Mantle heterogeneities, geoid, and plate motion: A Monte Carlo inversion
Mantle heterogeneities, geoid, and plate motion: A Monte Carlo inversion
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地幔异质性、大地水准面和板块运动：蒙特卡罗反演
DOI：
10.1029/jb094ib10p13739
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发表时间：
1989
影响因子：
--
通讯作者：
C. Froidevaux
中科院分区：
文献类型：
--
作者：
Y. Ricard;C. Vigny;C. Froidevaux
在上地幔和下地幔的地震层析成像，以及由地震活动定义的俯冲洋板块，已被转化为密度不均匀性，以产生地幔环流模型。这些模型可以预测地表速度和大地水准面，可以与板块构造和重力数据进行比较。一个给定的模型是由6个参数指定的3个地幔层的粘度和绝对振幅的密度变化在上，下地幔以及在板。在可接受的范围内随机选择这些参数的值。每个模型都要经过适当的测试，将观测结果与预测结果进行比较。显示了通过蒙特卡罗反演选择的最成功的模型的结果。它们产生优选的地幔粘度结构，在深度上表现出很大的变化。在上地幔和下地幔之间有一个物理界面，由于环流有可能穿透650公里的不连续面，两类粘度分布图十分突出。第一个是指岩石圈下地幔的粘度有规律地增加，密度参数具有合理的值。第二个是出乎意料的，因为它预测了上地幔的一个非常坚硬的底部。它还要求由层析成像确定的上地幔密度不均匀性的振幅非常小，因此必须是岩性而不是热成因。对于650 km处的化学界面，结果非常相似：同样的两类粘性结构确实产生了令人满意的大地水准面预测。然而，只有一类的模型与刚性层在中地幔深度预测可接受的表面速度。在60，000个经过测试的模型中，最好的模型只能解释1 - 6度球谐函数的三分之一的大地水准面和三分之二的地表发散。然而，这两个观察到的模式的主要特征存在于计算的地图中，6个相关系数中有4个接近90%的置信水平。这对于大地水准面和位移速度的表面发散都是正确的。然而，由于内部粘度结构已被假定为具有球对称性，表面速度的旋转分量无法预测。
Seismic tomography in both the upper and the lower mantle, as well as subducting oceanic slabs defined by seismicity, has been translated into density heterogeneities to generate models of mantle circulation. These models can predict both the surface velocities and the geoid, which can be compared with plate tectonics and gravity data. A given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs. The values of these parameters are chosen at random within an acceptable range. Each model is submitted to an appropriate test comparing observations with predictions. The results of the most successful models selected by this Monte Carlo inversion are displayed. They yield preferred mantle viscosity structures exhibiting large variations at depth. With a physical interface between upper and lower mantle, i.e., with the possibility for the circulation to penetrate the 650 km discontinuity, two classes of viscosity profiles stand out. The first one implies a regular increase of the viscosity in the sublithospheric mantle, with reasonable values for the density parameters. The second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle. It also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography, which would thus have to be of lithological rather than thermal origin. With a chemical interface at 650 km the outcome is very similar: the same two classes of viscosity structures do yield a satisfactory geoid prediction. However only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities. Altogether the best models out of some 60,000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6. Nevertheless the main features of these two observed patterns are present in the computed maps, and 4 out of 6 correlation coefficients lie close to the 90% confidence level. This is true for the geoid as well as for the surface divergence of the displacement velocity. However, as the internal viscosity structure has been assumed to have spherical symmetry, the rotational component of the surface velocities cannot be predicted.