Collaborative Research: Mantle dynamics and plate tectonics constrained by converted and reflected seismic wave imaging beneath hotspots

合作研究：热点下方转换和反射地震波成像约束的地幔动力学和板块构造

• 批准号: 2147923
• 负责人: Peter Shearer
• 金额: $ 21.45万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-02-01 至 2025-01-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2147923&HistoricalAwards=false
• 关键词: Collaborative; Research; Mantle; dynamics; plate

Since its formation billions of years ago, Earth has been slowly cooling via convection, where warmer material rises to the surface and cold tectonic plates plunge deep into the interior. Understanding this process is important for a wide range of problems, such as determining the factors that drive plate tectonics to sustaining deep water and carbon cycles that stabilize the atmosphere/hydrosphere and climate throughout Earth history. This is particularly relevant for society, both because plate tectonic processes are the driving forces behind hazards such as earthquakes and volcanoes and also because our climate and atmosphere make Earth habitable. This project studies large scale convection in the solid Earth by using earthquake data recorded at distant seismic stations in regions of upwelling. Upwellings are notoriously difficult to constrain. The focus will be on three end-member cases: 1) Hawaii, the classic example where deep upwelling occurs beneath ocean lithosphere, 2) Yellowstone, the classic example of deep upwelling that occurs beneath a continent, and 3) the equatorial mid-Atlantic Ridge, which is typically assumed to be a location without deep upwellings. The project will use classic techniques and also some newly developed approaches to better constrain the scale of the upwellings and their pathways. An outreach program will increase diversity in the Earth Sciences via public engagement and education and training of students and early career researchers. The outreach portion includes visits to core discipline classrooms at Morgan State University, a historically black university, with an established connection with two faculty members there. The goal of the visits and the materials is to increase awareness of career possibilities and also the societal relevance of the Earth Sciences. The approach is modeled on the success of the "Google in Residence" program that successfully placed Google engineers on HBCU campuses. Eventually a wider range of Earth scientists will be included by developing an archive of materials that can be scaled and adapted according to needs and also best-practice advice, both of which will be made publicly available to the broader scientific community. The project also provides training for undergraduate students, a graduate student, and a post doc in cutting-edge methodologies and use of seafloor seismic data. Earth’s convective system is important for understanding the evolution of the planet, including everything from the factors that drive and enable plate tectonics to sustaining deep water and carbon cycles that stabilize the atmosphere/hydrosphere and climate over billions of years. It is generally accepted that cool mantle sinks back into the Earth, e.g., at subduction zones, and hot mantle rises, e.g., beneath hotspots and mid ocean ridges. Although mantle tomography has resolved many seismically fast anomalies associated with subduction, imaging upwellings has proven more challenging, potentially because seismic waveforms have difficulty resolving thin slow conduits. Thus, the exact dimensions, locations, characteristics, origin depths and magnitudes of these thermal anomalies are poorly known, as well as their chemical and physical interaction with the surrounding mantle, such as in the transition zone and uppermost mantle. Similarly, the degree to which upwellings change in size and/or are deflected during their ascent and how they vary among tectonic environments is uncertain. Converted and reflected wave imaging of the transition zone discontinuities and the lithosphere-asthenosphere boundary should provide tighter constraints. However, there are some discrepancies in studies using these methods regarding hotspot character and location, perhaps because they have used different methodologies and approaches with different sensitivities in different locations. This study will examine this issue systematically at a varied suite of tectonic environments. The planned approach will use the complementary sensitives of converted and reflected seismic phases to image the lithosphere-asthenosphere boundary and transition zone discontinuities beneath three key regions that are representative of the range of tectonic environments where variability in upwelling characteristics might be expected. These include the iconic hotspot of Hawaii near the center of an old oceanic plate, the classic example of Yellowstone hotspot beneath a continental interior, and finally the mid-Atlantic Ridge where deep upwellings are not predicted by classic models. Analyses will include P-to-S imaging of the transition zone discontinuities and S-to-P imaging of the lithosphere-asthenosphere boundary, both sensitive to vertical changes in shear wave velocity, and also use S-reflections, which have the added advantage of high depth resolution. A systematic approach will allow comparisons among the regions and anisotropic testing and F-K full-waveform modelling will be performed to determine the influence of anisotropy and/or focussing/defocussing for any apparent discrepancies. Once a full range of possibilities is defined from the seismic waveforms, inversions for Earth properties based on experimental and ab initio constraints will determine the properties that can explain the observations.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.

自数十亿年前形成以来，地球一直在通过对流缓慢冷却，其中较温暖的物质上升到表面，而寒冷的构造板块则深入内部。了解这一过程对于一系列问题都很重要，例如确定驱动板块构造维持深水和碳循环的因素，这些因素在整个地球历史中稳定了大气层/水圈和气候。这与社会特别相关，因为板块构造过程是地震和火山等灾害背后的驱动力，也因为我们的气候和大气使地球适合居住。该项目利用在上升流区域的遥远地震台站记录的地震数据研究固体地球中的大规模对流。动荡是出了名的难以控制。重点将放在三个端元的情况下：1）夏威夷，深上升流发生在海洋岩石圈之下的经典例子，2）黄石公园，深上升流发生在大陆之下的经典例子，和3）赤道大西洋中脊，这是通常被认为是一个位置没有深上升。该项目将使用经典技术和一些新开发的方法来更好地限制剧变的规模及其路径。一项推广计划将通过公众参与以及对学生和早期职业研究人员的教育和培训来增加地球科学的多样性。外联部分包括访问摩根州立大学的核心学科教室，这是一所历史悠久的黑人大学，与那里的两名教师建立了联系。访问和材料的目的是提高人们对地球科学的职业可能性和社会相关性的认识。这种方法是模仿成功的“谷歌在驻地”计划，成功地把谷歌工程师在HBCU校园。最终，将通过建立一个可根据需要调整和调整的材料档案库以及最佳做法建议，将更广泛的地球科学家包括在内，这两种材料都将向更广泛的科学界公开提供。该项目还为本科生、一名研究生和一名博士后提供尖端方法和海底地震数据使用方面的培训。地球的对流系统对于理解地球的演变非常重要，包括从驱动和实现板块构造的因素到维持数十亿年来稳定大气层/水圈和气候的深层水和碳循环的一切。人们普遍认为，冷地幔下沉回地球，例如，在俯冲带，热地幔上升，例如，在热点地区和大洋中脊下虽然地幔层析成像已经解决了许多与俯冲有关的地震快速异常，成像隆起已被证明更具挑战性，可能是因为地震波形难以解决薄慢管道。因此，这些热异常的确切尺寸、位置、特征、起源深度和量级以及它们与周围地幔（如过渡带和上地幔）的化学和物理相互作用知之甚少。同样地，隆起的大小变化和/或在上升过程中偏转的程度以及它们在构造环境中的变化程度也是不确定的。转换波和反射波成像的过渡带的不连续性和岩石圈-软流圈边界应提供更严格的限制。然而，在使用这些方法的研究热点的特点和位置有一些差异，也许是因为他们使用了不同的方法和不同的敏感性在不同的位置的方法。本研究将在不同的构造环境中系统地研究这一问题。计划中的方法将使用转换和反射地震相的互补敏感度来对三个关键区域下方的岩石圈-软流圈边界和过渡带不连续面进行成像，这些区域代表了一系列构造环境，其中可能会出现上升流特征的变化。这些包括靠近古老海洋板块中心的标志性热点夏威夷，大陆内部之下的黄石热点的经典例子，最后是大西洋中脊，经典模型没有预测到那里的深部隆起。分析将包括P-to-S成像的过渡带的不连续性和S-to-P成像的岩石圈-软流圈边界，都敏感的剪切波速度的垂直变化，并使用S反射，这具有额外的优势，深度分辨率高。系统方法将允许在区域之间进行比较，并将进行各向异性测试和F-K全波形建模，以确定各向异性和/或聚焦/散焦对任何明显差异的影响。一旦从地震波形中定义了所有可能性，基于实验和从头开始约束的地球属性反演将确定可以解释观测结果的属性。该奖项反映了NSF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。