MARGINS: Collaborative Research: Modeling 3-D Wedge Flow with Complex Slab Geometries and Comparisons with Seismic Anisotropy

了解俯冲带中的大（板）尺度地幔流和小尺度浮力流的模式是俯冲带热结构、脱水反应和挥发分分布以及岩浆生成和迁移模型的关键。地幔楔中不同的流动模式可以在地震学和地球化学观测中产生不同的特征，大量的证据表明，一个简单的二维，板块驱动的角流模型是不够的，在相当多的地区。例如，在几个俯冲带观测到的剪切波快速偏振方向与基于二维楔角流的预测不一致，即使考虑到可能偏离标准A型橄榄石滑移系（例如马里亚纳群岛、中美洲、汤加、南美洲、堪察加、阿拉斯加）。此外，一些弧和弧后包含的熔岩表现出与附近热点火山岩非常相似的特征（例如汤加北方，哥斯达黎加-尼加拉瓜），这表明地幔沿着高度三维（3-D）（非角流）轨迹被夹带到楔中。最近的观测和三维模型的结合表明，俯冲引起的楔状环流具有更多的空间和时间的复杂性比以前的二维（2-D）模型预测。我们提出了两套实验室实验模型的3-D俯冲带流动和各向异性的基础上，PI的以前的努力，这是强烈的地震数据的约束。一组实验将调查地幔流是如何驱动的各种物理俯冲带参数与板块运动和俯冲板（沿走向板倾角变化，沟槽回滚，板边缘和撕裂，上板形态和变形）。在第二组实验中，第二个将考虑如何楔材料与异常粘度和/或浮力相互作用与流量和改变预测的各向异性;这种材料的来源包括水合或部分熔融地幔从板楔界面，挥发性贫化地幔减压熔融或富集地幔夹带到楔。在这两组实验中，参数将系统地变化，以量化不同因素的相对影响。将通过与在地球仪周围的俯冲带中观察到的地震各向异性进行比较，对流动模型进行测试;这涉及到将流场转换为晶体学取向，以及计算剪切波分裂。模型开发和数据模型比较将侧重于两个区域：尼加拉瓜-哥斯达黎加和马里亚纳群岛。这些系统在板块运动、板块几何形状和年龄以及上部板块变形方面有着关键的差异，并且都通过剪切波分裂以及其他地震和地球化学研究进行了良好的采样。这一提议将有助于理解驱动三维地幔楔流的物理过程，并将为解释俯冲系统中越来越多的数据集提供更好的背景。具体问题包括：1）沿弧板形态和下沉模式的空间复杂性对楔形体中的流动有什么影响？2)这些参数与上板形状和变形的组合是否能产生与观察到的各向异性一致的3D流动（例如，弧平行快速方向）？3)地幔楔的改变或化学性质不同的区域如何与三维俯冲引起的流动相互作用？4)这些特征的长期变形和夹带是否会产生与观测一致的LPO模式？5)这些模式如何改变与环境和蚀变地幔储层之间的密度/粘度对比，以及弧岩浆作用的地球化学模型的含义是什么？更广泛的影响。拟议的工作将有助于限制地幔流及其对尼加拉瓜-哥斯达黎加和伊苏-小笠原-马里亚纳俯冲带（MARGINS俯冲工厂的两个重点地区）熔融过程的影响。该项目将为布朗大学的研究生和URI的研究生提供实验室流体实验的培训，与地震观测的整合，以及他们的解释。URI流体实验室将用作URI和Brown课程的教学工具。

Understanding patterns of both large (plate) scale mantle flow and smaller scale buoyant flow in subduction zones is key to models of subduction zone thermal structure, dehydration reactions and volatile distribution, and magma generation and transport. Different patterns of flow in the mantle wedge can generate distinct signatures in seismological and geochemical observables, and ample evidence indicates that a simple two-dimensional, plate-driven corner flow model is inadequate in a significant number of regions. For example, observed shear-wave fast polarization directions in several subduction zones are inconsistent with predictions based on two-dimensional wedge corner flow even when taking into account possible deviations from the standard A-type olivine slip system (e.g. the Marianas, Central America, Tonga, South America, Kamchatka, Alaska). In addition, some arcs and back-arcs contain lavas that exhibit characteristics very similar to nearby hot spot volcanics (e.g. northern Tonga, Costa Rica- Nicaragua), suggesting mantle is entrained into the wedge along highly three-dimensional (3-D) (non-corner flow) trajectories. The combination of recent observations and 3-D modeling suggests subduction-induced wedge circulation has more spatial and temporal complexity than predicted in previous two-dimensional (2-D) modeling. We propose two sets of laboratory experiments to model 3-D subduction zone flow and anisotropy which build upon the PI's previous efforts and which are strongly constrained by seismic data. One set of experiments will investigate how mantle flow is driven by a variety of physical subduction zone parameters related to plate motions and the subducting slab (alongstrike slab dip variations, trench roll-back, slab edges and tears, and upper plate morphology and deformation). In a second set of experiments, The second will consider how wedge material with anomalous viscosity and/or buoyancy interacts with flow and alters predicted anisotropy; sources of such material include hydrated or partially melted mantle from the slab-wedge interface, volatile depleted mantle produced by decompression melting or enriched mantle entrained into the wedge. In both sets of experiments, parameters will be systemically varied to quantify the relative impact of different factors. Flow models will be tested through comparison with seismic anisotropy observed in subduction zones around the globe; this involves transformation of flow fields to crystallographic orientations, and calculation of shearwave splitting. Model development and data-model comparisons will focus on two regions: Nicaragua-Costa Rica and the Marianas. These systems have key differences in plate motions, slab geometry and age, and upper plate deformation, and both are well-sampled by shear-wave splitting, as well as other seismic and geochemical studies.Intellectual merit. This proposal will contribute to an understanding of the physical processes that drive 3D mantle wedge flow and will provide a better context for interpreting the growing number of data sets in subduction systems. Specific questions to be addressed include: 1) What effect does spatial complexity in along-arc slab morphology and sinking mode have on flow in the wedge? 2) Can combinations of these parameters and upper plate shape and deformation produce 3D flows that are consistent with observed anisotropy (e.g., arc-parallel fast directions)? 3) How do altered or chemically distinct regions of mantle wedge interact with 3- D, subduction-induced flow? 4) Does the long-term deformation and entrainment of these features produce LPO patterns that are consistent with observations? 5) How do these patterns change with density/viscosity contrasts between ambient and altered mantle reservoirs and what are the implications for geochemical models of arc magmagenesis?Broader impacts. The proposed work would help to constrain mantle flow and its implications for melting processes in the Nicaragua-Costa Rica and Izu-Bonin-Mariana subduction zones, the two MARGINS Subduction Factory focus areas. This project would provide a Brown graduate student and a URI graduate student with training in laboratory fluids experiments, their integration with seismic observables, and their interpretation. The URI fluids lab will be used as a teaching tool in URI and Brown courses.