Three-dimensional Subduction Models: Implications for Plate-Mantle Coupling and Length-scales of Seismic Anisotropy

板块构造理论预测，地球外层由岩石圈板块(0-250公里厚)组成，这些板块相互运动(以1-20厘米/年的速度运动)，大部分变形集中在板块边界。在地球表面，这种变形通常表现为地震活动增强和局部化、火山作用和造山。然而，在地幔中，板块下的板块边界带变形的表达式还没有被很好地理解。地球动力学模拟是一个有价值的工具，可以用来预测粘性地幔对构造板块的响应和与构造板块的相互作用，因为地幔不能直接进入。这项拟议的工作将使用高分辨率的三维地球动力学建模作为一种工具，来模拟地球粘性地幔如何响应俯冲带的构造板块并与之相互作用，俯冲带是指一个构造板块滑动到另一个板块下方并在下面的粘性地幔内引发运动的区域。最近的剪切波分裂地震学观测表明，位于俯冲板块和俯冲板块之间的地幔楔形部分通常具有地震快轴倾斜于板块运动的特征，表明许多俯冲带存在复杂的地幔流场。然而，远离板块边界的地震快轴通常与板块运动呈近平行的方向，这表明板块内部与下伏地幔流场之间存在耦合。二维俯冲机制不能解释剪切波分裂所隐含的沿走向和跨走向的地幔流动变化，因此需要一个三维框架。以前的俯冲带三维数值模拟预测，地幔楔体中的压力梯度可以驱动沟槽平行流动，而由于俯冲板块的陡峭或沟槽中的回滚，可以在侧板边缘周围产生环状流动。此外，地幔楔体内的小尺度对流模型预测了复杂的地幔流动和地幔楔体内的地震各向异性。然而，是什么控制了从俯冲带附近复杂的地幔覆盖板块运动到板块内部排列的地幔覆盖板块运动的转变，还没有得到研究。此外，尽管最近的工作表明，控制地幔变形的流变流动规律可能有助于控制俯冲引起的粘性降低和复杂的地幔流场的大小和长度尺度，但这一点在俯冲的三维模型中还没有被量化。在这项研究中，将建立、运行和分析三维数值模型，以系统地测试由于应变率相关粘度而导致的地幔楔体中板条驱动粘度降低的横向程度的控制。这些结果对俯冲带中观测到的复杂地震各向异性的长度尺度有一定的影响，并可能限制地幔与上覆板块之间的耦合程度。此外，了解流变学如何调节地幔的粘性流动，对于了解构造板块运动的速率、板块边界带变形的长度-尺度以及火山前缘内地球化学特征的三维传输具有重要意义。

The theory of plate tectonics predicts that the outer layer of the Earth is comprised of lithospheric plates (0 - 250 km thick) that are in motion with respect to one another (at rates on the order of 1 - 20 cm/yr), with the majority of deformation concentrated at the plate boundaries. At the Earth's surface, this deformation is commonly manifested in the form of increased and localized seismicity, volcanism, and mountain building. The expression of plate boundary zone deformation underneath the plates, in the Earth's mantle, however, is not well understood. Geodynamic modeling is a valuable tool that can be used to predict the Earth's viscous mantle response to and interaction with the tectonic plates, as the mantle cannot be accessed directly. The proposed work will use high-resolution, three-dimensional geodynamic modeling as a tool to simulate how the Earth's viscous mantle responds to and interacts with the tectonic plates at subduction zones, regions where one tectonic plate slides beneath another and induces motion within the underlying viscous mantle. Recent seismological observations of shear wave splitting indicate the portion of the mantle wedged between the overriding plate and subducting plate, the mantle wedge, is commonly characterized by seismic fast axes oriented oblique to plate motion, indicative of a complex mantle flow field in many subduction zones. However, far from the plate boundary, the seismic fast axes are commonly oriented sub-parallel to plate motion, indicative of coupling between the plate interior and underlying mantle flow field. A two-dimensional subduction regime cannot explain the along strike and across strike variations in mantle flow implied by the shear wave splitting, thus requiring a three-dimensional framework. Previous three-dimensional numerical simulations of subduction zones predict that trench-parallel flow can be driven by pressure gradients in the mantle wedge and that toroidal flow can be generated around lateral slab edges due to steepening of the subducting plate or rollback in the trench. In addition models of small-scale convection within the mantle wedge predict complex mantle flow and seismic anisotropy in the mantle wedge. However, what controls the transition from complex mantle-overriding plate motion near the subduction zone to aligned mantle-overriding plate motion in the plate interiors has not been investigated. Furthermore, although recent work indicates the rheological flow law governing deformation in the mantle may be instrumental in controlling the magnitude and length-scales of the subduction induced viscosity reduction and complex mantle flow field, this has not been quantified in three-dimensional models of subduction. For this research, three-dimensional numerical models will be constructed, run, and analyzed, to systematically test the controls on the lateral extent of the slab driven viscosity reduction in the mantle wedge due a strain-rate dependent viscosity. The results have implications for the length-scales of complex seismic anisotropy observed in subduction zones and may place constraints on the magnitude of coupling between the mantle and overriding plate. Moreover, understanding how the rheology modulates the viscous flow of the mantle has important implications for understanding the rates of tectonic plate motion, the length-scales of plate boundary zone deformation, as well as the three-dimensional transport of geochemical signatures within the volcanic front.