EAR-PF: Fault-rock trapped-charge and 4He/3He thermochronometry: new paleothermometers to assess scales and rates of fault slip

玛格丽特·奥德伦博士获得了NSF EAR博士后奖学金，在亚历克西斯·K·奥尔特和塔米·里特努尔教授的指导下，在犹他州立大学(USU)开展研究和专业发展计划。该项目的目标是开发新的方法，在断层岩中提取过去的地震指纹。查明古代地震活动的证据对于理解地震过程和机制以及建立更完整的断层活动记录非常重要，这些记录共同为地震活动断裂系统的危险性评估提供了信息。地震期间的大部分能量以热的形式释放出来，这在断层岩中产生了热和/或化学特征。拟议的研究使用矿物的发光特性和氦同位素测温与结构分析相结合的方法，获取地震事件在整个地震滑动速率范围内的热记录，以及时间和空间尺度，以告知断层滑动过程。这些方法提供的互补年龄信息将提供从历史到地质时间尺度上更连续的变形记录，这对于了解特定断层的地震周期是必不可少的。这项研究将在犹他州的瓦萨奇和飓风断层上进行。这些是年轻的、地震活跃的断层，分别毗邻犹他州沿瓦萨奇前线和圣乔治地区的两个最大的人口中心。这里开发的新方法将为研究人员量化其他断层系统中断层滑动行为的频谱创造途径。奥德伦博士的职业发展计划旨在增加STEM的劳动力，并让妇女参与地震科学研究和岩土技术技能建设。在犹他州，STEM中女性的比例较低。与犹他州地质调查局合作开发的地震灾害模块将影响参加新的USU GeoPath计划的学生，并通过与位于山间地震带和Wasatch Fronch地区的其他大学共享该模块，以及通过USU地球科学网页和GeoMinents YouTube频道在线共享该模块，从而扩大这项工作的覆盖范围和影响。这项研究的目标是开发新的方法，在全范围的地震滑移率(微米/秒到数十米/秒)内识别、量化和确定岩石记录中地震滑动引起的摩擦加热。地震期间能量收支的很大一部分(~90%)用于克服摩擦阻力的功，并以热量的形式消散。断层上的温升幅度是滑动速度、材料特性和应变局部化的函数。摩擦热反过来影响这些因素，而变量之间的相互作用控制着地震旋回中的断层强度。瞬时的高温使断层岩具有结构和/或地球化学特征。该项目将结合对石英和长石的俘获电荷分析与赤铁矿和磷灰石的4He/3He断裂面和下伏围岩的4He/3He热力学计时，以恢复与地震滑动有关的古温度。犹他州地震活跃的瓦萨奇断层和飓风断层将是应用这一新方法的第一个现场。这两条发震正断层都有过去同震摩擦热的结构证据，块状赤铁矿(U-Th)/He年龄反映了地震滑动期间的热重置或成矿作用，在捕获电荷热力学年代学的可行年龄范围内(0.5 Ma)，并对犹他州的大型人口中心构成了地震风险。来自断裂面的古温度和结构信息的耦合将为同震滑动过程中的动态弱化机制提供信息，并提高对伴随着断层热演化的断层物质的流变性变化以及上部孕震带的现场地震力学和断层强度的理解。来自俘获电荷和4He/3He热年代学的断层滑动的互补时间信息将填补历史、大地测量和古地震(1-10000年)和低温(10万-100万年)形变记录之间的关键时间数据空白，直接应用于地震灾害评估。这里应用的提取同震断层温度和变形时间约束的新的热计时方法可以移植到其他断层系统，为研究人员提供新的工具来访问跨越地震滑移率的断层滑动记录。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。

Dr. Margaret Odlum has been awarded the NSF EAR Postdoctoral Fellowship to carry out research as well as a professional development plan at Utah State University (USU) under the mentorship of Professors Alexis K Ault and Tammy Rittenour. The goal of the project is to develop new approaches to fingerprint past earthquakes in fault rocks. Pinpointing evidence of ancient seismicity is important for understanding earthquake processes and mechanics and building a more complete record of fault activity, which collectively inform hazard assessment in seismically active fault systems. Most of the energy during an earthquake is given off as heat, which imparts a thermal and/or chemical signature in fault rocks. Proposed research uses coupled luminescence properties of minerals and He isotope thermometry integrated with textural analysis to access the thermal record of earthquake events across the full range of seismic slip rates, as well as temporal and spatial scales to inform fault slip processes. Complementary age information provided by these methods will provide a more continuous record of deformation over historical to geological timescales which is essential to understand earthquake cycles in a particular fault. This research will be conducted on the Wasatch and Hurricane faults in Utah. These are young, seismically active faults that are adjacent to Utah’s two largest population centers along the Wasatch Front and in the Saint George region, respectively. New approaches developed here will create pathways for researchers to quantify the spectrum of fault slip behavior in other fault systems. Dr. Odlum’s professional development plan is aimed at growing STEM workforce and engaging women, an underrepresented group in Utah in STEM, in earthquake science research and geotechnical skill-building. A seismic hazards module developed in partnership with the Utah Geological Survey will impact students enrolled in the new USU GeoPaths program, and reach a wider undergraduate community by sharing the module with other universities located in the Intermountain Seismic Zone and Wasatch Front region, as well as online through the USU Geosciences webpage and GeoMinutes YouTube channel, thus extending the reach and impact of this work.The goal of this research is to develop new approaches to identify, quantify, and date frictional heating caused by seismic slip in the rock record across the full range of seismic slip rates (micrometers/sec to tens of meters/sec). Much (~90%) of the energy budget during an earthquake goes into work done overcoming frictional resistance and dissipates as heat. The magnitude of temperature rise across a fault is a function of slip velocity, material properties, and strain localization. Frictional heat, in turn, impacts these factors and the interplay between variables controls fault strength during the seismic cycle. Transient, elevated temperatures impart textural and/or geochemical signatures in fault rocks. This project will combine trapped-charge analysis on quartz and feldspar with hematite and apatite 4He/3He thermochronometry of fault surfaces and underlying host-rock to recover paleotemperatures associated with seismic slip. The seismically active Wasatch and Hurricane faults in Utah will be the first field sites for applying this novel approach. Both seismogenic normal faults have textural evidence of past coseismic frictional-heat, bulk hematite (U-Th)/He dates that reflect thermal resetting or mineralization during earthquake slip within the viable age range for trapped-charge thermochronometry (0.5 Ma), and pose seismic risk to large population centers in Utah. Coupled paleotemperature and textural information from fault surfaces will inform dynamic weakening mechanisms operative during coseismic slip and improve understanding of rheological changes to fault materials that accompany fault thermal evolution and in-situ earthquake mechanics and fault strength in the upper seismogenic zone. Complementary temporal information on fault slip from trapped-charge and 4He/3He thermochronometry will fill a critical temporal data gap between historical, geodetic, and paleo-seismic (1 - 10,000 years) and low-temperature thermochronometric (100,000 - 1,000,000 years) records of deformation, with direct applications to seismic hazards assessment. The new thermochronometric approach to extract coseismic fault temperatures and temporal constraints on deformation applied here is transferable to other fault systems, giving researchers new tools to access the record of fault slip across seismic slip rates.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.