Early structural and metamorphic evolution of the Scandinavian Caledonides: a study of the eclogite-bearing Seve Nappe Complex at the Arctic Circle, Sweden

Abstract: In the Seve Nappe Complex of the Swedish Caledonides, coronitic diabases pass into eclogites; sometimes the transition occurs within one and the same boudin. Calculated pressure-temperature conditions for the eclogite facies stage are 20-27 kbar and 650-750°. Protolith corona-dolerite dykes share the T-MORB signature with the rift-dolerites of the Sarek and Särv Nappes, indicating similar origin. Eclogites are hosted by metasedimentary rocks and a metavolcanic unit, the Tjidtjak Metavolcanite, serving as basement for deposition of sediments in rapidly subsiding rift-basins. Ion-probe datings of zircons from the metavolcanite give a preliminary intrusion age at 952±27 Ma. Dating of titanite from calc-silicate horizons within the Grapesvare Nappe, Seve Nappe Complex supports the existence of a Finnmarkian phase of subduction and eclogitization c. 475-500 Ma ago, which is about 100 Ma prior to the main Scandian thrusting. Structures which formed prior to, or early during eclogitization have been recrystallized but are occasionally preserved in domains between anastomosing shear zones. Formation of the latter, and of non-cylindrical folds was triggered by the presence of rigid eclogite boudins. Eclogitization was followed by extreme ductile deformation during rapid exhumation and retrogression of the eclogites. Folding is interpreted as a continuous process where early folds were consecutively refolded, sheared into tubular folds and deformed in new shear zones resembling shear bands at metre-scale. The overall deformational regime was vertical thinning during eastward general shear, possibly due to oblique compression during the exhumation phase. In the Scandian thrusts, strong lineations, lineation-parallel folds and occasional sheath folds indicate a component of oblique compression early during foreland-directed nappe emplacement. In schists and phyllites of lower nappes, horizons containing C'-type shear bands and asymmetric folds alternate, demonstrating a continuous interaction between thickening and thinning during foreland-directed translation. At the margin of the Nasafjäll Window, folded basal thrusts of both upper and lower tectonic units demonstrate that lower thrusts were active after movements on higher thrusts had stopped. North and west of the Nasafjäll Window, earlier SE-vergent structures have been overprinted by later N and NW vergent structures showing movements away from the centre of the window. This may have been an effect of increased topography caused by the stacking of the Nasafjället Window, indicating that this occurred late during the Scandian collision.