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。人类遇到的许多危险，包括地震，火山爆发和海啸，都是板块构造造成的。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局限于像加州的圣安德烈亚斯断层这样的结构中，是地球科学中的首要问题。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、变形的定位以及与地震和火山灾害的关系。然而，这样的数据提供的信息很少板块相互作用和变形在很大的深度。研究这些更深层次过程的最直接方法是在因隆起和侵蚀而被挖掘出来的古代板块边界区进行研究。地质学家使用一系列工具来评估这些挖出岩石的变形历史，但其中一些工具的有效性仍然需要测试。 这个项目的主要目标是测试一些微结构工具，用于从挖出的变形岩石中提取一个重要的量，称为运动学涡度。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学涡度。 这项工作将在诺伦贝加断层系统中进行，该断层系统横跨整个缅因州，是加州圣安德烈亚斯断层的极少数古代类似物之一。因此，我们的研究结果将适用于一个著名的地震活动断层。更具体地说，这些研究人员将调查倾斜的石英形状的首选方向和刚性碎屑旋转方法，以确定运动学涡度在一个良好的特征糜棱岩剪切带近似单斜应变对称。他们将提供一个详细的分析微观结构因素，可能会损害碎屑方法的运动学涡度分析，并确定是否其他方法之一，即使在碎屑的方法是妥协的一致的结果。此外，他们将制定标准，以估计在碎屑/基质边界的应变局部化程度，并进行数值敏感性分析，以更好地了解碎屑润滑对形成地震活动断层根部的糜棱岩剪切带的整体剪切强度的影响。详细的微观结构调查将利用光学，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用于调查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增加的演变如何有助于剪切带的长期弱化。初步的结果是新颖的，并建议谨慎使用碎屑旋转方法确定的运动学涡数，但它们打开了令人兴奋的新的可能性，调查应变相关的岩石强度的变化所产生的化学和机械过程中的反馈变形。

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.