Paleomagnetic evidence for post‐2.55‐GA tectonic tilting and 1.1‐Ga Reactivation in the Southern Kapuskasing Zone, Ontario, Canada

Abstract

We report paleomagnetic results from two traverses in the southern part of the Kapuskasing structural zone (KSZ) of north‐central Ontario, which is hypothesized to be an oblique cross section through upper and middle Archean crust exposed by uplift and tilting. Anorthosites and granulite‐grade gneisses were sampled to the east in the Shawmere traverse, while amphibolite‐grade tonalitic gneisses were sampled to the west in the Racine traverse. Three distinct remanent magnetization (RM) components were isolated by alternating field and thermal demagnetization. The A RM has mean directions D = 50°, I = +62° (k = 138, α95 = 5°, N = 5 sites (23 samples)) for the Shawmere traverse and D = 23° I = +53° (k = 49, α95 = 7°, N = 8 sites (46 samples)) for the Racine traverse. Because of the high (>500°C) laboratory unblocking temperatures of the A RM in the Shawmere granulite‐grade rocks, we believe this magnetization was acquired in early slow uplift, without tilting, from about 24‐km to about 16‐km depths, and has survived the later main KSZ uplift and tilting event. The Racine A RM has laboratory unblocking temperatures down to ≈400°C and was blocked prior to uplift from a shallower crustal level (about 8 km). With tilt corrections of 30° and 15°, respectively, about the 35° striking axis of the KSZ, the Shawmere and Racine A paleomagnetic poles fall on the Laurentian apparent polar wander path around 2.55 Ga, dating the time of initial uplift. Crustal tilting in the KSZ occurred after 2.55 Ga and was greater in the east than in the west. The B RM, found mainly in Shawmere anorthosites, has D = 115°, I = −68° (k = 17, α95 = 17°, N = 4 sites (14 samples)), with a paleopole at 144°W, 49°N, near 1.1 Ga Keweenawan reverse poles and the normal pole of the nearby 1.0 Ga Nemegosenda carbonatite. The B RM is probably a thermochemical overprint related to reactivation of the KSZ at the time of Keweenawan rifting and volcanism in the Lake Superior region. A novel C RM, with D = 64°, I = +2° (k = 18, α95 = 13°, N = 6 sites (10 samples)), is either an unresolved hybrid of the A and B RMs or an earliest Keweenawan (1.3 Ga) overprint.