The Sudbury dyke swarm and its bearing on the tectonic development of the Grenville Front, Ontario, Canada

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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 dykes, correlative in age and chemical composition with the Sudbury swarm, cut gneissic fabrics in the Grenville Province. Some irregularities in dyke trend, including marked deflections in strike at and southeast of the Grenville Front mylonite zone, are primary features, resulting from dyke propagation across pre-existing structure and an associated change in paleostress field. However, other irregularities, in particular the markedly sinuous traces of many dykes, analogous to classic buckle fold forms, are attributable to superimposed (Grenvillian) deformation. The deformation history of the dykes is constrained by a distinctive sequence of macroscopic and microscopic structures. Early deformation, related to northwest-southeast compression and accompanying overthrusting, resulted in layer-parallel shortening and buckling of the dykes in a regime with elements of both pure and simple shear, a process accommodated internally by alignment, bending and kinking of plagioclase and other primary minerals. Coeval metamorphism, manifested by reaction coronas around primary olivine and Fe-Ti oxide, outlasted early deformation. A later period of deformation resulted in tightening of the buckle folds, superseded by faulting, in response to further strain accumulation. The equivalent microscopic-scale structures are late fractures and brittle-ductile microfaults which post-date plastic strain. A cause-and-effect relationship, whereby pre-metamorphic microstructures strongly influenced the location and development of syn- to post-metamorphic microstructures, suggests that the two phases of dyke deformation represent more or less a continuum, separated only by a period of decreased strain rate coinciding with the peak of metamorphism. In relation to existing models for the tectonic development of the northwestern Grenville orogen, the progression of dyke structures and their relationship to metamorphism are best explained by three principal stages: (1) at ≧ 1035 Ma, an early stage of penetrative shortening and overthrusting, correlated with buckling of the dykes and their depression to lower crustal levels; (2) an intermediate stage, commencing some time before 1020 Ma, when compressive stresses waned, enabling thermal relaxation, and ductile extension took place in the interior of the orogen, initiating passive uplift of the dykes; (3) at 1010-980 Ma, a stage of renewed thrusting during which the dykes were further deformed as they were uplifted across the ductilebrittle transition, documenting the final advance of the Grenville orogen toward its foreland.