Collaborative Research: Evaluating the Roles of Melt Migration and Mantle Flow in Lithospheric Evolution: The Colorado Plateau as a Geodynamic Laboratory for EarthScope

1. 技术描述本项目重点研究岩石圈-软流圈边界（LAB）熔体的产生和分离，目的是(1)描述边界的物理性质，(2)限制决定其形式和演化的过程。具体来说，LAB可以通过主要的热机械过程（例如，通过滴或剪切对致密部件进行物理分层）或主要的热化学过程（例如，通过熔融浸渍和将耗尽材料转化为富集的，部分熔融的材料来恢复活力）进行实质性的修改。这项工作建立在最近的观察基础上，表明熔体在LAB上的偏析和迁移可以在“变形增强的热化学恢复”过程中大大改变边界的热、化学和机械性能。该项目通过建立基于实验观测的应力驱动熔体分离的现象学描述来探索这些思想的地球动力学含义，这将被纳入美国西部科罗拉多高原（CP）下地幔变形的流体动力学模型。这项研究建立在地质和地震观测的基础上，这些观测限制了CP地区融化的分布和几何形状，特别是将高原边缘与内部进行了比较。pi正在使用一系列过程的数值模型来解释这些观测结果：1)CP和周围区域板块变薄的空间变化，2)板块密集区域可能的对流不稳定性，以及3)CP岩石圈“龙骨”通过底层软流层的运动。这些研究与USArray表面波的地震各向异性观测相结合，以推断CP.2之下的地幔变形情况。对更广泛的影响和意义的非技术描述。科学界希望“地球望远镜”能够帮助实现的一项基本发现是更清晰地描绘出北美板块的“形状”；也就是说，对岩石圈厚度的测量和对板块热性质和化学性质的空间变异性的理解。岩石圈的底部是一个难以捉摸的边界，与一个容易成像的地震不连续不一致。在分析EarthScope的USArray组件数据以构建构造活跃的美国西部下地幔的地震模型时，从板块-地幔界面的结构和动力学角度解释地震观测结果是科学界的一个重要目标。困扰地震解释的一个基本的模糊性是温度、成分和流体（如熔体）在控制上地幔各向同性和各向异性地震结构中的相对重要性。该项目正在开发基于物理的北美板块底部变形模型，可用于帮助解释地震观测结果。该项目正在为研究生提供建立地球数量模型方面的重要培训，并正在扩大代表性不足的群体对地球科学的参与。流体动力学和基于实验的现象学的结合是一种创新，可以推广到其他地球动力学环境。由于北美和太平洋的结构和演变具有广泛的公众利益，pi将从地震观测、模型和具有教学意图的解释性插图中产生引人注目的图像。这些图像将用于西南、纽约和其他地方的博物馆和国家公园的公共教育和宣传（以及科学出版物）。

1. Technical descriptionThis project focuses on the generation and segregation of melt at the lithosphere-asthenosphere boundary (LAB), with the aims of (1) characterizing the physical nature of the boundary and (2) constraining the processes that determine its form and evolution. Specifically, the LAB may be substantially modified by processes that are dominantly thermomechanical (e.g., physical delamination of dense parts by drips or shear), or dominantly thermochemical (e.g. rejuvenation by melt impregnation and conversion of depleted material into enriched, partially molten material). This work builds on recent observations suggesting that melt segregation at and migration along the LAB can substantially modify the thermal, chemical, and mechanical properties of the boundary in a "deformation-enhanced thermo-chemical rejuvenation" process. This project explores the geodynamic implications of these ideas by developing a phenomenological description of stress-driven melt segregation based closely on experimental observations, that will be incorporated into fluid-dynamical models of mantle deformation beneath the Colorado Plateau (CP) in the western US. The research builds upon geologic and seismic observations that constrain the distribution and geometry of melt in the CP region, specifically comparing the plateau-margins to the interior. The PIs are interpreting these observations using numerical models of a suite of processes: 1) spatial variations in plate thinning of the CP and surrounding regions, 2) possible convective instability of dense regions of the plate, and 3) motion of the CP lithospheric "keel" through the underlying asthenosphere. These investigations are being coupled with seismic anisotropy observations from surface waves across the USArray to infer mantle deformation scenarios beneath the CP.2. Non-technical description of broader impact and significance.One of the fundamental discoveries that the scientific community hopes EarthScope will help achieve is a clearer picture of the "shape" of the North American plate; that is, a measurement of lithosphere thickness and an understanding of the spatial variability in both the thermal and chemical properties of the plate. The base of the lithosphere is an elusive boundary that does not coincide with an easily imaged seismic discontinuity. As data from the USArray component of EarthScope are analyzed to construct seismic models of the upper mantle beneath the tectonically active western US, an important goal for the scientific community is to interpret seismic observations in terms of the structure and dynamics of the plate-mantle interface. A fundamental ambiguity that plagues seismic interpretations is the relative importance of temperature, composition, and fluids (e.g., melt) in controlling the isotropic and anisotropic seismic structure of the upper mantle. This project is developing physics-based models of deformation at the base of the North American plate that can be used to help interpret seismic observations. The project is providing important training for a graduate student in building quantitative models of the Earth and is broadening participation of underrepresented groups in the earth sciences. The combination of fluid dynamics and the experimentally based phenomenology developed here is an innovation that can be generalized to other geodynamic settings. Because the structure and evolution of North America and the CP in particular is of broad public interest, the PIs will produce compelling images from seismic observations, models and interpretive illustrations with pedagogical intent. These images will be utilized in public education and outreach (as well as scientific publications) at museums and national parks in the Southwest, New York and elsewhere.