TECTONIC EVOLUTION OF THE NORTHEAST PORTION OF THE ARCHEAN ABITIBI GREENSTONE-BELT, CHIBOUGAMAU AREA, QUEBEC

The Chibougamau area, occupying the northeastern part of the Abitibi greenstone belt is a large synclinorium of volcanic and sedimentary rocks enclosed within tonalitic gneisses. Several east-west-trending regional folds within this synclinorium are responsible for the vertical attitude of the strata. Syniclinal structures, with youngest sediments within the core, possess axial-plane schistosity. Anticlines, on the other hand, either form domes with a core occupied by earlier tonalitic to dioritic plutons or are transected by a series of east-west-trending ductile faults (the Waconichi tectonic zone). An early deformation phase of low intensity (D1) generated broad, north-south folds without schistosity. The subsequent regional deformation, event D2, produced the large east-west folds. These deformations, in combination, produced the regional interference pattern of domes and basins. North-south horizontal shortening generated an east-west-trending schistosity associated with a vertical stretching lineation. Regional deformation at its climax produced a tightening of folds and rotation of fold axes parallel to the stretching lineation. Plutons deflected the regional east-west schistosity and formed concentric trajectories associated with "contact-strain aureoles." This produced small interaction zones or triple points characterized by strong vertical extension. These relations suggest an interference between regional stress field, which produced north-south horizontal shortening, and local stress fields, controlled or deflected by granitoid plutons acting as competent bodies. East-west trending ductile shear zones represent the final stage of the regional deformation. The observed northward and southward reverse movement along these east-west faults, their parallelism to the axial trace of folds, and the regional schistosity are probable evidence of a regime dominated by a coaxial strain.