Tectonic interpretations of systematic variations in quartz c-axis fabrics across the Thompson Belt

Abstract

The Thompson Belt is a linear NE-trending belt of Archean and early Proterozoic rocks forming the boundary between the Archean Superior and early Proterozoic Churchill provinces. The gneisses in the Belt record deformation which took place during the early Proterozoic Hudsonian orogeny. Quartz c-axis fabrics were measured from gneisses collected along a transect across the western portion of the Thompson Belt. Well developed quartz c-axis fabrics were found in virtually all gneisses, even in coarse-grained, quartz-poor and weakly lineated rocks. Several distinctive c-axis fabric types were found to occur throughout the area and can be related to specific deformation events. Different c-axis fabric types can be linked to the activation of separate glide systems within quartz at different stages of the orogeny. Polar asymmetry within fabrics that should be symmetric, if they had formed from a population of grains with random orientations, are evidence that the present c-axis fabric has overprinted an earlier preferred crystallographic orientation, c-axis fabric variations in the Thompson Belt show similarities to variations observed in the Saxony granulite region. Results of this study indicate that quartz c-axis fabric analysis can contribute to the knowledge of the regional deformation in the Thompson Belt. Regional quartz c-axis variations may prove to be useful for studying the deformation history of other high-grade gneissic terrains.