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

目前正在通过EarthScope开展一项协调一致的努力，以更好地了解圣安德烈亚斯断层沿线的地震活动，该断层对社会构成了来自大震级地震的严重威胁。深部的圣安德烈亚斯断层观测站有一个仪器钻孔，可以表征断裂带内的岩石、流体、运动学、应力方向和震级，深度约为3公里。这些研究的部分目的是调查圣安德烈亚斯断层的强度，这对地震预报至关重要。在进行这些研究的同时，重要的是我们要找到并研究今天地表类似断层的适当出土实例。与圣安德烈亚斯断层类似的极少数地表事件之一是诺鲁姆贝加断层系统(NFS)，它横跨整个缅因州，是本研究的现场地点。通过北冰洋的研究，我们希望阐明圣安德烈亚斯断层近地表应力测量作为断层强度指标的价值。这项工作是为了支持一位女博士生的论文，以及几篇本科生的论文。我们与缅因州地质调查局的密切合作，以及我们与缅因州K-12地球科学教师的参与，使这项工作具有更多的社会意义。由于固体地球材料的大弹性模数，弹性应力在地球内部传递到很远的距离和很深的地方。因此，在地表附近测量的应力方向和大小只提供了活动断层上应力状态的部分图像。为了更全面地了解，我们必须了解更深的断层上的应力状态，特别是在地壳最强部分发生的摩擦-粘滞过渡附近。出于这个原因，我们将重点放在深度挖掘的NFS上。为了对应力张量提供相对严格的约束，场址应包含无可辩驳的同震破裂(假地)的证据，并具有以下特征：(A)严格约束的运动边界条件，(B)适合差应力和平均运动涡度估计的微结构，以及(C)在应力和涡度估计中提供自然可变性的岩性(流变性)非均质性。在这样的系统中，三维数值实验可以为场导出的应力和涡量估计提供最佳拟合解，从而求解主应力和笛卡尔应力。我们在NFS场的工作包括场映射、使用光学和电子束技术的微结构分析，以及受场导出数据的严格约束的3D数值建模。我们的主要目标是：(A)系统地评估断裂带糜棱岩中跨应变梯度的微结构变化，(B)使用微结构测量来计算现场区域内大量样品的温度、差应力和平均运动涡度数，(C)使用这些计算值和3D数值实验来帮助约束当该糜棱岩带被地震活动断裂覆盖时的应力张量，以及(D)将我们的数值模拟扩展到地表，以探索运动边界条件和深部应力如何影响上面的活动断裂。

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.