Numerical Modeling of Three-dimensional Subduction Driven Mantle Wedge Weakening and Plate-Mantle Decoupling

板块构造理论预测地球外层是由运动的岩石圈板块组成的。俯冲带发生在构造板块汇聚的地方，一个板块俯冲到另一个板块之下，下降到下面的地幔。 在地球上观察到的大部分变形？地球的表面，以地震、火山活动和造山运动的形式，发生在俯冲带板块边界。然而，这种变形是如何表现在地下，在地幔和地幔楔区之间的俯冲板块和覆盖板块，是不太了解。拟议的研究将使用三维多板块地球动力学模型来解决地幔楔的流变学，地幔的耦合覆盖板岩石圈，以及相关的尺寸复杂的地震各向异性俯冲带板块边界附近的控制。这些模型将模拟地球作为一个高粘性流体体，在数百万年的时间尺度上变形，每个模型都在数百到数千个处理器上运行。这项研究的结果将具有广泛的科学意义，包括在火山喷发的俯冲带中化学信号的传输，地幔楔中环流的大小和规模，地幔如何耦合到覆盖的板块变形和造山运动，以及板块的粘性支撑和/或对板块下沉的粘性阻力，从而影响构造板块的速度。从实验室实验到半解析方法，再到完全数值模拟的俯冲三维模型表明，俯冲带的地幔流场在空间上是可变的，包括极向流、环形流和沟槽平行流。然而，以前的模型没有解决是什么控制着从一个局部解耦的地幔覆盖板块附近的俯冲带耦合地幔覆盖板块远离俯冲带的过渡。如果地幔运动不平行于俯冲带附近的表面板块，但如全球模型所示，在远离俯冲带的地方变得平行，那么这种转变必然发生。 将建立一系列三维多板块地球动力学模型，以系统地测试地幔楔中出现的低粘度的长度尺度，并量化地幔覆盖板块粘度、速度和牵引力的垂直和横向梯度。数值模型的复杂性将从广义3D模型系统地增加到两个目标地点的地理参考3D模型。 将测试地幔楔中橄榄石水化和熔体分数的流变学配方和变化的相对影响。为了正确地跨越从分离的地幔楔覆板块到耦合的地幔板块内部的过渡，模型域将比俯冲的3D区域模型更深入地延伸到板块内部。将使用C/C++代码SlabGenerator构建3D有限元网格、板边界界面和初始温度。一个热化学有限元地幔对流代码，CitcomCU，将用于运行三维粘性流模拟。矿物的晶格优选取向将从预测的流场计算，并用于计算模型域上的合成弹性常数。将合成剪切波分裂与目标研究中心剪切波分裂的实际观测结果进行比较。对于广义的三维模型，复杂的地幔流场的长度尺度将进行比较的长度尺度观察到的剪切波分裂在板块边界的全球分析。了解俯冲带中解耦带的机制和长度尺度，可以限制控制岩石圈底部沿着牵引力的横向变化、俯冲带附近观察到的复杂地震各向异性的长度尺度、垂直方向上覆盖的板幔耦合强度以及岩石圈-软流圈边界的流变学特征。

The theory of plate tectonics predicts the outer layer of the Earth is composed of lithospheric plates that are in motion. Subduction zones occur where tectonic plates converge and one plate subducts beneath the other, descending into the underlying mantle. The majority of the deformation observed at the Earth?s surface, in the form of earthquakes, volcanism, and mountain building, occurs at subduction zone plate boundaries. However, how this deformation is manifested in the subsurface, in the mantle and mantle wedge region between the subducting plate and overriding plate, is less understood. The proposed research will use 3D multi-plate geodynamic models to address the controls on the rheology of the mantle wedge, the coupling of the mantle to the overriding plate lithosphere, and associated dimensions of complex seismic anisotropy near subduction zone plate boundaries. The models will simulate the Earth as a highly viscous fluid body deforming on a timescale of millions of years, with each model run on hundreds to thousands of processors. Results from this research will have broad scientific implications, including the transport of chemical signatures in subduction zones which are then erupted in volcanoes, the magnitude and scale of circulation in the mantle wedge, how the mantle couples into overriding plate deformation and mountain building, and the viscous support of the slab and/or viscous resistance to slab sinking and thereby the velocity of the tectonic plates.Recent three-dimensional models of subduction ranging from laboratory experiments, to semi-analytic methods, to fully numerical simulations indicate the mantle flow field in subduction zones is spatially variable, containing poloidal, toroidal, and trench parallel flow. However, previous models do not address what controls the transition from a locally decoupled mantle-overriding plate near the subduction zone to a coupled mantle-overriding plate far from the subduction zone. Such a transition must occur if the mantle motion is non-parallel to the surface plates near the subduction zone but becomes parallel farther from the subduction zone as indicated by global models. A series of 3D multi-plate geodynamic models will be constructed to systematically test the length-scales of the emergent low viscosity in the mantle wedge and quantify the vertical and lateral gradients in mantle-overriding plate viscosity, velocity, and tractions. The numerical models will systematically increase in complexity from generalized 3D models to geographically referenced 3D models for two target sites. The relative effects of rheology formulation and variations in the olivine hydration and melt fraction in the mantle wedge will be tested. In order to properly span the transition from the decoupled mantle wedge-overriding plate to coupled mantle-plate interior, the model domain will extend farther into the plate interior than is typically done for 3D regional models of subduction. A C/C++ code, SlabGenerator, will be used to construct the 3D finite-element mesh, plate boundary interface, and initial temperature. A thermo-chemical finite-element mantle convection code, CitcomCU, will be used to run the 3D viscous flow simulations. The lattice preferred orientations of minerals will be calculated from the predicted flow field and used to calculate synthetic elastic constants over the model domain. The synthetic shear wave splitting will be compared with actual observations of shear wave splitting from the target study sites. For the generalized 3D models, the length-scales of the complex mantle flow field will be compared to the length-scales observed from a global analysis of shear wave splitting at plate boundaries. Understanding the mechanisms and length-scales of the decoupling zone in subduction zones can place constraints on what controls the lateral variability in tractions along the base of the lithosphere, the length-scales of complex seismic anisotropy observed near subduction zones, and the strength of overriding plate-mantle coupling in the vertical direction and, thus, the rheological signature of the lithosphere-asthenosphere boundary.