CAREER: Understanding Major Intracontinental Strike-slip Faults from the Lower Crust's Perspective: Tibetan Insights from the Canadian Shield

大陆碰撞与沿岩石圈尺度（数百公里长）的大陆内走滑断层的变形之间的关系仍然存在激烈的争论。争论还在继续，因为尽管我们对地表的这类重要断层有所了解，但我们对这些结构如何在下地壳中运作知之甚少。本项目将通过对加拿大地盾西部400公里长的陆内脂河剪切带（发掘于~30 ~ 40公里古深度）这一暴露良好的深部地壳剪切带及其围岩的研究，“就地”回答有关应变局部化程度、流变非均质横向程度以及陆内主要断层深部物理性质的问题。初步资料支持约1.92 ~ 1.90 Ga时，脂河剪切带北西侧下地壳熔融弱化，同时脂河剪切带东南侧下地壳脱水强的假设。这个吗?流变二分法?可能类似于青藏高原边缘陡峭地形梯度下的下地壳和主要的陆内断裂系统，例如阿尔金山山脉和龙门山山脉。该项目将从下地壳的角度对流变非均质性在促进大陆内主要走滑断层应变定位中的作用提供基于现场的约束。预期结果包括：(1)解决了脂河剪切带及其围岩中变质构造岩的压力-温度条件和路径；(2)通过EPMA对monazite进行原位Th-U-total Pb测年，测试了变形、剪切和部分熔融的同时代性；(3)通过对一套横切和可变变形岩脉的锆石进行高精度ID-TIMS U-Pb测年，限制了剪切带再活化的500万年历史。该项目的更广泛影响包括：(1)实施6 -8年级教师培训和提高计划；(2)通过课堂参观和小组讨论进行公众宣传，这些活动都集中在大陆内系统的断层和地震上。这些努力将直接影响住在新马德里地震带未暴露的大陆内断层附近的教师和学生。落基山脉以东地震最活跃的地区。两个专业发展暑期学院（工作坊+实地考察）将提供给教师使用由PI开发的基于探究的模块。这些模块的实施和评估将影响阿肯色州的中学地球科学框架（并通过模块的在线传播在全国范围内）。这些努力将与阿肯色大学数学和科学教育中心合作。培训和教育将提供给至少一名博士生，一名硕士学生和几名本科生，他们将全面参与研究和推广工作。这个项目直接影响到阿肯色大学新的地球科学博士项目。EarthScope社区也有望从中受益，因为该项目利用加拿大地盾中暴露的深部地壳剪切带，让我们了解大陆内主要走滑断层的结构、流变学和时间，因为它可能存在于莫霍附近的下地壳中。

The relationship between continental collision and deformation along lithosphere-scale (hundreds of km-long) intracontinental strike-slip faults remains intensely debated. Debate persists because despite what we know about this important class of faults at the surface, we know comparatively little about how these structures work in the lower crust. Questions regarding the degree of strain localization, the lateral extent of rheological heterogeneities, and the physical nature of major intracontinental faults at depth will be answered "in situ" in this project by studying a well-exposed deep crustal shear zone and its wall rocks, i.e., the 400 km-long intracontinental Grease River shear zone in the western Canadian Shield (exhumed from ~30 to 40 km-paleodepths). Preliminary data support the hypothesis that at ca. 1.92-1.90 Ga, lower crust NW of the Grease River shear zone was melt-weakened at the same time that lower crust SE of the Grease River shear zone was dehydrated and strong. This ?rheological dichotomy? may be analogous to lower crust beneath the steep topographic gradients and major intracontinental fault systems that mark the margins of the Tibetan Plateau, e.g., the Altyn Tagh and Longmen Shan ranges.This project will provide field-based constraints on the role of rheological heterogeneity in facilitating strain localization across a major intracontinental strike-slip fault from the lower crust's perspective. Anticipated results include: (1) resolving the Pressure-Temperature conditions and paths for metamorphic tectonites in the Grease River shear zone and its wall rocks, (2) testing the contemporaneity of deformation, shearing, and partial melting using in situ Th-U-total Pb dates on monazite via EPMA, and (3) constraining a 500 m.y. history of shear zone reactivation via high-precision ID-TIMS U-Pb dates on zircon from a suite of cross-cutting and variably-deformed dikes.Broader impacts of this project include: (1) implementing a program of 6th-8th grade teacher preparation and enhancement, and (2) public outreach via classroom visits and panel discussions that are all focused on faulting and earthquakes in intracontinental systems. These efforts will directly impact teachers and students who live in the immediate vicinity of unexposed intracontinental faults of the New Madrid seismic zone ? the most seismically active region east of the Rocky Mountains. Two professional development summer institutes (workshops + field trips) will be provided to teachers utilizing inquiry-based modules to be developed by the PI. Implementation and assessment of these modules will impact middle school Earth Science frameworks in Arkansas (and nationally via on-line dissemination of the modules). These efforts will be in collaboration with the University of Arkansas' Center for Math and Science Education. Training and education will be provided to at least one Ph.D. student, one M.S. student, and several undergraduate students who will be integrally involved in the research and outreach efforts. This project directly impacts the new geoscience Ph.D. program at the University of Arkansas. Benefits to the EarthScope community are also anticipated, as this project uses an exposed deep crustal shear zone in the Canadian Shield to inform our understanding of the structure, rheology, and timing of a major intracontinental strike-slip fault as it may have existed in the lower crust near the Moho.