Structure of the Palaeoproterozoic Nagssugtoqidian Orogen, South-East Greenland: Model for the tectonic evolution

Abstract

The Palaeoproterozoic Nagssugtoqidian Orogen extends over 250km along the east coast of Greenland around the settlement of Tasiilaq. The orogen includes Archaean rocks from the adjoining Rae Craton to the north and the North Atlantic Craton to the south, and Palaeoproterozoic rocks. The Rae Craton consists of orthogneiss and amphibolite included in the Schweizerland and Kuummiut Terranes, and is tectonically overlain in the Kuummiut Terrane by ca. 2100–2200Ma units that include marble, meta-pelite and -psammite and amphibolite assigned to the Helheim and Kuummiut units. The Kuummiut Terrane was probably subducted underneath the SE-trending ca. 1885Ma Ammassalik Intrusive Complex, which has a high-temperature metamorphic halo and is characterised by a change from sinistral faulting to pure shear deformation. The southern Isortoq Terrane consists of medium-pressure amphibolite facies bimodal meta-volcanic and <1910Ma meta-sedimentary rocks assigned to the Kap Tycho Brahe unit, which is in tectonic contact with orthogneiss and amphibolite of the North Atlantic Craton.The rocks of the Kuummiut Terrane were tectonically imbricated in ENE-verging structures during ca. 1870Ma high-pressure metamorphism. This was followed by NE–SW convergence and close to orthogonal extrusion in the weakened crust, which is characterised by partial melting during decompression. The rocks of the Isortoq Terrane were imbricated in a SE-vergent thrust and ramp system either during oblique subduction of the Kuummiut Terrane or an earlier tectonic stage elsewhere. NE–SW compression, as in the northern terranes, formed SW-vergent thrust systems and folds. This was most likely caused by a change in the regional stress field during collision between ca. 1870 and 1820Ma. In the north, the Schweizerland Terrane was juxtaposed to the Kuummiut Terrane in southeasterly direction, causing refolding of earlier structures in the lower amphibolite facies. This hinteland-type of deformation was possibly related to tectonism in western Greenland. The latest recognised deformation event was during ca. 1740–1680Ma associated with NE–SW extension, which is interpreted as orogenic collapse. The complex structural evolution of the orogen was caused by oblique convergence during WSW-directed subduction, the convergence of irregularly shaped cratons and the change of the regional stress field from ENE–WSW to NW–SE during progressive collisional tectonics between the two Archaean cratons.