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

低角度正断层在扩展的大陆和海洋岩石圈中得到了广泛的认识，但影响其在发震地壳中形成和在浅深部持续滑动的机制仍然存在争议。正常断层地震震源机制的汇编表明，只有一小部分地震发生在倾角小于30°的断层上，断层力学预测倾角小于30°的正常断层既不会形成也不会重新激活，除非它们异常微弱。相反，野外观测表明，在发震地壳内部发生了滑动。为了解决这一矛盾，我们正在对加利福尼亚州Chemehuevi山脉的一个非常暴露的低角度正断层进行多学科实地和实验室结构和同位素研究。我们的研究将提供天然断层岩石的宏观和微观结构特征，并记录流体在断层起始和滑动过程中的作用。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起爆有关的过程。研究地点是研究低角度正断层内部结构与力学行为关系的近乎理想的自然实验室，因为1)区域测绘已经建立了断层暴露的关键区域；2)下盘为未变形的花岗岩类，可以合理地认为原岩以石英和长石两相混合物为主；3)断层系统由多个低角度正断层组成，具有可变滑动（约2至18公里），并且暴露良好（倾角大于18公里，沿走向大于25公里），便于分析从开始到成熟的变形机制的空间变化。4)下盘的热历史记录良好，为研究脆性-塑性转变过程中初始环境温度约为150-400°C的岩石提供了机会。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石学、SEM （EBSD、微观结构的图像分析表征、阴极发光）和稳定同位素研究相结合。在盆地和山脉中暴露的低角度正断层为这类具有全球意义的断层提供了重要的例子，并突出了脆性地壳中缓倾滑动的问题，但这些断层系统的起源以前很少受到关注。离子探针原位稳定同位素分析允许我们？看穿?晚期滑动和流体运移过程中的次生蚀变，并将详细的微观结构与相关流体进行对比。通过将这些实地和实验室调查与欧洲同事的实验研究相结合，本研究跨越了学科界限，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。与挪威和瑞士科学家的合作将有助于扩大实验室实验的规模，为地震危害评估提供相关信息。本科生将参与整个项目的实地和实验室研究，获得进行个人研究和推广的经验。通过使用谷歌Earth和为小学到大学学生设计的相关教育材料开发虚拟实地考察，扩大到更广泛的社区。

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.