Exhumation of exposed deep continental crust, western Canadian Shield: Integrating structural analysis, petrology, and in situ geochronology.

摘要

High-pressure granulite terranes are important sources of information for understanding deep-crustal architecture and processes related to the evolution and stabilization of continental lithosphere. However, one of the most critical challenges is to understand how, and on what timescales, large exposures of lowermost continental crust are exhumed to the Earth's surface. In the East Lake Athabasca region of the western Canadian Shield, high pressure granulites (∼1.0+ GPa) make up one of the largest exposed deep crustal terranes in the world (>20,000 km2). Important insight into the exhumation history of this region come from the study of the kinematics, timing, and metamorphic evolution of a several km-wide, oblique-slip, thrust-sense mylonitic shear zone (Legs Lake shear zone) that forms a major boundary of the high-pressure terrane. The juxtaposition of crustal levels across this structure is more than 20 km. Integrated structural and petrologic analysis of the shear zone and its wallrocks, combined with in situ electron microprobe monazite Th-U-Pb geochronology and U-Pb isotope geochronology, suggest a multi-stage exhumation history for the high-pressure region that occurred over a period of >100 million years. Similar studies of a second and younger fault zone and of the cross-cutting relationships between the two shear zone systems (with displacements of up to 110 km), provide an explanation for the present-day distribution of high pressure rocks in the region and have important implications for the early growth of this part of Laurentia.