Collaborative Research: Lower Crustal Flow, Shallow Fabric Development, and Craton Assembly -East Athabasca Granulite Terrane, Canada

地质学家越来越多地得出结论，在适当的情况下，大陆地壳中下部的岩石可以横向流动。西藏、南美安第斯山脉和美国西南部的大型造山高原可能代表了相对较弱的中地壳或深部地壳流动的地形表现。地壳流动可能是山脉带高度和宽度的最终控制之一，它可能解释岩石远离原产地的横向平移。较低的地壳流动和由此产生的次水平组构也可以解释一些地质和地球物理观察，包括在深地震反射调查中常见的高反射率和片层结构。在将地壳流动完全纳入现在或过去地质地体的构造模型之前，了解深部地壳的性质、下地壳变形机制以及板块构造事件期间对岩石强度(流变性)的控制是很重要的。雪鸟构造带的中央部分可以说是世界上最大的下陆壳裸露部分，它仍然保留了大部分地壳深部的特征。最低大陆地壳的大片区域沿着一条主要断裂--莱斯湖剪切带--浮出水面。目前的暴露代表了超过20,000平方公里的太古宙到古元古代深部地壳。该地区的特征之一是早期麻粒岩级别、穿透性、近水平的组构(矿物和成分层的构造排列)。这项研究的目的是探索次水平组构是下大陆地壳在加拿大西部这一部分生长过程中流动的结果，并进一步探讨这种组构可能为现代大陆深部地壳提供洞察。东阿萨巴斯卡麻粒岩地体是北美最大、暴露最好的深部地壳露头之一，是EarthScope设施和了解大陆地壳性质的重要资源。目前的研究将为该地区的野外研讨会奠定基础，该研讨会将联合地球物理学家和地质学家讨论下地壳的暴露问题。其目标不仅是讨论深部地壳过程，而且是开始合作努力，将这一独特和可进入的研究区域发展为研究最低大陆地壳的成分和行为的天然实验室。这一合作项目的主要影响之一是正在进行的变形和变质过程地质年代学研究，旨在开发新的工具和程序，以确定诸如东阿萨巴斯卡麻粒岩地体等具有漫长和复杂构造历史的地区的地质事件、变质反应和地质过程的持续时间。麻省理工大学和麻省理工学院的学生和教职员工密切合作，以确定计时器相的岩石学和结构背景，然后通过整合两个机构的分析进行高分辨率测年。此外，研究生和本科生计划和协调加拿大丛林的实地研究。到目前为止，我们五名博士生中的两名、一名硕士研究生和三名现场助理都是代表性不足的团体的成员，他们是年轻学生研究人员的榜样。实地和实验室的经验通过课堂练习、学生项目和报告传递给两所大学的更大的学生群体，并使他们受益匪浅。

Geologists have increasingly concluded that, under the right circumstances, rocks in the middle to lower parts of continental crust can be made to flow laterally. The great orogenic plateaus of Tibet, the South American Andes, and the southwestern United States may represent the topographic expression of flow of relatively weak middle or deep crust. Crustal flow may be one of the ultimate controls on the elevation and width of mountain belts, and it may explain the lateral translation of rocks far from their region of origin. Lower crustal flow, and the resulting sub-horizontal fabric, may also explain a number of geological and geophysical observations including the common occurrence of high reflectivity and lamellar structures recognized in deep seismic reflection surveys. Before crustal flow can be fully incorporated into tectonic models of present or past geologic terranes, it is important to understand the nature of the deep crust, the mechanisms of lower crustal deformation, and the controls on the strength (rheology) of the rocks during plate tectonic events. The central portion of the Snowbird Tectonic Zone is arguably the world's largest exposure of lower continental crust that still preserves much of its deep crustal character. The large region of lowermost continental crust was brought to the surface along a major fault, the Legs Lake shear zone. The present exposure represents greater than 20,000 square kilometers of Archean to Paleoproterozoic deep crust. One of the characteristic features of the region is an early granulite-grade, penetrative, sub-horizontal fabric (tectonic alignment of minerals and compositional layers). The goal of this research is to explore the hypothesis that the sub-horizontal fabric resulted from flow of lower continental crust during growth of this portion of western Canada, and further, that this fabric may provide insight into modern deep continental crust. The East Athabasca granulite terrane, one of the largest and best exposed outcrops of deep crust in North America, is an important resource for the EarthScope facility and for understanding the nature of continental crust in general. The current research will lay the groundwork for a field workshop in the area that will unite geophysicists and geologists on exposures of the lower crust. The goal is not only to discuss deep-crustal processes, but to begin a collaborative effort to develop this unique and accessible study area as a natural laboratory for the study of the composition and behavior of lowermost continental crust. One of the major impacts of this collaborative project is the ongoing research on geochronology of deformation and metamorphic processes, aimed at developing new tools and procedures for determining the age of geologic events, metamorphic reactions, and the duration of geologic processes in regions with protracted and complex tectonic histories like the East Athabasca granulite terrane. There is a close collaboration among students and faculty from the University of Massachusetts and the Massachusetts Institute of Technology to characterize the petrologic and structural setting of chronometer phases, and then to carry out high-resolution dating by integrating analyses from both institutions. In addition, graduate and undergraduate students plan and coordinate the field research in the Canadian bush. To date, two of our five Ph.D. students, one M.S student, and three field assistants have been members of underrepresented groups, who serve as role models for younger student researchers. The field and laboratory experiences are transmitted to, and greatly benefit, the larger student body at both institutions through class exercises, student projects, and presentations.