The Sudbury Structure (SS) is an unusual crater structure which acquired its present oval surface shape during northwestdirected ductile thrusting. Lower amphibolite-facies metamorphism accompanied the thrusting which generated a major reverse shear zone. At least 50 km long, the South Range shear zone (SRSZ) transects the South Range of the Sudbury Structure and exhumes a low level of the Sudbury Igneous Complex (SIC). Assuming heterogeneous simple shear in the northwest-southeast vertical plane on northeasterly striking glide surfaces, minimal estimates of net displacement across the SRSZ exceed 8 krn. This displacement magnitude and the map pattern of the SIC require the southwest closure of the SS to be steeply plunging, in accord with a hypothetical funnel shape of the SIC. The rocks of the metasedimentary core of the SS are deformed into a family of second-order buckle folds, the tangent surface of which forms an upright open flexure within the first-order structure of the Sudbury synclinorium. The original orientation and bulk rotation of contacts in the SIC are unknown, so its participation in large-scale folding remains uncertain.La structure de Sudbury a une forme inhabituelle de cratere, dont la surface actuelle de forme ovale rksulte d'un chevauchement ductile de direction nord-ouest. Un mktamorphisme du facits infkrieur des amphibolites a accompagnk ce chevauchement qui a crkt une importante zone de cisaillement inverse. La zone de cisaillement de la rkgion South Range de la structure de Sudbury, longue d'au moins 50 km, recoupe la rkgion South Range et expose un niveau infkrieur du Complexe ignk de Sudbury. En supposant un cisaillement simple hktkrogene dans le plan vertical nord-ouest-sud-est sur des surfaces de glissement de direction nord-est, l'estimation minimum du dkplacement net en travers de la zone de cisaillement de la rtgion South Range dkpasse 8 km. Cette amplitude de dkplacement et la configuration du Complexe ignt de Sudbury sur la carte requikrent un plongement abrupt de la fermeture sud-ouest de la structure de Sudbury, ce qui concorde avec I'hypothtse d'une forme en entonnoir pour le Complexe ignk de Sudbury. Les roches de la partie mktaskdimentaire centrale de la structure de Sudbury sont dCformkes en une skrie de plis de flexion de deuxieme ordre. La surface tangentielle de celle-ci forme une flexure droite ouverte au sein de la structure de premier ordre du synclinorium de Sudbury. L'orientation originale ainsi que la rotation d'ensemble des contacts dans le Complexe ign6 de Sudbury sont inconnues et, par conskquent, leur participation dans la phase de plissement rkgional demeure incertaine.
The U–Pb isotopic ages of metamorphic zircons from deformed amphibolitized metadiabase dykes and associated unstrained anatectic pegmatites have been determined to constrain the times of postdyke ductile flow related recrystallization and deformation. Samples from the Central Gneiss Belt of the Grenville Province in the southern Georgian Bay region provide a means of linking areas formed during common episodes of ductile flow. Three zircon fractions from one attenuated dyke in the Moon River domain gave identical concordant ages of 1062, 1065, and 1064 ± 2 Ma, while those from another gave identical concordant ages of 1052, 1051, and 1050 ± 2 Ma for the time of metamorphism. Zircons from a folded dyke from the Go Home domain gave identical concordant ages of 1049, 1047, and 1046 ± 2 Ma, while a single overgrowth fragment from an unstrained anatectic pegmatite in the fold hinge gave a concordant age of 1047 ± 2 Ma, as if folding, metamorphic zircon growth and postfolding pegmatite are part of the same process. An anatectic pegmatite in a metadiabase boudin neck in the Ahmic domain gave two zircon fractions with identical concordant ages of 1079 and 1080 ± 2 Ma that match the concordant 1078 ± 2 Ma age of a single overgrowth type grain from the host migmatite. Zircons from the metadiabase have similar but less precise ages due to a low level of uranium and a low abundance. U–Pb results for the Love Lake granite situated at the Parry Sound–Ahmic domain boundary are equivocal but may imply folding of this structure after 1068 Ma, the age of contained titanite. Either the upper or lower end of the ca. 1050–1372 Ma zircon discordia may correspond to granite emplacement. Zircons from a dyke that postdates a granulite-grade migmatization in the Parry Sound domain record a metamorphism at 1114 ± 2 Ma but older inherited grains are present in other fractions. Ductile flow in the Go Home and Moon River domains occurred in the interval 1047–1064 Ma during which the latter was probably emplaced, whereas earlier (1080 Ma) ductile flow in the Ahmic domain may relate to the time of overthrusting of the Parry Sound domain. Parry Sound domain dyke metamorphic ages imply a lack of post 1114 Ma migmatization, and a predyke migmatization consistent with a more exotic derivation of this domain. A [Formula: see text] Ma age for a postmigmatite crosscutting pegmatite dyke sets an upper limit on the age of a late buckling event in the Ahmic domain.
Detailed structural maps of two granitoid complexes in the Wabigoon Subprovince are used to test three diapir hypotheses advanced in earlier papers. The gneiss masses of, and individual domes within, the complexes fail the test for solid-state diapirism. The gneiss domes also fail the test for tensile bending caused by hypothetical magmatic diapirs in the subsurface. An oval pluton located near the best-exposed gneiss dome proves to be a synformal sheet rather than a funnel-shaped magmatic diapir. This pluton could be a syenite–diorite phacolith emplaced into a concordant zone of dilation during the late upright folding of the gneiss mass about horizontal axes. Earlier tight folds were probably recumbent and south verging and, like the gneissosity, generated in a ductile shear regime with subhorizontal glide planes. These observations have important implications for Archean tectonics, especially the relative horizontal displacement of large greenstone masses (potentially allochthonous greenstone belts).
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