The influence of uncertain mantle density and viscosity structures on the calculations of deep mantle flow and lateral motion of plumes
The influence of uncertain mantle density and viscosity structures on the calculations of deep mantle flow and lateral motion of plumes
复制标题
不确定的地幔密度和粘性结构对深部地幔流和地幔柱横向运动计算的影响
DOI：
10.1093/gji/ggad040
复制
发表时间：
2023
影响因子：
2.8
通讯作者：
Li, Mingming
中科院分区：
文献类型：
--
作者：
Li, Mingming
地幔柱形成于热边界层，例如地球的核幔边界。当地幔柱向地表上升时，它们被周围的地幔流横向偏转，而地幔流受深部地幔密度和粘性结构的控制。地幔柱的横向运动携带着地幔深部结构和动力学的信息，可以用来建立重建板块绝对运动的参考架。在这项研究中，我们比较了两种方法来计算深部地幔流和地幔柱的横向运动。在地幔对流（MC）模型中，通过从给定的初始条件向前求解守恒方程来确定地幔流场和羽流的横向运动。在羽流平流（PA）模型中，使用近似粘度和当今密度结构来计算当今地幔流，然后假设热扩散为零，将其向后传播，羽流管道用连续线表示，并且被动平流在背景地幔流中。问题是PA模型中的假设如何影响深部地幔流和地幔柱横向运动的预测。在这里，我们执行纯热MC模型和热化学MC模型与固有的致密材料在最低地幔。在每个MC模型中，深部地幔流和地幔柱的横向运动被精确地确定。我们还使用这些MC模型中近似的现今粘度和温度结构来执行PA模型。我们发现，PA模型不考虑温度依赖性的粘度和/或只使用长波长的现代温度结构（高达20度）往往导致平均的50%-60%和60%-200%的差异，现代地幔流动速度比纯粹的热MC模型和热化学MC模型，分别。在PA模式中，通过将不准确的流场向后传播，往往会导致过去地幔流速度的更大误差。即使使用相同的参数，从相同的现代地幔流场在MC模式，PA模式仍然显示出的地幔流速度的平均100%-30%的失配后，400 Ma。此外，我们表明，地幔流场的错误在PA模式可以导致100%-600%的差异，在过去的60 Ma的地幔柱横向运动比MC模式的约束。即使我们使用MC模式中的地幔流来平流PA模式中的虚拟羽流，如果虚拟羽流的起始位置和/或形状与MC模式中的羽流不同，则虚拟羽流仍然可以显示出比MC模式中的动态羽流大50- 300%的横向运动差异。我们还发现PA模型中的虚拟羽流在不同的位置和/或具有不同的形状，可以稍后平流到类似的位置，这表明PA模型中的羽流的横向运动可以是非唯一的。因此，在解释PA模型对深部地幔流场和地幔柱横向运动的预测时，考虑PA模型的内在假设是很重要的。随着我们对地球深部地幔结构和动力学的更好理解，PA模型的准确性将会提高。
Mantle plumes form from thermal boundary layers, such as Earth's core–mantle boundary. As plumes rise towards the surface, they are laterally deflected by the surrounding mantle flow that is governed by deep mantle density and viscosity structures. The lateral motions of mantle plumes carry information of deep mantle structure and dynamics and are used to setup reference frames by which absolute plate motions are reconstructed. In this study, we compare two methods to compute deep mantle flow and lateral motion of plumes. In mantle convection (MC) models, the mantle flow field and lateral motions of plumes are determined by solving conservation equations forward-in-time from given initial conditions. In plume advection (PA) models, approximate viscosity and present-day density structures are used to calculate present-day mantle flow which is then propagated backward-in-time assuming zero thermal diffusion, and plume conduits are represented by continuous lines and are passively advected within the background mantle flow. The question is how assumptions in PA models influence the predictions of deep mantle flow and plume lateral motions. Here, we perform purely thermal MC models and thermochemical MC models with intrinsically dense materials in the lowermost mantle. The deep mantle flow and plume lateral motions are determined accurately in each MC model. We also perform PA models using the approximated present-day viscosity and temperature structures in these MC models. We find that PA models without considering temperature-dependence of viscosity and/or only using long wavelength present-day temperature structure (up to degree 20) often lead to an average of ∼50–60 per cent and ∼60–200 per cent differences of present-day mantle flow velocities than purely thermal MC models and thermochemical MC models, respectively. By propagating inaccurate flow fields backward-in-time in PA models often cause even larger errors of mantle flow velocities in the past. Even using the same parameters and starting from the same present-day mantle flow fields as in MC models, the PA models still show an average of ∼10–30 per cent misfit of mantle flow velocities after ∼40 Ma.In addition, we show that errors of mantle flow fields in PA models can cause ∼100–600 per cent differences of plume lateral motions than that constrained in MC models in the past 60 Ma. Even we use the mantle flow in MC models to advected virtual plumes in PA models, the virtual plumes could still show ∼50–300 per cent difference of lateral motions than dynamic plumes in MC models if the virtual plumes do not start with the same locations and/or shapes as plumes in MC models. We also find virtual plumes in PA models initiated at different locations and/or with different shapes can be later advected to similar locations, suggesting that the lateral motions of plumes in PA models can be non-unique. Therefore, it is important to consider the build-in assumptions of PA models when interpreting their predictions on deep mantle flow field and plume lateral motions. The accuracy of PA models would improve as we gain better understanding on Earth's deep mantle structure and dynamics.