Kinematic Vorticity Gauges and the Rheology of Mylonitic Shear Zones

该奖项是根据2009年美国复苏和再投资法案(公法111-5)资助的。人类遇到的许多灾害，包括地震、火山喷发和海啸，都是由板块构造造成的。构造板块如何移动和相互作用，以及在它们相互作用的边界发生的变形如何定位于加州的圣安德烈亚斯断层等结构，是地球科学的首要问题。在活跃的构造板块边界，GPS数据使人们能够更清楚地了解板块相互作用、变形的局部化以及与地震和火山灾害的关系。然而，这些数据提供的关于板块相互作用和深部变形的信息很少。研究这些更深层次过程的最直接方法是在古板块边界带工作，这些带已经被抬升和侵蚀挖掘出来。地质学家使用了一系列工具来评估这些出土岩石的变形历史，但其中一些工具的有效性仍需测试。这个项目的主要目标是测试一些微构造工具，这些工具用于从出土的变形岩石中提取一种重要的量，即运动学涡度。通过这项研究，研究人员希望澄清在什么条件下这些工具可以可靠地用于测量运动涡度。这项工作将在横跨整个缅因州的诺鲁姆贝加断层系统中进行，它是加利福尼亚州圣安德烈亚斯断层的极少数古代类似物之一。因此，我们的结果将适用于一个众所周知的地震活动断层。更具体地说，这些研究人员将研究倾斜石英形状择优取向和刚性碎屑旋转方法，以确定具有近似单斜应变对称性的特征良好的糜棱岩剪切带的运动涡度。他们将对可能影响碎屑运动涡度分析方法的微观结构因素进行详细分析，并确定其他方法中是否有一种方法即使在碎屑方法受到损害的情况下也能给出一致的结果。此外，他们还将制定评估碎屑/基质边界应变局部化程度的标准，并进行数值敏感性分析，以更好地了解碎屑润滑对构成地震活动断层根源的糜棱岩剪切带整体剪切强度的影响。详细的微观结构研究将利用光学、扫描电子显微镜和电子背散射衍射技术。二维和三维参数数值敏感性分析将被用来研究可能影响碎屑运动学的一系列参数，并评估碎屑润滑随应变的增加而演变如何有助于剪切带的长期减弱。初步结果是新的，并建议在使用碎屑旋转法确定运动涡度数时要谨慎，但它们为研究岩石强度随应变的变化打开了令人兴奋的新可能性，这些变化是由变形过程中化学和机械过程之间的反馈引起的。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Many hazards encountered by humans, including earthquakes, volcanic eruptions and tsunamis, result from plate tectonics. How tectonic plates move and interact with one another, and how deformation that occurs at their interacting boundaries is localized into structures like the San Andreas fault in California, are first-order questions in the Earth Sciences. At active tectonic plate boundaries, GPS data have allowed a much clearer understanding of plate interactions, localization of deformation, and relations to seismic and volcanic hazards. However, such data provide little information about plate interaction and deformation at great depth. The most direct way to study these deeper processes is to work in ancient plate boundary zones that have been exhumed by uplift and erosion. Geologists use a range of tools to evaluate the histories of deformation in these exhumed rocks, but the validity of some of these tools still needs to be tested. The primary goal of this project is to test some microstructural tools used to extract from exhumed, deformed rocks an important quantity known as kinematic vorticity. Through this study the investigators hope to clarify under what conditions these tools can be reliably used to measure kinematic vorticity. The work will be conducted in the Norumbega Fault System, which cuts across the entire State of Maine, and is one of the very few ancient analogs for the San Andreas Fault in California. Thus, our results will have applicability to a well-known seismically active fault. More specifically, these researchers will investigate the oblique quartz shape preferred orientation and rigid clast rotation methods for determining kinematic vorticity in a well-characterized mylonitic shear zone with approximately monoclinic strain symmetry. They will provide a detailed analysis of the microstructural factors that may compromise clast methods of kinematic vorticity analysis, and determine if one of the other methods gives consistent results even where clast methods are compromised. In addition, they will develop criteria for estimating the degree of strain localization at clast/matrix boundaries and conduct numerical sensitivity analyses to better understand the effects of clast lubrication on the bulk shear strength of mylonitic shear zones that form the roots of seismically active faults. Detailed microstructural investigations will utilize optical, scanning electron microscopy and electron backscatter diffraction techniques. 2D and 3D parametric numerical sensitivity analyses will be used to investigate a range of parameters that may affect clast kinematics, and assess how the evolution of clast lubrication with increasing strain may contribute to long-lived weakening of shear zones. The preliminary results are novel, and suggest caution when using clast rotation methods for determining the kinematic vorticity number, but they open exciting new possibilities for investigating strain-dependent changes in rock strength that arise from feedbacks among chemical and mechanical processes during deformation.