Collaborative Research: Seismic Imaging of Mid-Mantle Reflectors Associated with Geodynamical Processes and Compositional Heterogeneity

地球由几个不同的层组成：表面是地壳，地幔占了地球的大部分，中心是金属地核。地核释放的热量驱动热地幔的缓慢对流。这个过程导致了构造板块的运动，产生了火山活动，并重塑了大陆。最近的地球物理研究表明，中地幔的对流模式在800-1300公里深度发生了变化。主导地幔对流的上升流柱和俯冲板块在地震层析成像中表现为在中地幔发生偏转。目前尚不清楚这是由于成分还是粘度造成的。众所周知，上地幔也有类似的地震不连续性，这被认为是由于地幔温度升高和压力随深度增加而发生的化学变化或矿物相变造成的。本研究的目的是利用新的方法来了解中地幔中的大型反射体，通过开发小尺度地震特性的综合地图，并将其与不同类型的地球动力流域（如已知的板块或上升流区）的特征特征进行比较。该项目将支持研究生和本科生的研究。该项目将促进新墨西哥州立大学（一个为西班牙裔服务的机构）与欧洲和澳大利亚的大学之间的新的国际合作。这项工作将使地球科学界的其他学科受益，并向新墨西哥州的少数民族学生推广地球物理学。该项目由地球物理计划和促进竞争性研究的既定计划（EPSCoR）共同资助。该项目将确定中地幔地震反射体的不同起源，确定它们与地幔流动模式的空间关系，如下行冷板块或上升流羽流，并研究潜在的形成过程和成分起源。这项工作将生成地幔800-1300公里深度反射体的综合区域尺度地震图。新的和已建立的分析方法将应用于编译和评估互补数据集（SS和PP前体；ScS和PcP混响；接收函数），以约束反射器几何形状、表观阻抗对比度、清晰度和横向分辨率（100-1,000公里）多个长度尺度的异质性。这些观测结果将通过合成地震记录的正演模拟进行测试和联系。结果将在全球地震层析成像和地球动力学模型的背景下进行分析，使用地震速度作为地球动力学流动的代理。本研究将结合统计相关性和聚类分析，建立具有不同地球动力学背景的几个采样良好的速度域的中地幔反射体特征特征。将在全球地球动力学数值模拟的框架内评估每种域类型的反射器特性，并针对不同对流历史情景的影响进行测试。随后，这将为反射器可能的形成机制和热化学起源提供信息。由不同地球动力学模型生成的合成地震数据将提供与地震观测的直接比较。这种多学科的方法将告知地震转换对地幔对流的影响，反之亦然，并阐明流动在产生物理结构中的作用。该项目将有助于绘制全球中地幔环流的地震特征，表征上下地幔之间的热化学交换，并深入了解地幔混合和动力学的历史和风格。这些结果将为下一代地球动力学模拟和地幔矿物物理研究提供宝贵的输入，并具有广泛的意义，从绘制原始未混合地幔储层到追踪大陆火山作用的深层起源。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The Earth is composed of several distinct layers: the crust at the surface, the mantle which comprises the bulk of our planet, and the metallic core at its center. Heat released from the core drives slow convection of the hot mantle. This process results in the movement of tectonic plates, produces volcanism, and reshapes continents. Recent geophysical studies have suggested that the pattern of convection changes in the mid-mantle, from 800-1300 km depth. The upwelling plumes and subducting slabs which dominate mantle convection appear in seismic tomography images to become deflected in the mid-mantle. It is unknown whether this is due to composition or viscosity. The upper mantle is known to have similar seismic discontinuities that are thought to be caused by variations in chemistry or mineral phase changes as the mantle gets hotter and pressure increases with depth. The goal of this research is to use new methods to understand the large reflectors in the mid-mantle by developing comprehensive maps of the small-scale seismic properties, and comparing them to characteristic signatures for different types of geodynamic flow domains, such as known slab or upwelling regions. This project will support both graduate and undergraduate student research. The project will foster new international collaborations between New Mexico State University, a Hispanic-serving institution, and universities in Europe and Australia. The work will benefit various other disciplines across the wider Earth Sciences community, and promote Geophysics to minority students in New Mexico. This project is jointly funded by the Geophysics Program, and the Established Program to Stimulate Competitive Research (EPSCoR). This project will determine the varied origins of mid-mantle seismic reflectors, ascertain their spatial relationship to flow patterns in the mantle, such as downwelling cold slabs or upwelling plumes, and investigate potential formative processes and compositional origins. The work will generate comprehensive regional-scale seismic maps of reflectors from 800-1300 km depth in the mantle. Novel and established analytical methods will be applied to compile and evaluate complementary datasets (SS and PP precursors; ScS and PcP reverberations; receiver functions), to constrain reflector geometry, apparent impedance contrast, sharpness, and heterogeneity across multiple length-scales of lateral resolution (100-1,000 km). These observables will be tested and linked via forward modelling of synthetic seismograms. Results will be analyzed in the context of global seismic tomography and geodynamical models, using seismic velocity as a proxy for geodynamical flow. This research will establish the characteristic signatures of mid-mantle reflectors for several well-sampled velocity domains representative of diverse geodynamical settings, incorporating statistical correlations and clustering analysis. Reflector properties from each domain type will be assessed within the framework of global geodynamical numerical simulations, and testing against the influence of different scenarios of convection history. Subsequently, this will inform regarding likely formative mechanisms and thermochemical origins for the reflectors. Synthetic seismic data generated from different geodynamical models will provide a direct comparison to seismic observations. This multidisciplinary approach will inform regarding the influence of seismic transitions on mantle convection and vice versa and elucidate the role of flow for generating physical structures. The project will help to map the seismic signatures of global mid-mantle circulation, characterize thermochemical exchange between lower and upper mantle, and provide insight into the history and style of mantle mixing and dynamics. The results will provide valuable inputs for the next generation of geodynamical simulations and mineral physics studies in the mantle, and have wide implications ranging from the mapping of primordial unmixed mantle reservoirs, to tracing the deep origins of continental volcanism.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.