Thrust stacking in the inner Nordre Stromfjord area, West Greenland - Significance for the tectonic evolution of the Palaeoproterozoic Nagssugtoqidian orogen

The Nagssugtoqidian orogen is characterised by a predominance of Archaean g neisses that were reworked at moderate to deep crustal levels during the Pa laeoproterozoic, resulting in an ENE-trending structural grain. A detailed study in the central part of the orogen has shown that a lithotectonic unit containing Palaeoproterozoic supracrustal and igneous rocks, the Ussuit un it, was juxtaposed against Archaean gneisses along a tectonic contact. Subs equently, the sequence of Archaean gneisses overlain by the Ussuit unit was repeated by thrust imbrication. Tectonic contacts and thrusts are presentl y ductile shear zones, but some may have originated as brittle structures. Stretching lineations are consistently shallowly ESE- to E-plunging and kin ematic indicators show predominantly top-to-NW movements. Significant volum es of melt in the high-strain zones indicate that shearing occurred at high temperatures. While high temperatures and partial melting still prevailed, the thrusts/shear zones were deformed in kilometre-scale, open, upright fo lds. The change from thrusting to folding was accompanied by rotation of th e convergence direction from WNW-ESE to approximately N-S. This later defor mation resulted in the presently observed ENE-trending structural grain. Th e observation that a period of terrane accretion and crustal thickening by thrusting and shearing predated the formation of the ENE trend of the oroge n, adds further support to the hypothesis that the Nagssugtoqidian orogen i s a collisional orogen. (C) 1999 Elsevier Science B.V. All rights reserved.