Evidence from aeromagnetics on the configuration of Matachewan dykes and the tectonic evolution of the Kapuskasing Structural Zone, Ontario, Canada

Abstract

By digital image processing of federal–provincial aeromagnetic survey data for the south-central Superior Province, we have obtained an improved picture of the distribution of dykes in the huge Matachewan mafic dyke swarm (2454 Ma). We deduce from it a picture of post-emplacement deformation in the vicinity of the uplifted granulite gneisses of the Kapuskasing Structural Zone (KSZ). Matachewan dykes are emplaced in three subswarms. The two easterly subswarms are clearly truncated by the KSZ's eastern boundary faults. The western subswarm shows an open Z-bend as it crosses the KSZ, but it does not reveal any major fault offset. On the plausible assumption (supported by paleomagnetic data) that the subswarms were originally intruded radially, the horizontal strain suffered by the KSZ since emplacement of the dykes is mainly a northeast–southwest-trending band of dextral transcurrent deformation, which in the northeast is discontinuous and concentrated in a fault (horizontal offset 60–80 km) and in the southwest widens through a series of horsetail splays into an ~80 km wide zone of distributed strain. The KSZ is believed to have formed by a major, crustal-scale, thrust uplift along the KSZ's southeastern margin. Some thrusting is recorded by the dyke pattern, but this can account for only part of the ~20 km of differential uplift seen in the KSZ. Most likely, the mainly transcurrent deformation recorded by the dykes is a secondary event, and the primary period of thrust uplift predated dyke injection.