Constrictional deformation of the Koster dyke swarm in a ductile sinistral shear zone, Koster islands, SW Sweden

Abstract

The Koster-Kattsund dyke swarm is an important element in the Sveconorwegian province of the Baltic shield. Dyke intrusion took place in the period 1225-1015 Ma. Throughout most of the swarm the dykes are strongly deformed and thoroughly recrystallised into lineated amphibolites as a result of a Sveconor- wegian tectonic event about 1000 Ma ago. However, in the Koster archipelago fresh dolerites can be fol­lowed northwards in to partially recrystallised metadolerites and finally into the totally recrystallised, line­ated amphibolites that characterise the swarm. In the Koster archipelago intense dyking resulted in the formation of a multilayered rock sandwich con­sisting of alternating layers of gneiss and dolerite. The sandwich trends NNE and dips 67°W. The dolerite dykes have a mean thickness of2.2 m and they occupy 15-20% of the total rock mass. To the northeast the sandwich becomes progressively deformed and ultimately shows very high strain of pure constrictional type. The deformation took place in a steep NW-SE-trending ductile shear zone. During the initial shear zone deformation (D4,) the sandwich underwent anticlockwise bending and the large Kyrkosund synform was formed. The fold plunges 303/66 and has a NW-SE-trending axial surface. The bending took place by means of flexural-slip folding in which the layer-parallel shearing was located in incompetent dyke layers. Increasing shearing and recrystallisation in a NW-SE-trending belt crossing the northern limb of the Kyr­kosund synform resulted in a softening of this belt. The succeeding event (D4b) was localised in this initial soft belt, and involved sinistral simple shear combined with pure shear resulting in horizontal widening and vertical shortening of the belt. This composite deformation formed the pure constrictional fabric now seen in the rocks. The strong D4b stretching was followed by the formation of trains of asymmetric folds (D 4c and d4a). It is demonstrated that volume changes in the dyke rock during deformation were negligible, and that no competence contrast between gneiss and dyke rock existed during the D 4b stretching. The finite con­strictional strain ellipsoid has the dimensions X = 7.07, Y = Z = 0.18. The composite simple/pure shear deformation that presumably caused the constriction has a simple shear component y = 10.9, correspond­ing to an angular shear of 84. 7°. The pure shear deformation resulted in a 3.4 times horizontal widening of the initial soft belt. The horizontal sinistral displacement within the shear zone was at least 35 km.