Archaean structural evolution in the northwest of the Buksefjorden Region, southern West Greenland

Abstract

The Archaean structural evolution of northwest Buksefjorden is described in terms of four principal events. The first, D1, includes a series of phases of deformation and associated high-grade metamorphism that may be related to the injection of parental magmas of the Amîtsoq gneisses (ca. 3750 Ma) and consequent disruption of sediments and basic igneous rocks of the Akilia association. D1 fabrics suggest the finite strains were largely oblate. The D1 event was followed by injection of basic dyke swarms (Ameralik dykes) into the Amîtsoq gneisses. Malene supracrustal rocks appear to have been deposited on a foundation of Amîtsoq gneisses and the cover-basement association was subsequently disrupted by D2 thrusting: migmatisation and other fabrics suggest high-grade metamorphic conditions obtained by the close of D2, probably as precursors to the intrusion of large volumes of new crustal material in the form of tonalitic-granitic Nûk gneisses (ca. 2800 Ma). D3 deformation with syntectonic invasion of Nûk gneisses (uncommon in northwest Buksefjorden, but abundant elsewhere) gave rise to large fold nappes: D2 and D3 combined finite strain was oblate. D4 deformation superimposed on D3 nappes during continued high-grade metamorphism produced strongly compressed and elongate, large-scale domes and basins like flattened dunces' caps. An intense mineral lineation coaxial with D4 folds is broadly parallel to the elongation of the domes and basins and D4 finite maximum extension as indicated by deformed objects. Formation of the coaxial lineation pattern appears to have been controlled principally by rotation of earlier structures, D4 mineral growth, the extremely compressed and elongate shapes of D4 domes and basins and D4 strain superimposition with the total strain path culminating in the prolate field. High-grade metamorphism outlasted D4. No large-scale D1 structures have been recognised, but the D1 processes may have been similar to those of the younger D2 and D3 deformations associated with the syntectonic injection of Nûk gneisses. D2 thrusting may have been initiated in cool rocks at high crustal levels and gave rise to some crustal shortening. However, high-grade metamorphism associated with D2 indicates an association with D3 nappe formation which is related to the injection of large volumes of Nûk gneisses, possibly without significant crustal shortening. The origin of D4 structures may be attributed either to the continuation of D3 effects or to an intraplate shearing mechanism which concentrated strain in steep linear belts, the shearing being consequent on the stabilisation of more rigid areas of crust in the late Archaean.