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

断代脆性断层滑动是描述众多上地壳过程的重要研究前沿。对于重建构造历史、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险性、地震预报和断层力学，直接、稳健和有效地获得时间约束是必要的。然而，由于可应用的放射性同位素技术有限，直接测定上地壳断层活动的年龄是具有挑战性的，而且岩石记录中几乎没有关于过去地震滑动的强有力的地质标志。该项目测试了一种新的具有潜在变革性的新方法，即利用从活动和古老断裂中收集的断层表面铁氧化物涂层上的(U-Th)/He来直接测定断层滑动的年龄，并探索了该方法所依据的化学和物理基础。该项目通过以下方式推进预期的社会成果：(1)通过为K-12学生提供巡回展示，开发和展示小学教育模块，参与大学外联活动，以及为犹他州北部在家接受教育的学生制定项目和课程，提高公众的科学素养和公众对科学技术的参与度；(2)通过培训研究生和本科生，发展具有全球竞争力的STEM劳动力；(3)增加与其他学术机构和美国地质调查局的伙伴关系。该项目的主要研究目标是开发赤铁矿(U-Th)/He地质年代学，作为直接测定断层滑动日期的一种方法。来自赤铁矿覆盖的断层表面的(U-Th)/He测年记录了脆性变形事件，这是通过限制同运动型赤铁矿形成的时间或断层期间摩擦加热的快速冷却来实现的。在某些情况下，这些日期可能跟踪区域冷却，从而产生了一种新的工具，可以量化与更广泛的断裂带演化有关的构造折返或侵蚀。这种方法在北美科迪勒拉的三个活动或古老的断裂系统上进行了测试：犹他州的北部Wasatch断裂带；科罗拉多前缘山脉中部的断裂；以及育空地区的东部德纳利断裂带。该项目包括：(1)赤铁矿包裹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)4He/3He扩散实验的He扩散系数表征；(3)每个地点赤铁矿包裹断层的定向(U-Th)/He定年，包括来自同一断层表面的多个样品；(4)利用X射线光电子能谱和4He/3He热计时的过渡金属古温度法建立断层表面热历史的独立约束；(5)分别利用同运动方解石和富粘土断层泥的U-Pb定年和已有的40Ar/39Ar伊利石年龄数据，以及寄主岩石磷灰石(U-Th)/He和裂变径迹的区域冷却模式，建立了对断层活动时间的独立约束。

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.