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，而断层通常是在岩石深处强烈形成的。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掘出来的岩石中观察到，这些挖掘出来的断层可以用来了解发生在深处的过程。地震破裂应该会留下高温的证据，以及这些断层中高温的影响。由于热和流体的存在，断层及其周围发生了一系列的岩石变形，形成了断层滑动面。其中一些滑动面非常薄；确定断层滑动的温度，以及滑动过程中发生的过程，对于确定地震断层滑动是如何工作的很重要。进一步了解这一点可以更好地理解地震的物理原理。为了确定断层滑动的温度和过程，这项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(光学显微镜和扫描电子显微镜)和地球化学法进行的微观研究。此外，还将使用基于高能X射线的方法来研究与断层有关的岩石。这些X射线方法显示了元素在断裂带中的分布，一些元素可能是如何被断层加热转化的，以及矿物在断裂带中是如何转化的。这些X射线方法在变形岩石研究中的应用是新颖的。该项目通过为研究生和本科生提供分析方法方面的研究培训，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，并向学生传授一系列物理概念，从而促进预期的社会成果。本课程的目的是向学生提供基本概念，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，地震滑动应在地震震源区产生高温。对于一些断层，地震滑动集中在非常窄的滑动面上，沿着这些滑动面，高温是局部化的。因此，这些狭窄的滑动面应该显示出地震滑动导致的高温的证据。然而，几乎没有方法来确定结晶岩石中在孕震条件下形成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，确定天然断裂带中高温、变形机制和流体-岩石相互作用的时空分布，对于评估地震期间的断层滑动机制和能量分布至关重要。该项目的目标是：1)检验在滑动面上产生并保存高滑移温度的假设，2)发展用过渡元素测温确定断层温度的方法，3)确定导致滑移局部化和弱化的变形条件的物理和化学条件，4)研究滑动面及其附近的流体-岩石相互作用。峰值断层滑动温度将使用过渡元素测温法来估计，这是一种研究断层温度和与断层相关的变形的新方法。过渡元素测温是基于过渡元素(Fe、Mn、V)沿断层经历温度诱导还原的X射线吸收近边光谱。基于同步加速器的X射线荧光测绘和光谱分析将与扫描电子和全岩地球化学分析相结合，对圣安德烈亚斯、瓦萨奇、飓风和西萨尔顿拆离断层以及巴黎逆冲断层的挖掘和核心部分的断层滑动面进行分析。将确定与滑动有关的峰值温度，并寻找导致滑动定位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面上的峰值滑动温度，破译断层中可能的温度分布，并确定滑动局部化和弱化机制。这些努力需要在通常低于大多数光学分辨率技术的规模下对复杂变形的岩石进行检查。高分辨率、基于同步加速器的高能X射线荧光测绘和光谱分析将用于检查挖掘出的断层内的微米到毫米厚的滑动面，以及深度从150米到4公里的取心断层。从出土的正常断层中提取薄的“镜面”或抛光的过渡元素包裹的滑动面，以评估导致这些狭窄滑动面的热激活过程。高聚焦短波长X射线荧光测绘和光谱分析的使用，引入了一种检查极窄滑动面的创新方法，在这些滑动面上，地球化学性质和纹理无法通过常规形式的显微镜进行识别。这项研究的结果将能够确定断层滑动的条件，并检查这些断层中与地震滑动相关的变形的表现，提供变形机制和结构的基准，可以将实验岩石变形结果与之进行比较，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NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，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.