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.

低角度的正常断层在扩展的大陆和海洋岩石圈中得到了广泛认可，但是通过地震构成的地壳在浅水深处影响其主动性和旷日持久的机制仍然保持一致。正常断层地震局灶性机制的汇编仅显示出小于30°的断层上的所有地震的一小部分，并且断层力学预测，除非异常弱，否则降低的正常断层均不应形成或重新激活。相比之下，现场观察表明，在地震构成外壳内的滑动发生了。为了解决这一悖论，我们正在进行多学科领域以及实验室结构和同位素研究，以实现加利福尼亚州奇尤维维山脉中非常暴露的低角度正常断层。我们的投资将提供自然断层岩石的宏观和微观结构表征，以及在主动行动和沿断层滑落过程中烟道作用的文献。这些观察结果将与欧洲同事和最近的微症研究的实验室实验相结合，以解决与低角度正常断层倡议相关的过程。研究地点是近乎理想的天然实验室，用于解决低角度正常断层的内部结构与机械行为之间的关系，因为1）区域映射已经确定了断层暴露的关键领域； 2）足球是由未经构造的花岗岩的限制的，可以合理地假设原石是由石英和长石的两相混合物机械地主导的。 3）断层系统包括多个低角度正常故障，带有可变滑动（约2至18公里），并且暴露得很好（大于18 km的向下倾斜18 km，沿罢工沿撞车沿倾斜和25 km），支持分析空间变化的变形机制，从启动到成熟度到成熟度，以及在足球的热史之间进行15次调查的杂物，可衡量4个频繁的杂物，该速度是频繁的。脆性塑性过渡。要照亮导致低角度断层的微结构演化和断层弱化机制，我们将现场观测与岩石学，SEM（EBSD，微结构的图像分析和阴极发光的图像分析表征）和稳定的同位素研究整合在一起。在盆地和范围中暴露的低角度正常断层提供了这一全球重要类别的断层类别的关键示例，并突出了有关脆性外壳中温和倾斜的问题的问题，但是这些故障系统的主动性以前很少受到关注。通过离子微探针对原位稳定的同位素分析允许我们看到？后期滑移和流体迁移历史记录中的次要变化，并将详细的微观结构与相关的烟道相关联。通过将这些领域和实验室研究与欧洲同事的实验研究相结合，这项研究桥接了纪律界限，以改变人们对摩擦粘性过程的理解及其在地质记录中的表现。与挪威和瑞士科学家的合作将有助于实验室实验的扩展，从而为地震危险评估提供相关信息。本科生将在整个项目中参与实地和实验室研究，从而获得进行个人研究和外展的经验。通过使用Google Earth和相关的教育材料来开发虚拟实地考察，从而实现了对更广泛的社区的宣传。