FOLDING AND FRACTURING NEAR THE INTERFERENCE JUNCTION OF THREE ARCHEAN GRANITOID TERRANES

An Archean greenstone volcano-sedimentary pile at Black Island, L. Winnipeg, Canada is separated from granite-gneiss terranes (2.7 by, U-Pb) to the east and south by a narrow zone of strongely foliated quartz-diorite and tonalite gneiss (3.0 and 2.9 by, U-Pb) that is in part the basement for the pile. This pile now forms NNW- and WSW-trending arms at the junction of three greenstone belts squeezed between three granitoid masses that ascended within a field of NNE-trending maximum regional compression. The rising and expanding granitoid terranes induced subhorizontal axis folding parallel to their interference fronts (the greenstone belts), with the developement of a strong subvertical axial plane schistose foliation. As the granitioid masses stabilised, the regional compression dominated the deformation causing horizontal transport parallel to the two greenstone ‘arms’. This was manifested as a megaboxfold embracing the whole area, pervasive smaller bend-glide folding, mesosocopic fracturing and fracture cleavage, and finally major transcurrent faulting. This faulting developed as the two inner limbs of the megaboxfold failed and reversed the movement on the limbs. In the NNW-trending ‘arm’ left-lateral slip brought western side gneisses south and re-oriented earlier formed chevron axes. In the WSW-trending ‘arm’ most of the earlier structures were destroyed by strong right-lateral pervasive shearing with associated foliation development and some subsidiary fracturing. Some fracture sets parallel earlier foliations and the axial planes of bend-glide folds. In the clearer NNW-trending belt unequally developed Reidel fracture sets are associated with ductile faulting along the belt. Slight changes in attitude of some fracture sets suggest they formed before the full development of the bend-glide folds and were subsequently rotated. Due to rock mass heterogeneity and the formation of the megaboxfold, overall fracture set patterns appear complex though the local domains are simple.