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弱的中深部地壳流动的地形表现。地壳流动可能是山带高程和宽度的最终控制因素之一，它可以解释远离其原产地的岩石的横向平移。下地壳流动和由此产生的亚水平构造也可以解释许多地质和地球物理观测结果，包括在深地震反射调查中发现的高反射率和片状结构的常见现象。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，重要的是要了解深部地壳的性质、下地壳变形的机制以及板块构造事件中对岩石强度（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大陆地壳暴露区，它仍然保留了大部分的深部地壳特征。大陆地壳最下端的大片区域是沿着一条大断裂——莱格斯湖剪切带被带到地表的。目前暴露的太古宙至古元古代深地壳面积超过2万平方公里。该区的特征之一是早期麻粒岩级、渗透性、亚水平构造（矿物和成分层的构造排列）。本研究的目的是探索在加拿大西部这部分地区的生长过程中，由下大陆地壳流动形成的亚水平构造的假设，并进一步探讨这种构造可能为现代深部大陆地壳提供线索。东阿萨巴斯卡麻粒岩地体是北美深部地壳最大、暴露程度最好的露头之一，是EarthScope设备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一个实地研讨会奠定基础，该研讨会将联合地球物理学家和地质学家研究下地壳的暴露。我们的目标不仅是讨论地壳深部的过程，而且要开始合作开发这个独特的、可进入的研究区域，作为研究最下层大陆地壳组成和行为的天然实验室。这个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，以确定地质事件的年龄，变质反应，以及地质过程的持续时间，在具有漫长而复杂的构造历史的地区，如东阿萨巴斯卡麻粒岩地。马萨诸塞大学和麻省理工学院的学生和教职员工密切合作，对计时器阶段的岩石学和构造环境进行表征，然后通过整合两家机构的分析来进行高分辨率的测年。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，我们的五名博士生中有两名，一名硕士学生和三名现场助理是代表性不足的群体的成员，他们是年轻学生研究人员的榜样。实地和实验室经验通过课堂练习、学生项目和报告传递给两所院校的广大学生群体，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.