THE SUDBURY DYKE SWARM AND ITS BEARING ON THE TECTONIC DEVELOPMENT OFTHE GRENVILLE FRONT, ONTARIO, CANADA

In the Southern Province, Ontario, southeast-striking diabase dykes of the 1235 Ma Sudbury continental swarm intrude folded Huronian strata and Paleo-and Mesoproterozoic plutons. The dykes are truncated at the Grenville Front, southeast of which irregularly oriented metadiabase d ykes, correlative in age and chemical composition with the Sudbury swa rm, cut gneissic fabrics in the Grenville Province. Some irregularitie s in dyke trend, including marked deflections in strike at and southea st of the Grenville Front mylonite zone, are primary features, resulti ng from dyke propagation across pre-existing structure and an associat ed change in paleostress held. However, other irregularities, in parti cular the markedly sinuous traces of many dykes, analogous to classic buckle fold forms, are attributable to superimposed (Grenvillian) defo rmation. The deformation history of the dykes is constrained by a dist inctive sequence of macroscopic and microscopic structures. Early defo rmation, related to northwsst-southeast compression and accompanying o verthrusting, resulted in layer-parallel shortening and buckling of th e dykes in a regime with elements of both pure and simple shear, a pro cess accommodated internally by alignment, bending and kinking of plag ioclase and other primary minerals, Coeval metamorphism, manifested by reaction coronas around primary olivine and Fe-Ti oxide, outlasted ea rly deformation. A later period of deformation resulted in tightening of the buckle folds, superseded by faulting, in response to further st rain accumulation. The equivalent microscopic-scale structures are lat e fractures and brittle-ductile microfaults which post-date plastic st rain. A cause-and-effect relationship, whereby pre-metamorphic microst ructures strongly influenced the location and development of syn-to po st-metamorphic microstructures, suggests that the two phases of dyke d eformation represent more or less a continuum, separated only by a per iod of decreased strain rate coinciding with the peak of metamorphism. In relation to existing models for the tectonic development of the no rthwestern Grenville orogen, the progression of dyke structures and th eir relationship to metamorphism are best explained by three principal stages: (1) at greater than or equal to 1035 Ma, an early stage of pe netrative shortening and overthrusting, correlated with buckling of th e dykes and their depression to lower crustal levels. (2) an intermedi ate stage, commencing some time before 1020 Ma, when compressive stres ses waned, enabling thermal relaxation, and ductile extension took pla ce in the interior of the orogen, initiating passive uplift of the dyk es; (3) at 1010-980 Ma, a stage of renewed thrusting during which the dykes were further deformed as they were uplifted across the ductile-b rittle transition, documenting the final advance of the Grenville orog en toward its foreland. (C) 1997 Elsevier Science B.V.