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.

脆性断层滑动定年是研究许多上地壳过程的重要前沿。直接、可靠和有效地获得时间约束对于重建构造-构造历史、造山过程和景观演变，以及记录古代地震活动历史以评估现代地震灾害、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，上地壳断层活动的直接测年具有挑战性，而且岩石记录中很少存在过去地震滑动的可靠地质指标。该项目测试了一种潜在的革命性新方法，通过使用从活动断层和古老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促进了预期的社会成果：(1)通过为K-12学生提供巡回展览，开发和展示小学教育模块，参与大学外展活动，以及为北犹他州家庭教育学生开发项目和课程，提高了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伙伴关系。本项目的主要研究目标是发展赤铁矿(U-Th)/He年代学，作为断层滑动直接测年的方法。（U-Th）/He测年记录了赤铁矿包裹的断层表面的脆性变形事件，它限制了赤铁矿形成的同步时间或断层过程中摩擦加热的快速冷却时间。在某些情况下，这些日期可能会追踪区域冷却，从而产生一种新的工具来量化与更广泛的断裂带演化有关的构造挖掘或侵蚀。该方法在北美科迪勒拉的三个活跃或古老的断层系统上进行了测试：犹他州的北瓦萨奇断裂带；科罗拉多前山脉中部的断层；以及育空地区的德纳里断裂带东部。项目内容包括：(1)赤铁矿包覆断层的宏观表征和构造分析，赤铁矿产状、结构和粒度的微观测定；(2)通过4He/3He扩散实验表征He扩散率；(3)利用来自同一断层面的多个样品，对每个地点的赤铁矿包裹断层进行定向(U-Th)/He定年；(4)利用x射线光电子能谱和4He/3He热时法建立了断层表面热史的独立约束；(5)利用U-Pb定年和相关同动方解石和富泥断层泥的40Ar/39Ar伊利石年龄数据，以及主岩磷灰石(U-Th)/He和裂变径迹数据的区域冷却模式，分别建立断层活动时间的独立约束。