Quantifying Syntectonic Weakening in Deep Orogenic Crust

这个项目的主要智力影响将是提高我们对形成地壳的力学的理解。近年来，地球科学家利用越来越多的大地测量数据来实现这一目标，但对中下地壳的力学特性仍然知之甚少。本项目重点研究剪切带网络中应变减弱的幅度。具体而言，研究将探索导致这种弱化的矿物颗粒尺度和露头尺度的变形机制，然后对结果进行建模，以了解更大尺度上的弱化过程。这些概念和数值模型将使我们能够更好地预测大陆地壳将在哪里以及以多快的速度变形，反过来，这将有利于社会。在未来，那些最近冰川消退的地方，以及那些在构造活跃的地区，地震的可能性，可能是进一步应用这一研究的有趣的目标。该项目还可作为继续和加强K-16和研究生阶段正在进行的教育和推广活动的工具。为参加缅因大学教学科学硕士课程的学生提供支持，将允许K-12职前教师参与积极的研究项目，并参与创造一个更容易获得科学的环境。PI及其同事将围绕超级计算机、可视化和现场视频库项目开发新的内容，这些项目的综合结果将加强专业发展、研究生和本科生课程，并向缅因州农村地区的K-12学生提供服务。在整个岩石圈中，应变局部化在影响地震活动、挖掘、流体迁移和成矿、岩浆运输、地形和板块边界演化等构造过程中起着重要作用。以往的研究包括野外观测和数值模拟，对应变局部化的原因和后果产生了许多限制，但研究人员对深部地壳强度变化的幅度缺乏透彻的认识。由于特殊的暴露和深度挖掘，该研究小组将使用安大略省南部格伦维尔省的Parry Sound区域作为自然实验室。在该区一侧，沿米级剪切带，麻粒岩相矿物组合在上角闪岩相条件下发生了转化。在实地测绘之后，PI将通过使用自然几何框架的模型计算，量化与相互连接的米尺度剪切带发展相关的强度变化。此外，为了开发更好的工具来预测其他地方这种强度的减弱，他将确定剪切带发展和减弱发生的机制。PI假设，其中一种机制涉及伟晶岩衍生的流体渗入相邻带，允许矿物变化，剪切带可以沿着这些变化成核。要确定强度变化在地壳尺度上的意义，就需要确定米尺度的过程是否产生了，例如，区域尺度的、域边界的十二哩湾剪切带。初步观察表明，在渐进变形过程中，没有横向偏移的裂缝有助于构造的完全移位，因为它们演变成相互连接的米级剪切带。该研究小组将通过绘制沿12里湾剪切带推断长度的热气压和地质年代学模式来评估这些初步解释是否有效。

The primary intellectual impact of this project will be in improving our understanding of the mechanics that shape the Earth's crust. In recent years, earth scientists have used the increasing body of geodetic data towards that end, but the mechanical properties of the middle and lower crust remain only loosely constrained. This project focuses on the magnitude of strain weakening in shear zone networks. In detail, the research will explore the grain-scale and outcrop-scale deformation mechanisms in minerals that lead to this weakening, followed by modeling of the results to understand the weakening process on the larger scale. These conceptual and numerical models will allow better prediction of where and how fast the continental crust will deform and in turn this will benefit society. In the future, locations that have recently been deglaciated, and those in tectonically active areas where earthquakes are likely, may be interesting targets for further application of this research. This project also serves as a vehicle to continue and enhance ongoing educational and outreach initiatives at the K-16 and graduate levels. Support for students enrolled in the University of Maine's Master of Science in Teaching program will allow pre-service K-12 teachers to become involved in an active research project and to participate in creating an environment where science is more accessible. The PI and colleagues will develop new content based around supercomputer, visualization, and field video library projects, the combined results of which will enhance professional development, graduate and undergraduate courses, and outreach to K-12 students in Maine's rural areas.Throughout the lithosphere, strain localization plays a fundamental role in tectonic processes affecting, for example, seismicity, exhumation, fluid migration and mineralization, magma transport, and topographic and plate boundary evolution. Previous research involving field observations and numerical modeling has produced many constraints on the causes and consequences of strain localization, but researchers lack a thorough understanding of the magnitude of strength variation in the deep crust in particular. Due to exceptional exposure and deep exhumation this research group will use the Parry Sound domain of the Grenville Province, southern Ontario, as a natural laboratory. Along one margin of the domain, granulite facies mineral assemblages have been transformed under upper amphibolite facies conditions along meter-scale shear zones. After field-based mapping, the PI will quantify the change in strength associated with the development of the interconnected meter-scale shear zones through model calculations using the natural geometric framework. In addition, to develop better tools to predict weakening of this magnitude elsewhere, he will determine the mechanisms by which the shear zones developed and the weakening occurred. The PI has hypothesized that one such mechanism involved pegmatite-derived fluids that infiltrated adjacent zones, allowing mineralogical changes along which shear zones could nucleate. Establishing the crustal-scale significance of strength changes requires determining if meter-scale processes produced, for example, the regional-scale, domain-bounding Twelve Mile Bay shear zone. Preliminary observations indicate that fractures with no lateral offset assisted full transposition of the fabric as they evolved into interconnected meter-scale shear zones during progressive deformation. This research group will evaluate whether these preliminary interpretations are valid through the mapping of thermobarometric and geochronologic patterns along the inferred length of the Twelve Mile Bay shear zone.