Abstract

An extensive paleomagnetic study of the 2.45 Ga Matachewan dyke swarm of the North American Superior Province suggests that the interior of an Archean shield can undergo broad-scale distortion as a result of later (Proterozoic) orogenic activity around the craton margins. Data collected from over 300 sites, of which 137 are reported here for the first time, reveal that the dykes contain a dual-polarity primary remanence that varies across the swarm in both inclination and declination. These regional variations are statistically significant at the 95% confidence level, and cannot be attributed to remagnetization or to magnetic anisotropy. Inclination variation is probably due to real or apparent polar wander during the emplacement of the swarm, and may in part explain the declination variation as well. However, for dykes within and northwest of the Kapuskasing Structural Zone (KSZ) a positive correlation is found between regionally averaged values of declination and dyke trend. Here the dykes appear to have suffered differential rotations about vertical axes of up to 40° since emplacement. The Matachewan swarm radiates northwards from a broad focus situated approximately in northern Lake Huron but the trend of the western half of the swarm follows a broad Z-shaped pattern where it crosses the KSZ. Our data suggest that this changing trend is a secondary feature and that the western dykes, like their eastern counterparts, originally had a more uniform trend. This large-scale distortion of the western Matachewan swarm and Archean host rocks within and north of the KSZ is probably the result of broad-scale deformation during the Trans-Hudson Orogeny at about 1.95 Ga, coeval with uplift along the KSZ.

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