Polyphase Deformation in the Canadian Shield of Northeastern Alberta

The Canadian Shield of northeastern Alberta, within the Athabasca obile Belt, consists largely of reworked Archean basement. Two generations of structures can be distinguished in the granite gneisses and intercalated high-grade metasediments of the basement neiss complex, which has been shown to be Archean by radiometry dating. The older generation (D1), characterized by tight to isoclinal folds in the foliation, is assigned to the Kenoran orogeny. The younger (D2) folds are more open, and have affected D1 axial traces. In some areas, D2 folds form parts of domes within the neisses.

Most granitoid masses give radiometry ages of about 1900 Ma and show a dome and basin geometry. These domes are generally immature diapirs and are correlated with D2 structures in the gneisses. The predominance of D2 structures in the granitoids is the basis for assigning these structures to the Aphebian. The environment of deformation, indicated by metamorphic minerals within the enclosed high-grade metasediments, shows temperatures of 740 + 30 degrees elsius and pressures of 5 + 0.7 kbar. Some of the D2 folds fit a conical fold model. Post D2, large scale fault zones are accompanied by extensive mylonitization. Jointing is related to regional stress fields.