Examining temperatures and microgeochemical processes on fault slip surfaces with synchrotron methods

地震是由地壳深处的断层滑动引起的，而断层通常是在深处坚硬的岩石中形成的。研究地震对岩石的影响的机会是有限的；有时，地震的影响可以在从深处挖出的岩石中观察到，这些挖出的断层可以用来了解发生在深处的过程。地震破裂应该会在这些断层中留下高温和高温影响的证据。由于热量和流体的存在，断层内部和周围发生了一系列的岩石变形，断层滑动面就形成了。有些滑动面非常薄；确定断层滑动的温度，以及在滑动过程中发生的过程，对于确定地震断层滑动是如何发生的非常重要。进一步加深这种认识可以使我们更好地了解地震的物理性质。为了确定断层滑动的温度和过程，本研究使用了一系列方法在非常小的尺度上检查岩石。这些方法包括用标准显微镜方法（光学和扫描电子显微镜）进行微观研究和用地球化学方法进行研究。此外，基于高能x射线的方法将用于观察与断层有关的岩石。这些x射线方法显示了元素是如何在断裂带中分布的，一些元素可能是如何通过断层加热而转变的，以及矿物是如何在断裂带中转变的。这些x射线方法在变形岩石研究中的应用是新颖的。该项目通过对研究生和本科生进行分析方法的研究培训，以及为本科生开发短期课程和教学模块，将x射线分析应用于地质问题，并教授学生一系列物理概念，从而促进预期的社会成果。其目的是为学生提供基本概念，并介绍材料科学分析。由于岩石的高摩擦强度和克服这些强度所需的应力，地震滑动应该在地震的震源区域产生高温。对于某些断层，地震滑动集中在非常狭窄的滑动面上，高温沿着这些滑动面局部分布。因此，这些狭窄的滑动面应该显示出由地震滑动引起的高温的证据。然而，目前很少有方法来确定结晶岩中在发震条件下发育的断层滑动的峰值温度。估计断层峰值温度，记录同震滑动定位的证据，确定高温的时空分布、变形机制和自然断裂带的流体-岩石相互作用，对于评估地震期间断层滑动力学和能量分布至关重要。该项目的目标是：1)验证在滑动表面产生并保持高滑动温度的假设，2)发展用过渡元素测温法确定断层温度的方法，3)确定导致滑动局部化和减弱的变形条件的物理和化学条件，以及4)检查滑动表面及其附近的流体-岩石相互作用。将使用过渡元素测温法估计断层滑动峰值温度，这是一种研究断层温度和断层相关变形的新方法。过渡元素测温是基于x射线吸收近边光谱的过渡元素（Fe, Mn， V）沿断层经历温度诱导还原。基于同步加速器的x射线荧光测绘和光谱学将与扫描电子和全岩石地球化学分析相结合，对圣安地列斯断层、瓦萨奇断层、飓风断层、西索尔顿剥离断层和巴黎逆冲断层的挖掘和核心部分的断层滑动面进行分析。将确定与滑动相关的峰值温度，并寻找导致滑动局部化和断层演化的高温流体-岩石相互作用的证据。这项工作的变革性在于努力确定滑动表面的峰值滑动温度，破译断层中可能的温度分布，并确定滑动局部化和减弱机制。这些工作需要在通常低于大多数光学分辨率技术的尺度上检查复杂变形的岩石。高分辨率、基于同步加速器的高能x射线荧光测绘和光谱学将用于检查挖掘断层内微米至毫米厚的滑动表面以及深度从150米到4公里的核心断层。从挖掘出的正常断层中获得薄的“镜像”或抛光的过渡元素涂层滑动表面，以评估造成这些狭窄滑动表面的热激活过程。高度聚焦的短波x射线荧光测绘和光谱学的使用引入了一种创新的方法，可以检查通过传统显微镜无法识别的极窄滑动表面的地球化学和结构。研究结果将能够确定断层滑动的条件，并检查这些断层中地震前后滑动相关变形的表现，提供变形机制和纹理的基准，与实验岩石变形结果进行比较，有助于约束断裂带加热和滑动定位的理论模型。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。

Earthquakes result from slip along faults at depth in the Earth's crust, and faults typically form in rocks at depth that are strong. Opportunities to examine the effects of earthquakes in rocks are limited; sometimes the effects of earthquakes can be observed in rocks that are exhumed from depth and these exhumed faults can be used to learn about processes that occur at depth. Earthquake rupture should leave evidence for high temperatures and the effects of high heat in these faults. Because of the heat and the presence of fluids, a range of rock transformations occur in and around faults, and fault slip surfaces form. Some of these slip surfaces are very thin; determining the temperatures of fault slip, and the processes that occur during slip, are important for determining how earthquake fault slip works. Furthering this understanding can lead to better understanding of the physics of earthquakes. In order to determine the temperatures and processes of fault slip, this study uses a range of methods to examine rocks at very small scales. These methods include microscopic studies with standard microscopy methods (optical light and scanning electron microscopy) and geochemical methods of study. In addition, high-energy X-ray based methods will be used to look into fault-related rocks. These X-ray methods show how elements are distributed in the fault zones, how some of the elements might have been transformed by fault heating, and how minerals are transformed in the fault zones. The applications of these X-ray methods are novel in the study of deformed rocks. The project advances desired societal outcomes through research training in analytical methods for graduate and undergraduate students and development of short courses and teaching modules for undergraduate students that use X-ray analyses applied to geological questions and teach students a range of physics concepts. The aims are to provide fundamental concepts to the students and to provide an introduction into materials science analysis.Seismic slip should produce high temperatures in the focal region of earthquakes due to the high frictional strength of rocks and the stresses required to overcome these strengths. For some faults, seismic slip is focused on very narrow slip surfaces along which high temperatures are localized. Thus, these narrow slip surfaces should exhibit evidence for high temperatures that result from seismic slip. Yet, few methods exist to determine the peak temperatures of fault slip developed at seismogenic conditions in crystalline rocks. Estimating peak fault temperatures, documenting the evidence for coseismic slip localization, and determining the temporal and spatial distribution of high temperatures, deformation mechanisms, and fluid-rock interactions in natural fault zones are critical for assessing fault slip mechanics and energy distribution during earthquakes. The objectives of this project are to: 1) test the hypothesis that high slip temperatures are generated and preserved on slip surfaces, 2) develop methods of determining fault temperatures with transition element thermometry, 3) determine the physics and chemistry of the conditions of deformation that result in slip-localization and weakening, and 4) examine fluid-rock interactions in and near slip surfaces. Peak fault slip temperatures will be estimated using transition element thermometry, a new and novel method of investigating fault temperatures and fault-related deformation. Transition element thermometry is based on X-ray Absorption Near Edge Spectroscopy of transition elements (Fe, Mn, V) along faults that experience temperature-induced reduction. Synchrotron-based X-ray fluorescence mapping and spectroscopy will be integrated with scanning-electron and whole-rock geochemical analyses of fault slip surfaces from exhumed and cored parts of the San Andreas, Wasatch, Hurricane, and the West Salton detachment faults, and the Paris thrust fault. Peak slip-related temperatures will be determined and evidence of high-temperature fluid-rock interactions that lead to slip localization and fault evolution will be sought. The transformative nature of this work lies in the efforts to determine peak slip temperatures on slip surfaces, to decipher the possible temperature distribution in faults, and to identify slip localization and weakening mechanisms. These efforts require examination of complexly deformed rocks at scales typically below most optical resolution techniques. High-resolution, synchrotron-based high-energy X-ray fluorescence mapping and spectroscopy will be used to examine micrometer- to millimeter-thick slip surfaces within exhumed faults and from cored faults at depths ranging from 150 meters to 4 kilometers. Thin 'mirrored' or polished transition element-coated slip surfaces from exhumed normal faults to evaluate thermally activated processes responsible for these narrow slip surfaces. The use of highly focused short-wavelength X-ray fluorescent mapping and spectroscopy introduces an innovative approach of examining extremely narrow slip surfaces where geochemistry and textures cannot be discerned through conventional forms of microscopy. The results of the study will enable determination of the conditions of fault slip and examination of the manifestation of co- and post-seismic slip related deformation in these faults, providing benchmarks of deformation mechanisms and textures against which experimental rock deformation results can be compared, helping constrain theoretical models of fault zone heating and slip localization.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.