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

合作研究：开发赤铁矿 (U-Th)/He 年代学以直接测定断层滑移和古地震活动的年代

• 批准号: 1419745
• 负责人: Peter Reiners
• 金额: $ 20.02万
• 依托单位: University of Arizona
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-09-01 至 2018-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1419745&HistoricalAwards=false
• 关键词: Collaborative; Research; Development; Hematite; Th

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.

脆性断层滑动的年代测定是表征众多上地壳过程的重要研究前沿。直接、稳健和有效获得的时间约束对于重建构造构造历史、造山过程和景观演化以及记录古代地震活动历史以评估现代地震危害、地震预报和断层力学是必要的。然而，由于适用的放射性同位素技术有限，对上地壳断层活动的直接测年具有挑战性，并且岩石记录中很少存在过去地震滑移的可靠地质指标。该项目测试了一种新的潜在变革方法，通过使用从活动断层和古代断层收集的断层表面氧化铁涂层上的 (U-Th)/He 来直接确定断层滑动的年代，并探索支持该方法的化学和物理原理。该项目通过以下方式推进预期的社会成果：(1) 通过为 K-12 学生的巡回展示、小学教育模块的开发和演示、参与大学外展活动和发展做出贡献，提高公众科学素养和公众对科学技术的参与。为犹他州北部在家接受教育的学生提供项目和课程； (2) 通过研究生和本科生的培训，培养具有全球竞争力的 STEM 劳动力； (3) 加强与其他学术机构和美国地质调查局的合作。该项目的主要研究目标是开发赤铁矿（U-Th）/He地质年代学作为直接断层滑动年代测定的方法。 (U-Th)/He 源自赤铁矿覆盖的断层表面，通过限制同运动赤铁矿形成的时间或断层期间摩擦加热的快速冷却来记录脆性变形事件。在某些情况下，这些日期可能会追踪区域变冷，从而产生一种新工具来量化与更广泛的断层带演化相关的构造折返或侵蚀。该方法在北美科迪勒拉山脉的三个活跃或古老的断层系统上进行了测试：犹他州的北部瓦萨奇断层带；科罗拉多前岭中部的断层；和育空地区的东部迪纳利断裂带。项目内容包括：（1）赤铁矿包覆断层的宏观表征和构造分析，以及赤铁矿产状、结构和粒度的微观测定； (2) 4He/3He扩散实验的He扩散率表征； (3) 对每个地点的赤铁矿覆盖断层进行有针对性的 (U-Th)/He 定年，包括来自同一断层表面的多个样本； (4) 利用X射线光电子能谱和4He/3He热测时法的过渡金属古测温法开发断层表面热史的独立约束； （5）分别使用 U-Pb 测年和相关联动方解石和富粘土断层泥的现有 40Ar/39Ar 伊利石年龄数据，以及主岩磷灰石的区域冷却模式（U- Th)/He 和裂变径迹数据。