Kathryn M. Bethune

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

Abstract 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 A later period of deformation resulted in tightening of the buckle folds, superseded by faulting, in response to further strain accumulation.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.

Anton terrane revisited: Late Archean exhumation of a moderate-pressure granulite terrane in the western Slave Province

The Anton terrane of the western Slave Province has been interpreted as a remnant of a meso-Archean (3500–2800 Ma) microcontinent. The largest coherent entity of the terrane is a previously unrecognized 50 by 250 km granulite complex, herein termed the Snare domain. The granulite facies mineral assemblages give the domain a characteristic magnetic signature that is distinct from areas of exposed meso-Archean basement in the Slave Province. The domain consists of interlayered 3100–2624 Ma orthogneisses and paragneisses and 2600–2585 Ma granitoid rocks; its structure was largely established during 2610–2590 Ma orogeny throughout the province that culminated in ca. 2590 Ma granulite facies metamorphism. Pressure-temperature ( P-T ) determinations yield peak conditions of 775–875 °C at 6–7 kbar. Shortly thereafter, 2–4 kbar of decompression occurred before the domain cooled below 450 °C. Decompression and retrogression textures are widespread but are most pervasive adjacent to late extensional detachment faults. U-Pb zircon age data suggest that the Snare domain was partially exhumed from mid-crustal levels ca. 2585–2580 Ma. We attribute exhumation to tectonic denudation of an overthickened crustal welt in the western Slave Province. Localization of the crustal welt in this part of the Slave Province may be related to the presence there of meso-Archean crust.

Synchronous egress and ingress fluid flow related to compressional reactivation of basement faults: the Phoenix and Gryphon uranium deposits, southeastern Athabasca Basin, Saskatchewan, Canada

Previous studies on unconformity-related uranium deposits in the Athabasca Basin (Canada) suggest that egress flow and ingress flow can develop along single fault systems at different stages of compressional deformation. This research aims to examine whether or not both ingress and egress flow can develop at the same time within an area under a common compressional stress field, as suggested by the reverse displacement of the unconformity surface by the basement faults. The study considers the Phoenix and Gryphon uranium deposits in the Wheeler River area in the southeastern part of the Athabasca Basin. Two-dimensional numerical modeling of fluid flow, coupled with compressional deformation and thermal effects, was carried out to examine the fluid flow pattern. The results show that local variations in the basement geology under a common compressional stress field can result in both egress and ingress flow at the same time. The fault zone at Phoenix underwent a relatively low degree of deformation, as reflected by minor reverse displacement of the unconformity, and egress flow developed, whereas the fault zone at Gryphon experienced a relatively high degree of deformation, as demonstrated by significant reverse displacement of the unconformity, and ingress flow was dominant. The correlation between strain development and location of uranium mineralization, as exemplified by Gryphon and Phoenix uranium deposits, suggests that the localization of dilation predicted by numerical modeling may represent favourable sites for uranium mineralization in the Athabasca Basin.