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.
