Structural analysis of the central and southwestern Sudbury Structure, Southern Province, Canadian Shield

Abstract

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.

Emplacement geometry of the Sudbury Igneous Complex: Structural examination of a proposed impact melt-sheet

“…These strain patterns, cannot explain the sharp curvature of the SIC at these localities. Similarly, large strike separations on faults transecting the southwestern SIC are attributed to northwestward translation of the South Range on thrust surfaces of the South Range Shear Zone (Shanks and Schwerdtner, 1991a).…”

“…Evidence for large degree of solid-state strain, however, is clearly lacking in the North Lobe. Shanks and Schwerdtner (1991a); those in Huronian rocks are from Riller et al (1996) and Riller and Schwerdtner (1997). Ellipses represent the arithmetic mean of the normalized horizontal sectional aspect ratio of measured primary markers and Sudbury Breccia fragments per station.…”

“…In the light of our structural results, such models and the fractionation mechanism of the SIC require revision. (Riller and Schwerdtner, 1997); MF = Murray Fault; SRSZ = South Range Shear Zone (Shanks and Schwerdtner, 1991a). B, Northwest-southeast profile of the Sudbury Basin and the Sudbury Dome based on the Lithoprobe seismic profile (Milkereit et al, 1992).…”

Structural characteristics of the Sudbury impact structure, Canada: Impact-induced versus orogenic deformation-A review

“…The most prominent structure in the Sudbury area, that is likely associated with Penokean deformation, is the South Range Shear Zone (SRSZ in Fig. 2b : Wilson 1956;Rousell 1975;Shanks and Schwerdtner 1991a), a south-dipping ductile thrust that displaced the South Range and its underlying Huronian rocks toward the NW. This zone transects the Main Mass of the SIC south of the SE-lobe ) and at the southwestern terminus, where strain is concentrated at the interface between Archean basement and Huronian cover rocks (Riller et al 1998).…”

“…Unfortunately, their work leaves open how the 2D strain field on which the reconstruction is based was acquired and to what extent discontinuous deformation was considered. Unlike the finite-element model by Shanks and Schwerdtner (1991a), the reconstruction does not take into account vertical stretching of rock contained in (1) steeply inclined mineral lineations of ductilely deformed rocks, (2) folds in the Whitewater Group and the SIC and (3) zones of reverse shearing. Thus, the reconstruction is based on the unrealistic assumption that the pre-deformational erosion surface coincides with today's peneplain.…”