Cooling During Deformation: An Overlooked Scenario with Implications for the Analysis of Ductily Deformed Rocks

地壳应力值的时空变异性是地球动力学、地震力学等多个地学分支的活跃和长期研究领域。这项研究调查了一种广泛使用的工具，它将岩石样本的粒度与应力水平联系起来，进行了一系列实验，其中岩石在变形时被冷却，模拟了天然岩石被断层带到地球表面时的冷却过程。该项目探索了这种样品中粒度分布的统计数据可以用来提取关于这种岩石的应力和变形历史的有用信息的可能性。该项目正在通过以下方式促进预期的社会成果：(1)提高对地震过程的了解；(2)妇女和代表性不足的少数民族充分参与科学、技术、工程和数学(STEM)；以及(3)通过培训地球科学中代表性不足的群体的研究生和本科生以及支持两名早期职业科学家，发展一支多样化的、具有全球竞争力的STEM劳动力队伍。这种“晶粒度测压”需要假设微结构处于稳定状态，然而大多数暴露的韧性剪切带在冷却过程中都经历了变形。冷却导致岩石强度增加，因此，稳定的再结晶颗粒尺寸是一个移动的目标。最近的岩石变形实验表明，当冷却速率与应变速率之比相当于典型的天然剪切带时，再结晶颗粒尺寸明显失衡--再结晶颗粒尺寸是达到稳定状态时的两倍。这一结果推动了这项建议的主要目标：1)确定识别变形过程中受冷却影响的微结构的可靠标准，以及2)开发用于约束此类岩石的应力历史的工具包。目标一号的初步工作表明，冷却斜坡实验和恒温实验的粒度分布形状存在系统差异。在Griggs试验台上进行了500°C温度范围内的普通剪切冷却斜坡实验。这些实验将与电子背向散射衍射(EBSD)扫描电子显微镜数据以及在恒温下进行的严格限制的自然样品的扫描电子显微镜数据相结合，探索将实验室结果外推到低温地质条件，以及应变、初始颗粒大小、碎斑和次生相的存在的潜在影响。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The spatial and temporal variability of stress magnitudes in Earth’s crust are active and longstanding research areas in several branches of the geosciences, e.g. geodynamics and earthquake mechanics. This study investigates a widely used tool which relates grain size of rock samples to stress levels by conducting a series of experiments in which rocks are cooled as they are deformed, mimicking the cooling of natural rocks as they are carried by faults towards Earth’s surface. The project explores the possibility that the statistics of grain size distributions in such samples can be used to extract useful information about the stress and deformation history of such rocks. The project is advancing desired societal outcomes through: (1) improved understanding of processes involved in earthquakes; (2) full participation of women and underrepresented minorities in science, technology, engineering and mathematics (STEM); and (3) the development of a diverse, globally competitive STEM workforce through training of graduate and undergraduate students from underrepresented groups in the Earth sciences and support of two early career scientists.The recrystallized grain size of rocks deformed in the middle crust is routinely used to constrain differential stress magnitudes and rock rheology. Such “grain size piezometry” requires an assumption of microstructural steady state, however most exposed ductile shear zones experienced deformation while cooling. Cooling causes an increase in rock strength, and as a result, a stable recrystallized grain size is a moving target. Recent rock deformation experiments demonstrate that, at ratios of cooling rate to strain rate equivalent to those of typical natural shear zones, recrystallized grain size is markedly out of equilibrium—recrystallized grain sizes are twice as large as would occur if a steady-state were achieved. This result motivates the primary goals of this proposal: 1) determine robust criteria for recognizing microstructures that are affected by cooling during deformation, and 2) develop a toolkit for constraining stress histories from such rocks. Preliminary work on goal one demonstrates that there are systematic differences in the shapes of grain size distributions of cooling ramp experiments versus experiments at constant temperature. A suite of Griggs Rig, general-shear cooling-ramp experiments over a 500°C range of temperature are conducted. These experiments will--in conjunction with Electron backscatter diffraction (EBSD) scanning electron microscopy data from well-constrained natural samples and archival experiments carried out at constant temperature—explore extrapolation of laboratory results to low temperature geologic conditions, and potential effects of strain, initial grain size, and the presence of porphyroclasts and secondary phases.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.