The radial viscosity of the mantle is generally thought to increase by ∼10–100 times from the upper to lower mantle with a putative, abrupt increase at 660 km depth. Recently, a low viscosity channel (LVC) between 660 and 1,000 km has been suggested. We conduct a series of time-dependent flow models with viscosity either increasing or decreasing at 660 km depth while tracking slab structure, state-of-stress, and geoid. We find that a LVC will lower the amplitude of long wavelength (>5,000 km) geoid highs over slabs, with amplitudes <10 m in height, while increasing the slab dip angle and downdip tension in the upper 300 km of slabs. A viscosity increase at 660 km gives rise to strong downdip compression throughout a slab and this pattern will largely go away with the introduction of the LVC. In addition, the endothermic phase change at 660 km depth can substantially affect the stress distribution within slabs but has a minor influence on the geoid. Models that fit the observed long wavelength geoid and observed focal mechanism in the western Pacific favor models without the presence of the LVC between 660 km and 1,000 km depths.

中文翻译：

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板坯动力学对地幔粘度的约束

通常认为地幔的径向粘度从上地幔到下地幔增加约 10-100 倍，并在 660 公里深度处突然增加。最近，有人建议在 660 至 1,000 公里之间建立一个低粘度通道 (LVC)。我们在跟踪板片结构、应力状态和大地水准面的同时，进行了一系列随时间变化的流动模型，其粘度在 660 公里深度处增加或减少。我们发现 LVC 会降低长波长（> 5,000 km）大地水准面高度的幅度，幅度小于 10 m，同时增加板坯上 300 km 的板倾角和下倾张力。660 公里处的粘度增加会导致整个板坯产生强烈的下倾压缩，随着 LVC 的引入，这种模式将在很大程度上消失。此外，660 公里深度处的吸热相变可以显着影响板坯内的应力分布，但对大地水准面影响较小。适合在西太平洋观测到的长波长大地水准面和观测到的震源机制的模型有利于在 660 公里和 1,000 公里深度之间不存在 LVC 的模型。