Mixing in a 3D spherical model of present-day mantle convection
Mixing in a 3D spherical model of present-day mantle convection
复制标题
在当今地幔对流的 3D 球形模型中进行混合
DOI：
10.1016/s0012-821x(99)00181-8
复制
发表时间：
1999
影响因子：
5.3
通讯作者：
S. Zhong
中科院分区：
文献类型：
--
作者：
P. V. Keken;S. Zhong
我们提出的第一个研究现今的混合效率的地球是基于一个三维球形模型的对流在地球的地幔。我们采用的固定模型是来自现今板浮力使用近似的板块流变学，弱上地幔以上的过渡区和粘度增加到下地幔。该模型在预测表层板块速度和大地水准面方面是相当成功的。我们认为这个模型是现今地幔内部速度结构的适当近似。通过研究颗粒轨迹、评价庞加莱截面和量化拉伸效率来研究混合效率。这使得确定区域在现今地幔的特点是层流或混乱的混合，并表明不同的地幔区域之间的连接。模拟结果表明，现今地幔速度场存在多种混合尺度。只有某些区域表现出层状单细胞混合。大多数其他地区的对流的特征是螺旋状的颗粒轨迹，可以将颗粒输送到远离其来源的地方，并可能进行混乱的混合。控制现今地幔混合效率的关键因素是板块的驱动力，特别是那些沿着太平洋边缘的板块，以及由旋转和表面板块的走滑运动引起的涡度（或环向运动）。我们的方法是保守的，因为板块构造的内在时间依赖性和更年轻、更热的地球上更剧烈的对流会导致更强的混合。基于这个简单的模型方法，我们得出的结论是，在今天的地球混合是相对有效的，这是不可能的大部分地球地幔保持孤立在地球的生命时间。
We present the first study of present-day mixing efficiency of the Earth that is based on a 3D spherical model of convection in the Earth's mantle. The stationary model we employ is derived from the present-day slab buoyancy forces using an approximate plate rheology, a weak upper mantle above the transition zone and an increase of viscosity into the lower mantle. The model is quite successful in predicting surface plate velocities and the geoid. We consider this model an appropriate approximation of the present-day internal velocity structure in the Earth's mantle. The mixing efficiency is studied by studying particle traces, evaluating Poincaré sections, and quantifying stretching efficiency. This allows for determining regions in the present-day mantle that are characterized by laminar or chaotic mixing and indicate the connection between different mantle regions. The modeling indicates that a variety of mixing scales exist in the present mantle velocity field. Only certain regions exhibit laminar single cell mixing. Convection in most other regions is characterized by corkscrew-like particle tracks that allow for transport of particles far from their source and possibly for chaotic mixing. The key components governing the efficiency of mixing of the present-day mantle are the driving forces of the slabs, in particular those along the Pacific rim, and the vorticity (or toroidal motion) induced by the rotation and resulting strike-slip motion of surface plates. Our approach is a conservative one, as the intrinsic time-dependence of plate tectonics and more vigorous convection in a younger and hotter Earth would cause stronger mixing. Based on this simple model approach, we conclude that the mixing in the present-day Earth is relatively efficient, and that it is not possible for large portions of the Earth mantle to remain isolated over the life time of the Earth.