STYLE AND INFLUENCE OF SUPERIMPOSED SE-DIRECTED DEFORMATION ON TACONIAN STRUCTURES AT THE FORELAND-HINTERLAND BOUNDARY, SOUTHERN QUEBEC APPALACHIANS

The Appalachian Belt, about 50 km south of Quebec City, encompasses the transition zone between the polytectonized hinterland and the essentially monophased foreland sequence of the Laurentian margin. Two main tectonic events are documented. The Taconian D₁ phase (462±3) is associated with a regionally developed foliation, northwest-directed thrust sheet emplacement, and regional metamorphism. The metamorphic grade increases from prehnite-pumpellyite to greenschist grade toward the southeast as the S₁ fabric evolves from a slaty cleavage into a penetrative schistosity in the same direction. A major D₁ fault, the Richardson thrust, is delineated by the Sainte-Marguerite Complex (fault slivers of Precambrian basement) and the Rivière Boyer tectonic mélange, and is affected by a regional-scale F₂ fold. In the Sainte-Marguerite Complex, the unconformable relationship between the Precambrian basement and Cambrian volcanic and sedimentary cover is locally preserved. The second tectonic event (D₂, 431 to 411 Ma) corresponds to a reversal of the structural vergence of regional deformation, which is manifested by hinterland-directed faulting (backthrusting) and southeast-verging folding. D₂ structures are discrete in the northeastern part of the study area but their intensity and influence on D₁ structures increase southeastward. The S₂ fabric evolves from a fracture cleavage or a crenulation cleavage in the northwestern part of the study area, to a penetrative foliation, which obliterates older fabrics along and southeast of the D₂ Bennett fault. The latter fault apparently represents the dominant structure marking the transition between the foreland and hinterland of the Laurentian Margin sequence. The ongoing study aims at characterizing the tectonic significance of the Richardson fault, Rivière Boyer Mélange, and the influence and structural style of the D₂ event on D₁ structures. For example, the Richardson fault is most probably truncated by the Bennett backthrust but field evidence of this truncation has been previously lacking.