Some border relations between supracrustal and infracrustal rocks in South-West Greenland

Abstract

Three types of border relation between supracrustal and intracrustal rocks are described from South-West Greenland. In the Ravns Storø area a largely volcanic series, possibly equivalent in age to the Tartoq Group, passes downwards into the underlying gneisses via a transitional migmatite front. The supracrustal bedding passes conformably downwards into the gneissic foliation and there is a progressive downward increase in metamorphic grade and degree of mineral orientation. In the northern part of the Ivigtut region Ketilidian supracrustals lie with a major unconformity on pre-Ketilidian gneisses (and Tartoq Group) supracrustals; between which there is a migmatite front relation. Passing southwards the autochthonous unconformity between the Ketilidian supra­crustals and the underlying rocks is affected by a series of progressive tectonic modifications. Faulting parallel to the axial planes of Ketilidian second phase folds in the supracrustals is followed by thrusting along the unconformity, giving rise to a parautochthonous border, which further south passes into an allochthonous border modified by gneissification and associated with the formation of granitic rocks. Earlier basic dykes together with the gneissic foliation are dragged along the transitional border which simulates a transition zone between a superstructure and an infrastructure. In association with this increase in tectonisation there is a concomitant increase in grade of metamorphism of the supracrustal rocks from an almost unmetamorphosed state through greenschist facies and epidotemphibolite facies to amphibolite facies.