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。西藏大造山高原、南美安第斯山脉和美国西南部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高度和宽度的最终控制因素之一，它可以解释远离其起源地区的岩石的横向平移。下地壳流动和由此产生的近水平结构也可以解释一些地质和地球物理观测结果，包括深地震反射勘测中发现的高反射率和层状结构的常见现象。在地壳流动可以完全纳入到现在或过去的地质构造模型，重要的是要了解地壳深部的性质，下地壳变形的机制，以及在板块构造事件的岩石强度（流变学）的控制。雪鸟构造带的中心部分可以说是世界上最大的下陆壳暴露，仍然保留了大部分的深地壳特征。大面积的最低大陆地壳被带到表面沿着一个主要的断层，腿湖剪切带。目前的暴露代表了超过20，000平方公里的太古代至古元古代地壳深部。该地区的一个特征是早期麻粒岩级、渗透性、近水平组构（矿物和成分层的构造排列）。本研究的目的是探讨假设，亚水平组构造成的流动下大陆地壳在加拿大西部的这一部分的增长，并进一步，这种结构可能提供洞察现代深部大陆地壳。东阿萨巴斯卡麻粒岩是北美最大和最好的地壳深部露头之一，是地球镜设施和了解大陆地壳一般性质的重要资源。目前的研究将为在该地区举办一次实地讲习班奠定基础，该讲习班将使地球物理学家和地质学家联合起来研究下地壳的暴露问题。目标不仅是讨论地壳深部过程，而且开始合作努力，开发这一独特和可访问的研究区，作为研究最低大陆地壳组成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的地质年代学研究，旨在开发新的工具和程序，用于确定地质事件的年龄，变质反应，以及在东阿萨巴斯卡麻粒岩等具有长期和复杂构造历史的地区的地质过程的持续时间。马萨诸塞州大学和马萨诸塞州理工学院的学生和教师密切合作，确定计时器相位的岩石学和结构背景，然后通过综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在加拿大丛林的实地研究。到目前为止，我们五个博士中的两个。学生，一个硕士生，和三个现场助理一直是代表性不足的群体，谁作为年轻的学生研究人员的榜样成员。现场和实验室的经验被传输到，并大大受益，在两个机构通过课堂练习，学生项目和演示的更大的学生团体。

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.