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The Swedish Nuclear Fuel and Waste Management Company (SKB) has recently completed investigations in the Forsmark and Laxemar-Simpevarp areas in the Fennoscandian Shield, south-eastern Sweden, aimed at locating and characterizing a site for the disposal of highly radioactive nuclear waste. The safety analysis of a selected site requires an understanding of the past geological evolution, from the time of crystallization of the bedrock up to historical time with its low seismic activity. With this aim in mind, a synthesis of the geological evolution inside the Fennoscandian Shield in south-eastern Sweden has recently been completed. This contribution aims to present the results of the synthesis for the time interval 1.91 to 1.75 Ga.
The bedrock geology in south-eastern Sweden is divided into six major tectonic domains. The boundaries between these domains consist, at least partly, of belts that are affected by high ductile strain. Furthermore, two of these belts are sufficiently wide to merit definition as separate tectonic entities. These domains trend WNW more or less parallel to the Archaean continental nucleus to the north-east. The different domains have been distinguished primarily on the basis of differences in the character and timing of ductile deformation in combination with the timing of igneous activity. It is proposed that different phases of igneous activity and ductile deformation in this part of the shield belong to three different tectonic cycles. Each of these cycles extended over c. 50 Ma in time from 1.91 to 1.86 Ga, from 1.87 to 1.82 Ga and from 1.83 to at least 1.79 Ga. Furthermore, the focus of tectonic activity migrated in space from one tectonic domain (or groups of domains) to another between the different cycles. The geodynamic regime involved an accretionary orogenic setting, with oblique subduction of oceanic crust north-eastwards beneath an active continental margin, conceptually very reminiscent of that envisaged by studies during the 1970's and 1980's.
A conceptual model for each tectonic cycle involves migration of the subduction hinge away from and, at a later stage, towards the overriding continental plate. This gave rise to alternating extensional and compressional tectonics, respectively. It corresponds to the model referred to as tectonic switching in the younger accretionary orogens of eastern Australia (Lachlan orogen) and New Zealand. In south-eastern Sweden, continental back-arc basins developed during the extensional phase. Compressional deformation was transpressive in character with dextral strike-slip, ductile deformation along the boundary high-strain belts. This was combined with shortening in a north-east direction across these belts and strongly constrictional strain with folding and stretching, with variable plunge to the south-east, between them. Crustal behaviour in the form of a "moving concertina" summarises the diachronous ductile deformation and igneous activity in the different domains.
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