The Niflheim thrust: a tectonic contact between granulite and amphibolite facies gneisses, South-East Greenland

Abstract

The Niflheim thrust forms part of the northern boundary zone of the Proterozoic mobile belt of the Ammassalik region and defines the southernmost extent of granulite facies gneisses north-west of Sermilik. The thrust sharply separates grey amphibolite facies gneisses (footwall) from a thick and extensive unit of brown granulite facies gneisses, suggesting considerable lateral as well as vertical transport of the brown gneisses. Above the contact, the brown gneisses have only been weakly affected by deformation, whilst below the contact intensely folded and sheared grey gneisses indicate strong deformation of the upper part of the footwall sequence during thrusting. Asymmetry of folds below the contact and the E–W trending, gently north dipping thrust contact indicate a sinistral transpressional sense of movement with an up-to-the-south main component of transport. Three undeformed, discordant basic Proterozoic dykes in the grey gneisses of the footwall are truncated by the thrust and the thrust plane has been gently folded during a late stage of the regional Proterozoic deformation. Contrasts between high-grade mineralogy of Proterozoic dykes in the northern part of the Ammassalik region and lower grade, high crustal-level dykes of the grey gneiss terrain in the south are related to the regional thrusting from the north.