Geodynamic setting and deformation history of an Archaean terrane at mid-crustal level: The Tasiusarsuaq terrane of southern West Greenland

The Neoarchaean Tasiusarsuaq terrane is situated in the North Atlantic craton approximately 50. km south of the capital Nuuk, and extends for 400. km to the south to the Frederikshåb Isblink. It mainly consists of granulite facies and commonly migmatitic grey gneiss, narrow supracrustal belts and volumetrically minor late-tectonic granites. The supracrustal belts have a lithological association of picritic, komatiitic and basaltic rocks together with ultramafic dykes and sills, with geochemical characteristics typical of an arc-type setting. The grey gneiss is tonalitic to granodioritic in composition, and was emplaced between ca. 2.92 and 2.82. Ga into crust that is probably represented by the supracrustal rocks. The gneiss, in turn, was intruded by granite dykes at ca. 2830. Ma-2800. Ma, and by tonalitic dykes at ca. 2770. Ma. Both probably are the result of partial melting of the thickened crust. Late-tectonic tonalitic pegmatites intruded at 2673 ± 5. Ma at amphibolite-facies conditions. The structural evolution from the granulite-facies peak to amphibolite-facies levels is characterised by three deformation stages. D ₁ fabrics form only relics in low-strain domains of the dominant NW-vergent D ₂ fold-and-thrust structures. Syn-tectonic granite and tonalitic dykes date the D ₂ thrusting to have occurred between ca. 2800Ma and 2700Ma, contemporaneously with near-isobaric cooling. The subsequent D ₃ deformation, still in the amphibolite-facies, is marked by E-W to NE-SW shortening, forming upright folds and near-vertical strike slip shear zones at ca. 2670-2630Ma. The final exhumation of the terrane likely occurred first during the Proterozoic, and was probably mainly driven by erosion. The granulites of the Tasiusarsuaq terrane formed at an orogenic margin in a compressive regime. A first orogenic cycle (2.8-2.7. Ga), here termed the Tasiusarsuaq orogeny, resulted in thrusting of the terrane on a rigid foreland represented by the Færingehavn terrane in the north. The Tasiusarsuaq granulites were further affected by a second Archaean orogeny (2.67-2.58. Ga), the Kapisilik orogeny, which caused transcurrent deformation and only insignificant exhumation to mid-crustal levels. Significantly, there is no evidence that the exhumation of the granulite facies Tasiusarsuaq terrane was driven by Archaean extension. In contrast to many modern situations, where granulites are exhumed by extension related to magmatic underplating and delamination of the lithosphere, the thickened Tasiusarsuaq terrane was possibly stabilised due to the low density differences and equilibration by melt extrusion during granulite-facies metamorphism. Stabilisation of the crust and lack of lithospheric delamination is, therefore, regarded as being more common in Archaean orogens compared to younger orogens.