Onaping fault system: age constraints on deformation of the Kapuskasing structural zone and units underlying the Sudbury Structure

Abstract

The deformation of the Matachewan dyke swarm of the Superior Province, which includes a 60 km right-lateral offset and moderate uplift localized along the Kapuskasing structural zone, represents an important time marker in the evolution of the zone. However, the age of this deformation is poorly constrained. Here, it is shown that the prominent Mattagami River – Upper Wanapitei River fault of the Onaping fault system, which offsets 2167 Ma Biscotasing dykes south of the Kapuskasing structural zone, can be aligned with a fault of similar offset north of the zone after the Matachewan swarm is restored to its predeformational configuration. Thus, the deformation of the Matachewan swarm in the vicinity of the Kapuskasing structural zone must postdate the 2167 Ma emplacement and the subsequent faulting of the Biscotasing dykes. Southward extensions of faults of the Onaping fault system appear to offset aeromagnetic anomalies associated with highly magnetic units in the basement beneath the Sudbury Structure and Huronian sediments of the Southern Province, but do not substantially offset the Sudbury Structure itself. This suggests that the 1850 Ma Sudbury Structure was emplaced after most of the displacement on Onaping faults, while the unit underlying it, variously interpreted as the Archean Levack Gneiss Complex or a hidden ultramafic body of unknown age, predates Onaping faulting.