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

目前，通过EarthScope进行的一项协同工作正在进行中，目的是更好地了解圣安德烈亚斯断层（San Andreas Fault）沿线的地震活动，该断层对社会构成了大地震的严重威胁。圣安地列斯断层观测站的特点是有一个仪器钻孔，可以对断层带内的岩石、流体、运动学、应力方向和震级进行表征，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，这对地震预测至关重要。结合这些研究，重要的是我们要定位和研究今天在地表出土的类似断层的适当例子。诺鲁姆贝加断层系统（NFS）是为数不多的与圣安德烈亚斯断层类似的地表断层之一，它横跨整个缅因州，是本研究的实地地点。通过NFS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。这项工作旨在支持一名女博士生的论文，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地球科学教师的合作，为这项工作增添了更多的社会相关性。由于固体地球材料的弹性模量大，弹性应力在地球内部的距离和深度上都能传播。由于这个原因，在地球表面附近测量的应力方向和大小只能提供活动断层应力状态的部分图像。为了获得更完整的图像，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-粘性过渡附近。因此，我们将重点放在深入挖掘的NFS上。为了对应力张量提供相对严格的约束，应力场局部应该包含同震破裂（伪水杨酸盐）的无可辩驳的证据，并具有以下特征：(a)严格约束的运动学边界条件，(b)适合于差应力和平均运动涡度估计的微观结构，以及(c)岩性（流变）非均质性，提供应力和涡度估计的自然可变性。在这样的系统中，三维数值实验可以为场源应力和涡度估计提供最佳的拟合解，从而求解主应力和笛卡尔应力。我们在NFS的工作包括现场绘图，使用光学和电子束技术进行微观结构分析，以及受现场派生数据严格约束的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在应变梯度上的微结构变化，(b)使用微结构测量来计算整个野外地区众多样品的温度、差应力和平均运动涡量，(c)使用这些计算值和3D数值实验来帮助约束该糜棱岩带被地震活跃断层覆盖时的应力张量。(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度应力如何影响上面的活动断层。

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.