NAPPE EMPLACEMENT DIRECTION IN THE CENTRAL GNEISS BELT, GRENVILLE PROVINCE, ONTARIO, CANADA - EVIDENCE FOR OBLIQUE COLLISION

The Proterozoic Grenville Province in Canada is widely regarded as a compressional orogen in which thrust transport occurred towards the northwest, perpendicular to the orogenic front. New structural data from an area near Parry Sound, Ontario, lead to a re-evaluation of this interpretation. Nappe structures in this area are consistent with northeast rather than northwest-directed thrusting and imply that the southwestern part of the Province experienced an early, sinistral oblique collision which predated the emplacement of the 1.24 Ga Sudbury Dike swarm. Two nappe piles, which contain different rock types and have distinct stacking sequences, are separated by a lateral ramp or ductile tear fault that coincides with the interior of the Parry Sound Domain of Davidson et al. (1982). Early nappes and associated structures (F1) are overprinted by NW-SE-striking upright folds (F2) that formed between about 1. 1 6 and 1. 12 Ga. These later folds partially reorient rotated feldspar porphyroclast systems that formed during nappe emplacement, and have axes that are consistently parallel to a well-developed mineral lineation. The orientation of this lineation reflects the two-stage deformation history of the area rather than the thrusting direction during a single orogenic event.