The Sensitivity of Joint Inversions of Seismic and Geodynamic Data to Mantle Viscosity
The Sensitivity of Joint Inversions of Seismic and Geodynamic Data to Mantle Viscosity
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DOI：
10.1029/2019gc008648
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发表时间：
2020-04
期刊：
影响因子：
3.7
通讯作者：
Chang Lu;A. Forte;N. Simmons;S. Grand;M. Kajan;Hongyu Lai;E. Garnero
中科院分区：
文献类型：
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作者：
Chang Lu;A. Forte;N. Simmons;S. Grand;M. Kajan;Hongyu Lai;E. Garnero
地震层析成像揭示了地幔中大尺度速度非均质性的存在。用温度和化学成分来解释地震速度异常并不是唯一的。我们利用地球动力学观测，包括重力、板块运动、动态地形和核幔边界的超椭圆度，结合地震观测，通过联合反演来研究地幔的热化学结构。然而，一个突出的问题是地幔密度异常与地表地球动力学观测之间的物理联系，这需要了解地幔粘度结构。在这里，我们执行联合反演假设不同的粘度剖面，并检查结果对粘度的依赖。我们首先假设地幔非均质性是由于热变化，这对地震速度和密度之间的关系施加了限制，随后我们放宽了限制，以允许潜在的非热效应。在我们所有的联合反演中，需要密度异常的非热源来解释地球动力学数据，尽管其数量随假定的粘度结构而变化。一种常见的观测结果是，在地幔底部大的低剪切速度区域的中心存在高密度的化学信号，导致那里的总体浮力接近中性或略密集。利用导出的密度模型和相应的黏度曲线，我们还计算了瞬时地幔流场。联合反演得到的流场与纯地震层析成像模型后验标度得到的密度模型预测的流场大致相似，但有很大不同。
Seismic tomography has revealed the existence of large‐scale velocity heterogeneities in the mantle. The interpretation of seismic velocity anomalies in terms of temperature and chemical composition is nonunique. We use geodynamic observations including gravity, plate motions, dynamic topography, and excess ellipticity of the core‐mantle boundary combined with seismic observations to investigate the thermo‐chemical structure of the mantle through joint inversions. An outstanding issue, however, is the physical connection between mantle density anomalies and the surface geodynamic observations, which requires knowledge of the mantle viscosity structure. Here we perform joint inversions assuming different viscosity profiles and examine the dependence of the results on the viscosity. We first assume that mantle heterogeneity is due to thermal variations, which places a constraint on the relation between seismic velocity and density, and we subsequently relax the constraint to allow for potential nonthermal effects. In all of our joint inversions, a nonthermal origin of density anomalies is required to explain the geodynamic data, though the amount varies with the assumed viscosity structure. A common observation is a high‐density chemical signal in the center of the large low‐shear‐velocity provinces at the base of the mantle resulting in a near neutral or slightly dense overall buoyancy there. Using the derived density models and their corresponding viscosity profiles, we also calculate instantaneous mantle flow fields. The predicted flow fields derived from joint inversions are generally similar but are quite different from flow fields using density models derived from a posteriori scaling of pure seismic tomography models.