 0.61 between the modeled and observed geoid at degrees 4–12, the lower mantle viscosity is ∼1.3–2.5 × 10²² Pa⋅s and is ∼30 and ∼600–1,000 times higher than that in the transition zone and asthenosphere, respectively. Slab structures and the geoid are also strongly affected by slab strength, and the observations prefer moderately strong slabs that are ∼10–100 times stronger than the ambient mantle. Finally, a thin weak layer below the 670‐km phase change on a regional scale only in subduction zones produces stagnant slabs in the mantle transition zone as effectively as a weak layer on a global scale.

中文翻译：

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具有板块运动历史的地幔对流模型中波长大地水准面异常对地幔粘度的约束

地球的长波和中波大地水准面异常是地幔对流的表面表现，并且对地幔粘度敏感。尽管先前对大地水准面的研究对地幔径向粘度变化提供了重要的约束条件，但这些研究中的地幔浮力可能来自地震层析成像或平板密度模型，但可能存在很大的不确定性。在这项研究中，我们自白垩纪以来就利用板块运动历史来公式化3D球形地幔对流模型，该模型生成动态自洽的地幔热力和浮力结构，并且首次使用动态生成的平板结构和观测到的大地水准面来进行定位地幔粘度的重要制约因素。我们发现，不均匀的弱板边缘和强内部板块对于再现观测到的大地水准面和表面板块运动，特别是净岩石圈自转（即1度环面板块运动）至关重要。在最佳拟合模型中，建模和观察到的大地水准面在4–12度之间的相关性为0.61，下地幔粘度为〜1.3–2.5×10²²  Pa·s，分别比过渡带和软流圈高30〜600倍和1000〜1000倍。平板结构和大地水准面也受到平板强度的强烈影响，观测结果偏爱中等强度的平板，其强度是周围地幔的10至100倍。最后，仅在俯冲带中，在670 km相变以下的薄弱层才在区域尺度上产生，而在地幔过渡带则产生停滞平板，就像在全球范围内薄弱层一样有效。