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

板块构造理论预测地球的外层是由运动中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，下降到下面的地幔中。在地球上观测到的大部分变形是什么？S表面以地震、火山活动和造山活动的形式发生在俯冲带板块边界。然而，这种变形是如何在地下，在俯冲板块和上覆板块之间的地幔和地幔楔区表现出来的，人们还不太清楚。该研究将使用三维多板块地球动力学模型来研究地幔楔流变控制、地幔与上覆板块岩石圈的耦合以及俯冲带板块边界附近复杂地震各向异性的相关维度。这些模型将模拟地球作为一个高度粘性的流体体，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，包括俯冲带中化学特征的传输，然后在火山中爆发，地幔楔中循环的大小和规模，地幔如何耦合到覆盖的板块变形和造山，以及板块的粘性支撑和/或对板块下沉的粘性阻力，从而产生构造板块的速度。从实验室实验到半解析方法，再到全数值模拟，近年来的三维俯冲模型表明，俯冲带的地幔流场在空间上是可变的，包括极向流、环向流和海沟平行流。然而，以前的模型并没有解决是什么控制了从俯冲带附近局部解耦的幔覆板块到远离俯冲带的耦合幔覆板块的转变。如果地幔运动与俯冲带附近的地表板块不平行，但在远离俯冲带的地方变得平行，就像全球模型所显示的那样，这种转变必然发生。构建一系列三维多板块地球动力学模型，系统测试地幔楔中突出低黏度的长度尺度，量化覆幔板块黏度、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三维模型系统地增加到两个目标地点的地理参考三维模型。将测试流变配方的相对影响以及地幔楔中橄榄石水化和熔体分数的变化。为了适当地跨越从解耦的地幔楔覆板块到耦合的地幔-板块内部的过渡，模型域将比通常的三维区域俯冲模型延伸到板块内部更远。将使用C/ c++代码SlabGenerator来构建三维有限元网格、板边界界面和初始温度。热化学有限元地幔对流程序CitcomCU将用于运行三维粘性流动模拟。根据预测的流场计算矿物的晶格优选取向，并用于计算模型域上的合成弹性常数。合成剪切波分裂将与目标研究地点实际观测到的剪切波分裂进行比较。对于广义三维模型，将复杂地幔流场的长度尺度与从板块边界剪切波分裂的全球分析中观测到的长度尺度进行比较。了解俯冲带中解耦带的机制和长度尺度，可以对控制岩石圈底部牵引力横向变化的因素、在俯冲带附近观测到的复杂地震各向异性的长度尺度、在垂直方向上覆盖板块-地幔耦合的强度以及岩石圈-软流圈边界的流变特征进行约束。

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.