Paleomagnetism of the Great Slave Supergroup, Northwest Territories, Canada: multicomponent magnetization of the Kahochella Group

Abstract

Results are reported from 59 stratigraphic horizons spanning an aggregate of some 2000 m of sedimentary rocks of the Great Slave Supergroup, mostly located in the Kahochella Group. Partial demagnetization experiments and vector differences indicate that the magnetization involved is multicomponent, with a well-defined overprint masking an underlying magnetic component. The overprint has now been recognized in many formations of the Great Slave Supergroup and is interpreted as having been acquired in response to uplift and cooling following orogeny in the Coronation Geosyncline. It corresponds to a paleomagnetic pole at 9I0W, 2I0N (A95 = 5'). Removal of this overprint reveals the underlying, presumed primary, magnetization, which has a mean direction of D = 129", I = + 2 l o (N = 18 sites, k = 12, a95 = 10") and a corresponding pole at 62OW, 7"s (K = 15, Ag5 = 9"). The ages of these poles cannot be fixed precisely, but the available data suggest that the "primary" remanence was acquired about -1.8 Ga, and the overprint about -1.7 Ga. The two poles thus help define the Precambrian polar wander curve for Laurentia. In particular they help define the so-called Coronation loop, which reflects the response of Laurentia to the major orogeny in the Coronation Geosyncline.On rapporte les rksultats pour 59 horizons stratigraphiques qui recoupent au total quelques 2000 m de roches skdimentaires dans le supergroupe de Great Slave, localiskes surtout dans le groupe de Kahochella. Les experiences de dksaimantation partielle et les diffkrences entre les vecteurs indiquent que I'aimantation de ces roches possede plusieurs composantes avec une surimpression bien dkfinie qui masque une composante magnktique sous-jacente. On a maintenant reconnu cette surimpression dans plusieurs formations du supergroupe de Great Slave et on I'interprete comme ayant kt6 acquise lors du soultvernent et du refroidissement qui ont suivi I'orogkntse dans le gkosynclinal de Coronation. Elle correspond ?i un pBle palkomagnktique ?i 91'0, 21°N (A95 = 5"). La skparation de cette surimpression rkvele I'aimantation sous-jacente, prksumkment primaire, qui a une direction moyenne de D = 12g0, I = +21° (N = 18 sites, k = 12, ag5 = 10') et un pBle correspondant ?i 62'0, 7"s (K = 15, Ag5 = 9'). On ne peut dkterminer avec prkcision I'bge de ces pBles mais les donnkes disponibles suggtrent que la rkmanence "primaire" a kt6 acquise il y a environ 1,8 Ga et la surimpression il y a 1,7 Ga environ. Les deux p6les aident ainsi ?i dkfinir la courbe de migration polaire de la Laurentie au Prkcambrien. En particulier, ils aident ?i dkfinir la soi-disant boucle de Coronation qui refltte la rkponse de la Laurentie ?i I'orogkntse majeure dans le gkosynclinal de Coronation.