Constraints on upper mantle viscosity from the flow‐induced pressure gradient across the Australian continental keel
Constraints on upper mantle viscosity from the flow‐induced pressure gradient across the Australian continental keel
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澳大利亚大陆龙骨上流动引起的压力梯度对上地幔粘度的限制
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
F. Simons
中科院分区：
文献类型：
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作者：
C. Harig;S. Zhong;F. Simons
大陆岩石圈的厚度从构造活动区到地壳运动区变化很大，厚度可达250-300 km。大陆龙骨像船一样嵌入上地幔，当被表面施加的速度驱动移动时，预计会在地幔中引起压力梯度。我们假设，软流圈的粘度或低岩石圈和软流圈之间的剪切耦合应控制这种压力效应，从而产生的动态地形。我们进行了三维有限元计算，以澳大利亚地区为例，研究了通过施加表面速度迫使大陆龙骨的影响。当上地幔较强但仍弱于下地幔时，在龙骨前缘附近产生正的动力地形，在龙骨后缘附近产生负的动力地形，分别用正大地水准面异常和负大地水准面异常来测量。对于一个软弱的上地幔，这种影响要小得多。我们分析大地水准面和重力异常在澳大利亚地区的空间光谱定位使用Slepian函数。该方法使我们能够删除一个最佳的拟合估计的地理本地化的低球谐度的贡献。由此过滤的区域大地水准面异常在整个澳大利亚大陆上大约为±10 m，其空间模式与模型预测的相似。模拟和观测大地水准面异常的比较对地幔粘性结构产生了约束。具有双层地幔的模型不能充分约束上地幔和下地幔之间的粘度比。第三层较弱的上地幔层--软流圈的加入，放大了龙骨的影响。我们的三层模型，下地幔粘度为3 × 1022 Pa·s，表明上地幔（软流圈）比下地幔弱300倍，而过渡带（400-670 km深处）的粘度在1021和1022 Pa·s之间变化。
The thickness of continental lithosphere varies considerably from tectonically active to cratonic regions, where it can be as thick as 250–300 km. Embedded in the upper mantle like a ship, when driven to move by a velocity imposed at the surface, a continental keel is expected to induce a pressure gradient in the mantle. We hypothesize that the viscosity of the asthenosphere or the shear coupling between lower lithosphere and asthenosphere should control this pressure effect and thus the resulting dynamic topography. We perform three‐dimensional finite element calculations to examine the effects of forcing a continental keel by an imposed surface velocity, with the Australian region as a case study. When the upper mantle is strong but still weaker than the lower mantle, positive dynamic topography is created around the leading edge, and negative dynamic topography is created around the trailing edge of the keel, which is measurable by positive and negative geoid anomalies, respectively. For a weak upper mantle the effect is much reduced. We analyze geoidal and gravity anomalies in the Australian region by spatiospectral localization using Slepian functions. The method allows us to remove a best fit estimate of the geographically localized low spherical harmonic degree contributions. Regional geoid anomalies thus filtered are on the order of ±10 m across the Australian continent, with a spatial pattern similar to that predicted by the models. The comparison of modeled and observed geoid anomalies places constraints on mantle viscosity structure. Models with a two‐layer mantle cannot sufficiently constrain the ratio of viscosity between the upper and lower mantle. The addition of a third, weak, upper mantle layer, an asthenosphere, amplifies the effects of keels. Our three‐layer models, with lower mantle viscosity of 3 × 1022 Pa s, suggest that the upper mantle (asthenosphere) is 300 times weaker than the lower mantle, while the transition zone (400–670 km depths) has a viscosity varying between 1021 and 1022 Pa s.