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

在地球动力学和地震力学等地球科学的多个分支中，地壳应力震级的时空变化是一个活跃且长期存在的研究领域。这项研究调查了一种广泛使用的工具，该工具通过进行一系列的实验，将岩石样品的晶粒尺寸与应力水平联系起来，在这些实验中，岩石在变形时被冷却，模拟天然岩石在被断层带到地球表面时的冷却。该项目探索了这样一种可能性，即这些样品中粒度分布的统计数据可以用来提取有关这些岩石的应力和变形历史的有用信息。该项目正在通过以下方式推进预期的社会成果：(1)提高对地震过程的理解；(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）；(3)通过培训来自地球科学领域代表性不足群体的研究生和本科生，并支持两名早期职业科学家，发展一支多元化的、具有全球竞争力的STEM劳动力队伍。中地壳变形岩石的再结晶粒度通常用于约束应力差大小和岩石流变学。这种“晶粒尺寸压力测量法”需要假设微观结构处于稳定状态，然而大多数暴露的韧性剪切带在冷却时都会发生变形。冷却引起岩石强度的增加，因此，稳定的再结晶晶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与典型的自然剪切带相当时，再结晶晶粒尺寸明显不平衡——再结晶晶粒尺寸是达到稳态时的两倍。这一结果激发了本提案的主要目标：1)确定识别变形过程中受冷却影响的微观结构的可靠标准；2)开发约束此类岩石应力历史的工具包。目标1的初步工作表明，冷却斜坡实验与恒温实验在晶粒尺寸分布形状上存在系统差异。一套格里格斯钻机，在500°C的温度范围内进行了通用剪切冷却斜坡实验。这些实验将结合电子背散射衍射（EBSD）扫描电镜数据，这些数据来自约束良好的自然样品和在恒温下进行的档案实验，探索实验室结果对低温地质条件的外推，以及应变、初始晶粒尺寸、卟岩碎屑和次级相的存在的潜在影响。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 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.