Developing a Seismic Model for Investigating Layering in Cratonic Lithosphere beneath Africa

该奖项全部或部分由《2021年美国救援计划法案》（公法117-2）资助。岩石圈是地球坚硬的外壳，是板块构造的刚性板块。在大陆最古老和最稳定的地区（克拉通），岩石圈被认为是寒冷和厚的，然而我们观察到岩石圈内部有一个广泛分布的、几乎恒定的层，几乎类似于地壳-地幔边界，但表现为速度降低。对“岩石圈中层分层”的这种令人困惑和普遍存在的观察导致了不同的、有时是相互矛盾的解释（即部分熔融、各向异性、化学分层或短期流变弱化）。在这项研究中，研究人员将开发一种新的岩石圈地震模型，用于测试提出的非洲岩石圈分层模型。多年来对创新的非洲阵列计划的投资，克拉通的多样性，以及大陆上越来越多的永久和临时地震网络，以及公开可用的数据，为我们解决非洲大陆精细岩石圈分层的能力开辟了新的前景。同样，利用环境噪声的光谱相干性、机器学习和转换体波的概率分析的新型无源成像技术可以确保存档地震数据集的最大效用。该项目将吸引早期职业科学家，培养少数族裔博士生，并通过设计新课程，使学生了解高性能计算资源的最佳使用和管理，促进教育和推广目标，以招募代表性不足的本科生。该项目将填补目前在研究大陆岩石圈分层方面的空白。调查人员将对大陆上所有公开的地震数据进行分析，对地壳和上地幔岩石圈敏感，并准备和公布以下测量结果：(1) Love面波和Rayleigh面波的宽带环境噪声和地震记录的群速度和相速度；(2)岩石圈不连续面的纵波和横波反射率的深度和清晰度；(3)量化相速度图的不确定性；(4)提供更新的非洲地壳和上地幔的全大陆参考模型（LithoAFR+）。使用概率联合逆方法报告测量和模型构建的每个阶段的不确定性（即1-4）。预期的同行评议研究，当与互补的实验和地球物理约束（即电导率和矿物物理）解释时，将促进我们对克拉通岩石圈分层和大陆地质演化的理解。利用其他地球物理约束条件的综合建模也将促进对以下问题的理解：(1)非洲高架地形的起源；(2)孤儿震、板块内地震和火山活动的起源，以及它们与众所周知的非洲超级羽流的联系；(3)大陆裂谷的大规模演化和新海洋盆地的形成；(4)冈瓦纳大陆的多阶段组装和分裂。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。

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).The lithosphere is the stiff outer shell of our planet - it is the rigid plate of plate tectonics. In the oldest and most stable regions of continents (cratons) the lithosphere is expected to be cold and thick, yet we observe a wide-spread, near-constant layer internal to the lithosphere, almost akin to the crust-mantle boundary, but expressed as a velocity reduction. This puzzling and ubiquitous observation of ‘mid-lithospheric layering’ has resulted in different, sometimes contradictory, explanations (i.e., partial melt, anisotropy, chemical stratification, or short-term rheological weakening). In this study, the investigators will develop a new seismic model of the lithosphere useful for testing proposed models of lithospheric layering beneath Africa. The multi-decadal investment in the innovative AfricaArray initiative, the diversity of cratons, and a growing number of permanent and temporary seismic networks on the continent, with publicly available data, has opened a new vista on our ability to resolve fine-scale lithospheric layering on the continent. Similarly, new passive-source imaging using spectral coherence of ambient noise, machine learning, and probabilistic analysis of converted body waves is poised to ensure maximum utility from the archived seismic datasets. This project will engage early career scientists, train minority Ph.D. students, and promote education and outreach goals for recruiting under-represented undergraduate students by designing new courses that expose students to the optimal use and management of high-performance computing resources. The project will address gaps in current efforts to study lithospheric layering on the continent. The investigators will conduct analysis of all publicly available seismic data on the continent, with sensitivity to the crust and upper mantle lithosphere, and prepare and publishing measurements of: (1) group and phase velocity of broad-band ambient noise and earthquake records for Love and Rayleigh surface waves, (2) depth and sharpness of compressional and shear body wave reflectivity of lithospheric discontinuities, (3) quantify uncertainty in phase velocity maps and, (4) provide an updated continent-wide reference model of the African crust and upper mantle (LithoAFR+), using a probabilistic joint inverse approach to reporting uncertainty in each stage of measurement and model-construction (i.e., 1-4). The expected peer-reviewed studies, when interpreted with complementary experimental and geophysical constraints (i.e, conductivity and mineral physics), will advance our understanding of cratonic lithospheric layering and the geological evolution of continents. Integrated modeling, using other geophysical constraints, will also advance the understanding of (1) the origin of the elevated African topography, (2) the origin of orphan tremors, intra-plate earthquakes, and volcanism, and their connection to the well-known African superplume, (3) the broad-scale evolution of continental rifting and formation of new ocean basins, and (4) multi-stage assembly and break up of Gondwanaland.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.