Aspects of the regional tectonics of eastern central Sweden

Abstract

Abstract This paper deals with the use of differently trending macrostructures (supracrustal contra infracrustal layers) in trying to solve some important tectonic problems in the Svecokarelian orogenic zone of eastern central Sweden. For this purpose, a small type area has been investigated, in order to shed light on the interference patterns formed between old structures oriented north—south (axial planes, synforms or antiforms) and later superimposed structures oriented east—west. As a result of this investigation, two main phases of folding could be distinguished within the region, one caused by east—west stress (F 1) and the other by compression between north and south (F 2). This is fully in accordance with my earlier interpretation of the tectonic evolution in the neighbouring Stockholm region. During the first (synorogenic) deformation, the strata were overturned isoclinally more or less constantly in a westerly direction. The later (serorogenic), north—south compression, superimposed on the axial planes now dipping to the east, caused the present, almost constant, easterly plunge of the lineations and fold axes of the area. The serorogenic folding took place under conditions of low viscosity during the culmination of regional metamorphism and the formation of veined gneisses. The observed interference pattern could probably not have been developed if pressures in different directions had acted simultaneously. A distinct time interval of unknown duration may thus be assumed to have existed between the two periods of folding. This time interval or intra-orogenic period is characterized by intrusions of basic dikes, which have been deformed only by the second phase of folding (F 2). A model for the regional tectonics of eastern central Sweden is proposed. This model is based on the fact that the axial planes of the coastal areas are somewhat steeper than those in the areas to the west. These conditions probably explain the open style of fold arcs along the coastal line and the more closed configuration of fold arcs to the west.