Collaborative Research: Theoretical and Experimental Investigation of Grain Damage and the Formation of Plate Boundaries

板块构造学描述了地球最上层岩层的运动，并支配着地球上几乎所有的地质活动，比如地震和火山。板块构造只发生在地球上，而没有发生在金星和火星上，而且地球在其45亿年的历史中是否一直有板块构造尚不清楚。地球上板块构造的出现取决于寒冷的地幔最上层——岩石圈——的岩石如何变形。当岩石变形时，它们会在破坏过程中变弱，从而集中变形，导致更多的破坏，以此类推。这种正反馈将岩石圈变形集中到微弱的狭窄板块边界，这是大多数地震和火山活动的发生地。该项目将通过理论和实验室相结合的方法，发展和应用岩石中矿物颗粒如何受损的物理学（称为“颗粒损伤”）。当矿物颗粒变形时，它们会积累缺陷，最终导致颗粒分解成更小的颗粒，这反过来又使岩石变弱。这一过程在被称为糜棱岩的岩石中表现得很明显，糜棱岩通常位于板块构造边界。这个项目将使用颗粒损伤物理学来了解板块构造是如何以及何时在早期地球上出现的。该项目还将研究颗粒的破坏如何影响板块边界的地震周期，特别是岩石如何减弱、恢复并将应力传递给其他岩石，最终引发更多地震。板块边界过程具有重要的人类影响，包括板块边界断裂系统的演化和地震的复发。该项目通过对两名早期职业女性科学家的支持和专业发展，特别是在理论和实验方法方面的跨学科培训，促进了多样性。该项目还将包括为学生和青年科学家组织一次关于板块边界演变的专题讨论会，并将为一个已建立的科学可视化设施的教育和推广活动提供新的科学材料。本项目将推进岩石圈颗粒级物理和变形机制的研究，并将其应用于板块构造的产生和作用。PI将扩展岩石圈弱化和板块边界形成的颗粒损伤理论，包括矿物相混合和多矿物材料中的位错动力学，并通过岩石变形实验进行校准和测试。这些新的发展对于解决两个主要的科学问题是必要的：板块构造的出现：古太古代地球的板块边界是如何形成的？地球早期的热学和岩石学演化，即冷却、地壳生产减少和岩石圈成分的变化，是否影响了岩石圈变形的局部化和板块构造的出现？具体来说，PI的假设是温度和熔融影响矿物组成，然后通过颗粒损伤和岩石学阶段之间的混合的正反馈影响岩石的弱化。震后蠕变和岩石圈剪切带：岩石圈中的瞬态韧性行为，以及相关的微观结构变化，如晶粒尺寸和位错密度，如何影响地震后应力的积累、传递和释放？这种反应如何影响地震的复发周期和触发？继承下来的岩石圈软弱，以及由此产生的板块边界年龄，是否会影响震后恢复和地震周期？为了解决这些问题，研究人员将利用实验确定的流变定律，将颗粒级物理的新发展与板块级地球动力学模型联系起来。这些多尺度模型将为板块边界形成与上地幔热化学演化以及地壳孕震行为的耦合提供平台。该项目的最终科学目标是了解早期地球板块构造的出现，以及板块边界的演化和行为如何影响震后反应和地震周期。这些问题跨越了地球表面深时演化的时间尺度，新构造环境的发展，以及地震活动带的行为。这项提议的工作将利用理论和实验来更好地理解地质和人类时间尺度上的地球构造过程。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。

Plate tectonics describes the motion of Earth's upper-most rocky layers and governs almost all geological activity on Earth, such as earthquakes and volcanoes. Plate tectonics is only known to occur on Earth, but not on Venus and Mars, and whether Earth always had plate tectonics in its 4.5 billion-year history is unknown. The emergence of plate tectonics on Earth depends on how rocks in the cold uppermost layers of the mantle - the lithosphere - deform. As rocks deform they can weaken through the process of damage, which concentrates deformation, resulting in more damage, and so on. This positive feedback focuses lithospheric deformation into weak narrow plate boundaries, which are the locus of most seismic and volcanic activity. This project will develop and apply the physics of how mineral grains in rocks are damaged (termed "grain-damage") through combined theoretical and laboratory approaches. When mineral grains are deformed they accumulate defects, which eventually leads to grains breaking down into smaller grains, and this in turn makes the rocks weaker. This process is evident in rocks called mylonites that are often found at plate tectonic boundaries. This project will use grain damage physics to understand how and when plate tectonics arose on early Earth. The project will also study how damage in grains influences cycles of earthquakes at plate boundaries, specifically how rocks weaken, recover and transmit stress to other rocks and eventually trigger more earthquakes. Plate boundary processes have a significant human impact, including evolution of plate boundary fault systems and earthquake recurrence. The project promotes diversity through the support and professional development of two early career female scientists, specifically with interdisciplinary training in theoretical and experimental methods. The project will also involve organization of a symposium on the evolution of plate boundaries for students and young scientists, and will contribute new scientific materials to education and outreach activities at an established scientific visualization facility. This project will advance the study of lithospheric grain-scale physics and deformation mechanisms, and their application to the generation and operation of plate tectonics. The PI's will extend the grain damage theory for lithospheric weakening and plate boundary formation to include mineralogical phase mixing and dislocation dynamics in polymineralic materials, with calibration and testing by rock deformation experiments. These new developments are necessary to address two major scientific questions:1. Emergence of plate tectonics: How did tectonic plate boundaries form in the ancient Archean Earth? Did thermal and petrological evolution in the early Earth, namely cooling, decreasing crustal production, and changes in lithospheric composition, affect localization of deformation in the lithosphere and the emergence of plate tectonics? Specifically, the PI's hypothesize that temperature and melting affect mineral composition, which can then influence rock weakening through the positive feedback of grain damage and mixing between petrological phases.2. Post-seismic creep and lithospheric shear zones: How does transient ductile behavior in the lithosphere, with associated changes in microstructure like grain-size and dislocation density, influence the accumulation, transmission and release of stress following an earthquake? How does this response affect earthquake recurrence cycles and triggering? Does inherited lithospheric weakness, and thus age of a plate boundary, influence post-seismic recovery and earthquake cycles?To address these questions, the investigators will connect new developments in grain-scale physics to plate-scale geodynamic models using experimentally determined rheological laws. These multi-scale models will provide the platform for coupling plate-boundary formation to thermo-chemical evolution of the upper mantle, and to seismogenic behavior in the crust. The ultimate scientific goal of this project is to understand emergence of plate tectonics on early Earth, and how plate boundary evolution and behavior influences post-seismic response and earthquake cycles. These issues span the time scales of the evolution of Earth's surface in deep time, the development of the neotectonic environment, and the behavior of seismically active zones. The proposed work will use theory and experiments to better understand Earth's tectonic processes on the geological and human time scales.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.