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

低角度正断层广泛存在于伸展大陆和大洋岩石圈中，但其在发震地壳浅部的启动和长期滑动机制仍存在争议。正断层地震震源机制的解释表明，在倾角小于30°的断层上发生的所有地震中，只有一小部分发生在断层上，断层力学预测，倾角小于30°的正断层既不会形成，也不会重新激活，除非它们非常弱。与此相反，现场观测表明，地震地壳内发生了滑动。为了解决这个矛盾，我们正在进行多学科领域和实验室结构和同位素研究的一个非常好的暴露在加州的Chemehuevi山脉的低角度正断层。我们的研究将提供天然断层岩的宏观和微观结构特征，并记录流体在断层形成和沿着断层滑动过程中的作用。这些观测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有关的过程。该研究场地是解决低角度正断层内部结构和力学行为之间关系的近乎理想的天然实验室，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类，可以合理地假设原岩在机械上由石英和长石的两相混合物控制; 3）断裂系统由多条低角度的正断层组成，断层的滑动量是可变的（约2至18公里），并充分暴露（大于18 km的下倾和25 km的沿着走向），有利于分析从开始到成熟的变形机制的空间变化，和4）下盘的热历史有据可查，提供了一个机会，以检查岩石与初始环境温度之间约150-400°C的脆塑性转变。为了阐明微观结构的演变和断层弱化机制，导致低角度的正常断层，我们正在整合现场观察岩相，扫描电镜（EBSD，微观结构的图像分析表征和阴极发光）和稳定同位素研究。在盆地和山脉中暴露的低角度正断层提供了这类全球重要断层的关键例子，并突出了关于脆性地壳中缓倾角滑动的问题，但这些断层系统的启动以前很少受到关注。离子探针原位稳定同位素分析使我们能够？看穿了吗在晚期滑动和流体迁移历史中的次生蚀变，并将详细的显微结构与相关流体相关联。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，这项研究弥合了学科界限，以改变对摩擦粘性过程及其在地质记录中表现形式的理解。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，获得进行个人研究和推广的经验。正在通过利用谷歌地球和为小学至大学生设计的相关教材开展虚拟实地考察，与更广泛的社区进行外联。

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.