Collaborative Research: Dynamics at the Base of a Pseudotachylyte-bearing Fault System

合作研究：含假速石断层系统基础的动力学

• 批准号: 0810039
• 负责人: Scott Johnson
• 金额: $ 21万
• 依托单位: University of Maine
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-15 至 2012-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0810039&HistoricalAwards=false
• 关键词: Collaborative; Research; Dynamics; Base; Pseudotachylyte

A concerted effort through EarthScope is currently underway to better understand seismicity along the San Andreas Fault, which poses serious threats to society from large-magnitude earthquakes. The San Andreas Fault Observatory at Depth features an instrumented drill hole, allowing characterization of the rocks, fluids, kinematics, and stress orientations and magnitudes within the fault zone to depths of approximately 3 km. These studies are designed partly to investigate the strength of the San Andreas Fault, which is critical for seismic forecasting. In conjunction with these studies it is important that we locate and study appropriate exhumed examples of similar faults at the surface today. One of the very few surface occurrences of an analog for the San Andreas Fault is the Norumbega Fault System (NFS), which cuts across the entire State of Maine and is the field site for this study. Through the NFS study we hope to clarify the value of near-surface stress measurements on the San Andreas Fault as indicators for the strength of the fault. This work is intended to support the dissertation of a female PhD student, and several undergraduate student theses. Our close cooperation with the Maine Geological Survey, and our involvement with K-12 Earth Science teachers in Maine, lends additional societal relevance to the work.Due to the large elastic moduli of solid Earth materials elastic stresses are transmitted over great distances and depths within the Earth. For this reason, the stress orientations and magnitudes measured near Earths surface provide only a partial picture of the state of stress on active faults. For a more complete picture, we must understand the state of stress on faults at greater depths, particularly near the frictional-to-viscous transition where the strongest part of the crust occurs. For this reason we are focusing on the deeply exhumed NFS. To provide relatively tight constraints on the stress tensor the field locality should contain irrefutable evidence for coseismic rupture (pseudotachylyte), and be characterized by: (a) tightly constrained kinematic boundary conditions, (b) microstructures amenable to estimates of differential stress and mean kinematic vorticity, and (c) lithological (rheological) heterogeneity providing natural variability in the stress and vorticity estimates. In such a system, 3D numerical experiments can provide best-fit solutions for the field-derived stress and vorticity estimates, and in doing so solve for the principal and Cartesian stresses. Our work in the NFS involves field mapping, microstructural analysis using optical and electron-beam techniques, and 3D numerical modeling that is tightly constrained by the field-derived data. Our primary aims are to: (a) systematically evaluate the microstructural variation across the strain gradient in the fault zone mylonites, (b) use microstructural measurements to calculate temperatures, differential stresses and mean kinematic vorticity numbers for numerous samples across the field area, (c) use these calculated values and 3D numerical experiments to help constrain the stress tensor at the time when this mylonite zone was overlain by a seismically active fault, and (d) extend our numerical modeling to the surface to explore how kinematic boundary conditions and stresses at depth influence active faults above.

目前正在进行通过Earthscope的一致努力，以更好地了解圣安德烈亚斯断层沿线的地震性，这构成了大型地震对社会的严重威胁。深度的圣安德烈亚斯断层天文台具有一个仪器的钻孔，可以表征岩石，流体，运动学以及压力方向以及断层区域内的压力方向以及大小约3 km。这些研究的设计部分是为了研究圣安德烈亚斯断层的强度，这对于地震预测至关重要。结合这些研究，我们必须在当今的表面找到和研究适当的相似断层的散布示例，这一点很重要。 San Andreas断层类似物的表面出现很少的是Norumbega断层系统（NFS），该系统在整个缅因州的整个状态下切断，并且是本研究的现场地点。 通过NFS研究，我们希望阐明在圣安德烈亚斯断层上近表面应力测量值的价值，作为断层强度的指标。这项工作旨在支持一位女博士生的论文和几个本科生论文。我们与缅因州地质调查局的密切合作，以及与缅因州的K-12地球科学教师的参与，为工作提供了额外的社会相关性。对于固体地球材料的巨大弹性模量弹性压力是在地球内部较大的距离和深度上传播的。因此，在地球表面附近测得的应力取向和大小仅提供了主动断层应力状态的部分图片。为了进行更完整的情况，我们必须了解更大深度的断层压力状态，尤其是在地壳中最强部分发生的摩擦到粘性过渡附近。 因此，我们专注于深度挖掘的NFS。 要在应力张量上提供相对较严重的约束，场地应包含无可辩驳的证据证明coseiscic破裂（伪thylyte），并以：（a）严格约束的运动边界条件的特征，（b）微观结构可与差异应力和平均差异性均匀性（C）自然差异（c）的估计（b））。和涡度估计。在这样的系统中，3D数值实验可以为现场衍生的应力和涡度估计提供最佳拟合解决方案，并为主要和笛卡尔压力做出解决方案。我们在NFS中的工作涉及现场映射，使用光学和电子梁技术的微观结构分析以及受现场衍生数据严格约束的3D数值建模。 Our primary aims are to: (a) systematically evaluate the microstructural variation across the strain gradient in the fault zone mylonites, (b) use microstructural measurements to calculate temperatures, differential stresses and mean kinematic vorticity numbers for numerous samples across the field area, (c) use these calculated values and 3D numerical experiments to help constrain the stress tensor at the time when this mylonite zone was overlain通过地震活跃的断层，（d）将我们的数值建模扩展到表面，以探索运动的边界条件和深度时的应力如何影响上面的活动断层。