Paleomagnetic evidence for proterozoic tectonism in the Kapuskasing Structural Zone, Ontario

Abstract

The Kapuskasing Structural Zone (KSZ) in the Superior Province of north‐central Ontario is thought to be an oblique cross‐section through Middle Archean crust. Samples from 30 sites in granulite gneisses and anorthosites of the KSZ have been analyzed using alternating field and thermal demagnetization methods. Three remanent magnetization components were found with mean directions of: D = 23°, I = 50° (k =52, α95 = 11°, N = 5 sites) for the A component; D = 73°, I = −68° (k = 50, α95 = 5°, N = 18) for the B component; and D = 332°, I = −59° (k = 53, α95 = 17°, N = 3) for the C component. V. Constanzo‐Alvarez and D. J. Dunlop found similar A and B components in less metamorphosed rocks from the same units to the immediate west. They attributed the A component to uplift at 2.55 Ga with subsequent 15° to 30° WNW tilt and the B component to thermochemical overprinting at 1.1 Ga as the KSZ was reactivated during Keweenawan rifting and volcanism. We also attribute the A component to uplift during the 2.55 Ga Kenoran Orogeny with concomitant 10°±5° WNW downward tilting before intrusion of the 2.45 Ga Matachewan dikes, to give its present paleopole of 33°E, 67° (dp = 6°, dm = 9°, N = 13) using data from both studies. We attribute acquisition of the B component to uplift at about 1.88 Ga, as suggested by recent thermal modeling and tectonic syntheses, to give a paleopole of 124°W, 30°N (dp = 7°, dm = 8°, N = 20). The C component is found in only three sites and may be either an hybrid of a 1.1 Ga Keweenawan CRM with the B component or a fault‐block tilted B component near the Ivanhoe Lake cataclastic zone.