Collaborative Research: Development of Hematite (U-Th)/He Chronology to Directly Date Fault Slip and Ancient Seismicity

脆性断层滑动定年是表征上地壳众多过程的重要研究前沿。直接的，强大的，有效地获得的时间约束是必要的重建结构构造的历史，造山过程和景观演变，以及记录古代地震活动的历史，以评估现代地震灾害，地震预报和断层力学。然而，由于有限的适用放射性同位素技术，直接测定上地壳断层活动的年龄是具有挑战性的，并且在岩石记录中存在过去地震滑动的可靠地质指标。该项目测试了一种新的潜在的变革性方法，通过使用从活动断层和古断层收集的断层表面上的氧化铁涂层上的（U-Th）/He来直接测定断层滑动的日期，并探索了该方法的化学和物理基础。该项目通过以下方式推进预期的社会成果：（1）通过为K-12学生的巡回展览、小学教育模块的开发和介绍、参与大学外联活动以及为北方犹他州在家接受教育的学生开发项目和课程，提高公众的科学素养和公众对科学技术的参与;（2）通过培养研究生和本科生，培养具有全球竞争力的STEM人才队伍;（3）加强与其他学术机构和美国地质调查局的合作。本项目的主要研究目标是发展赤铁矿（U-Th）/He地质年代学，作为直接测定断层滑动日期的方法。（U-Th）/He日期从赤铁矿涂层的断层表面记录脆性变形事件，通过约束的时间无论是同运动的赤铁矿形成或快速冷却的摩擦加热断层。在某些情况下，这些日期可以跟踪区域冷却，产生一个新的工具来量化与更广泛的断层带演化有关的构造折返或侵蚀。这种方法是在北美科迪勒拉三个活跃的或古老的断层系统进行测试：北方瓦萨奇断层带在犹他州;故障在中央科罗拉多前线山脉;和东部德纳里断层带在育空地区。该项目涉及：（1）赤铁矿覆盖断层的宏观表征和结构分析以及赤铁矿产状、结构和粒度的微观测定;（2）来自4 He/3 He扩散实验的He扩散率表征;（3）每个地点的赤铁矿覆盖断层的目标（U-Th）/He定年，包括来自同一断层面的多个样品;（4）利用X射线光电子能谱和~ 4 He/~ 3 He热年代学的过渡金属古温度计对断层表面热历史进行独立约束;（5）分别利用同运动断层泥中方解石和富粘土断层泥的U-Pb测年和已有的40 Ar/39 Ar伊利石年龄数据，建立断层活动时间的独立约束条件，和区域冷却模式从主岩磷灰石（U-Th）/He和裂变径迹数据。

Dating brittle fault slip is a research frontier essential for characterizing numerous upper crustal processes. Direct, robust, and efficiently obtained timing constraints are necessary to reconstruct structural-tectonic histories, mountain building processes, and landscape evolution, as well as to document ancient seismicity histories to assess modern seismic hazards, earthquake forecasting, and fault mechanics. However, direct dating of fault activity in the upper crust is challenging due to limited applicable radioisotopic techniques, and few robust geologic indicators of past seismic slip exist in the rock record. This project tests a new potentially transformative method to directly date fault slip by using the (U-Th)/He on iron oxide coatings on fault surfaces collected from active and ancient faults as well as explores the chemistry and physics underpinning the method. The project advances desired societal outcomes through: (1) increased public scientific literacy and public engagement with science and technology through contributions to traveling displays for K-12 students, development and presentation of elementary school educational modules, participation in university outreach activities, and development of projects and lessons for northern Utah home-schooled students; (2) development of a globally competitive STEM workforce through training of graduate and undergraduate students; (3) increased partnerships with other academic institutions and the U.S. Geological Survey.The principal research objective of this project is to develop hematite (U-Th)/He geochronology as a method to directly date fault slip. (U-Th)/He dates from hematite-coated fault surfaces record brittle deformation events by constraining the timing of either synkinematic hematite formation or the rapid cooling from frictional heating during faulting. In some cases, these dates may track regional cooling, yielding a new tool to quantify tectonic exhumation or erosion linked to broader fault zone evolution. This method is tested on three active or ancient fault systems in the North American Cordillera: the northern Wasatch fault zone in Utah; faults in the central Colorado Front Range; and the eastern Denali fault zone in the Yukon. The project involves: (1) macroscopic characterization and structural analysis of hematite-coated faults and microscopic determination of hematite occurrence, texture, and grain size; (2) characterization of He diffusivity from 4He/3He diffusion experiments; (3) targeted (U-Th)/He dating of hematite-coated faults at each locale including multiple samples from the same fault surface; (4) development of independent constraints on fault surface thermal histories using transition metal paleothermometry from X-ray photoelectron spectroscopy and 4He/3He thermochronometry; and (5) establishment of independent constraints on fault activity timing using U-Pb dating and existing 40Ar/39Ar illite age data of related synkinematic calcite and clay-rich fault gouge, respectively, and regional cooling patterns from host-rock apatite (U-Th)/He and fission track data.