Collaborative Research: Constraints on Initiation of Low-Angle Normal Faults Within the Seismogenic Regime

合作研究：发震区内低角度正断层萌生的制约因素

• 批准号: 1305610
• 负责人: Craig Grimes
• 金额: $ 13.63万
• 依托单位: Ohio University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-08-27 至 2017-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1305610&HistoricalAwards=false
• 关键词: Collaborative; Research; Constraints; Initiation; Low

Low-angle normal faults are widely recognized in extended continental and oceanic lithosphere, yet the mechanisms influencing their initiation and protracted slip at shallow depths through the seismogenic crust remain controversial. Compilations of normal faulting earthquake focal mechanisms show only a small fraction of all earthquakes on faults dipping less than 30°, and fault mechanics predicts that normal faults with dips less than 30° should neither form nor be reactivated unless they are anomalously weak. In contrast, field observations indicate that slip within the seismogenic crust has occurred. To address this paradox, we are conducting multidisciplinary field and laboratory structural and isotopic studies of a very well exposed low-angle normal fault in the Chemehuevi Mountains of California. Our investigation will provide macro- and microstructural characterization of natural fault rocks and documentation of the role of fluids during initiation and slip along the fault. These observations will be combined with laboratory experiments by colleagues in Europe and recent microseismicity studies, to address processes allied with low-angle normal fault initiation. The study site is the near-ideal natural laboratory to address the relationship between internal structure and mechanical behavior of low-angle normal faults, because 1) regional mapping has established key areas of fault exposure; 2) the footwall is underlain by undeformed granitoids, and it can reasonably be assumed that the protolith is dominated mechanically by a two-phase mixture of quartz and feldspar; 3) the fault system comprises multiple low-angle normal faults with variable slip (about 2 to 18 kilometers) and is well exposed (greater than 18 km down dip and 25 km along strike), facilitating analysis of spatial variations in deformation mechanisms from initiation to maturity, and 4) the thermal history of the footwall is well documented, providing an opportunity to examine rocks with initial ambient temperatures between approximately 150-400°C across the brittle-plastic transition.To illuminate the microstructural evolution and fault weakening mechanisms that lead to low-angle normal faulting, we are integrating field observations with petrographic, SEM (EBSD, image analysis characterization of microstructures, and cathodoluminescence), and stable isotope studies. low-angle normal faults exposed in the Basin and Range provide key examples of this globally significant class of faults and highlight questions about slip at gentle dips in the brittle crust, yet initiation of these fault systems has previously received little attention. In situ stable isotope analyses by ion microprobe allow us to ?see through? secondary alteration during the late slip and fluid migration history and correlate detailed microstructures with associated fluids. By combining these field and laboratory investigations with experimental studies by colleagues in Europe, this research bridges disciplinary boundaries in an effort to transform the understanding of frictional-viscous processes and their manifestation in the geologic record. Collaboration with scientists in Norway and Switzerland will contribute to scaling of laboratory experiments, providing relevant information for earthquake hazard assessment. Undergraduate students will be engaged throughout the project in field and laboratory studies, gaining experience conducting individual research and outreach. Outreach to the broader community is being achieved through the development of virtual field trips using Google Earth and associated educational materials designed for grade school through college students.

低角度正断层在扩展的大陆和海洋岩石圈中得到广泛认可，但影响其在发震地壳浅层启动和长期滑动的机制仍然存在争议，正断层地震震源机制的汇编仅显示了所有地震的一小部分。断层倾角小于 30°，断层力学预测倾角小于 30° 的正断层不会形成，也不会重新活动，除非它们异常弱。相反，现场观察表明：为了解决这一悖论，我们正在对加利福尼亚州切梅韦维山脉的一个暴露良好的低角度正断层进行多学科现场和实验室结构和同位素研究，我们的研究将提供其宏观和微观结构特征。天然断层岩石以及流体在断层启动和滑动过程中的作用的记录，这些观察结果将与欧洲同事的实验室实验和最近的微震活动研究相结合，以解决与低角度正断层启动相关的过程。研究地点是研究低角度正断层内部结构与力学行为之间关系的近乎理想的自然实验室，因为 1) 区域测绘已经确定了断层暴露的关键区域；2) 下盘下方是未变形的花岗岩，并且可以合理地假设原岩在力学上主要由石英和长石的两相混合物组成；3）断层系统包括多个具有可变滑移的低角度正断层（约2至18个）；公里）并且暴露良好（下倾超过 18 公里，沿走向超过 25 公里），有助于分析从起始到成熟的变形机制的空间变化，4）下盘的热历史有详细记录，提供了一个机会检查脆塑性转变过程中初始环境温度约为 150-400°C 的岩石。为了阐明导致低角度正断层的微观结构演化和断层弱化机制，我们将现场观测与岩相学相结合， SEM（EBSD、微观结构图像分析表征和阴极发光）和盆地和山脉中暴露的稳定同位素研究提供了此类全球重要断层的关键示例，并突出了有关盆地和山脉中缓倾滑移的问题。脆性地壳，但这些断层系统的起源以前很少受到关注，通过离子微探针进行的原位稳定同位素分析使我们能够“洞察”晚期滑移和流体运移历史过程中的次生蚀变。通过将这些现场和实验室研究与欧洲同事的实验研究相结合，这项研究弥合了学科界限，努力改变对摩擦粘性过程及其在地质记录中的表现的理解。挪威和瑞士的项目将有助于扩大实验室实验，为地震灾害评估提供相关信息，本科生将参与整个项目的现场和实验室研究，获得进行个人研究的经验，并扩大到更广泛的社区。通过虚拟技术的发展使用 Google 地球和专为小学至大学生设计的相关教育材料进行实地考察。