Since I have found several nice papers describing the geological history of the Baltic shield, and therefore also of Sweden and Scania I had the great idea of writing an overview of these papers. Later on I just may focus more in depth on some of the interesting events that has occurred, but it is reasonable to begin with an overview. My hope is that it will be rather short, but I am afraid that it will not. This is the first installment of this series.
This will also highlight the complexity of the Swedish crystalline bedrock, a result of numerous accretion events, collisions and orogens.
Archean to paleoproterozoic
I have not seen a good overview of the history of the Baltic shield from Archean or Paleopterozoic time, maybe I have missed it. But Bogdanova et al (2008) mention that time period in a few short paragraphs.
The oldest parts of the Baltic shield has an age of 3.5 – 3.2 Ga and are primarily known from the southeast part, with a few segments also in current Finland. The Fenno-Karelian protocontinent was later formed by accretion during 3.1 – 2.7 Ga . After this time of accretion there followed a time of rifting and breakup during 2.5 – 2.0 Ga ending in passive margins in the south and southwest end of the craton. The Svecofennian orogeny complex are several accretion events during 1.95 and 1.85 Ga that added new continental crust to the southwest, in current Sweden. At the end of this orogeny the Fennoscandian protocontinent collided with the Volga-Saramatia continent to the easy, creating what is now known as the Baltic shield. Several event of magmatism and deformation in eastern Fennoscandia – Finland and Gulf of Finland – between 1.84 and 1.75 Ga are associated with this semi-collision, but with no clear evidence.
At the same time there are three recorded accretion events to the southeast of the Fennoscandian protocontinent, resulting in new continental crust to the south and southwest. These happens between 1.83 – 1.82, 1.81 – 1.78 and 1.77 – 1.75 Ga.
At about this time there was also a collision to the north called the Lapland-Kola orogen, with no age given.
During Paleoproterozoic the Baltic shield had been a part of the Columbian supercontinent, and now in Mesoproterozoic it is part of the Nena supercontinent. In both these time periods the western margin are associated with a subduction zone and regular accretion.
A marked difference of the Mesoproterozoic accretion of continental crust ar the change in direction, in Paleoproterozoic accretion was primarily from the south, now it is instead from the west. The eastern edge of this accretion are today identified as the Protogine zone trending N-S through south-central Sweden. Between 1.73 and 1.67 Ga Ättran (to the south) and Klarälven (to the north) was accreated, with the Mylonite Zone as its western edge. West of the Mylonite Zone there are three identified formations together named the Idrefjord terranes. Horred formation to the southeast at 1.66 G, Åmål belt to the east 1.65 – 1.59 Ga and Östfold-Marstrand belt to the west in Norway 1.59 – 1.55 Ga. Even further to the east in southern Norway a few more slightly younger terranes has been identified.
The reason for these accretions are in doubt with at least two possible explanations. Either as the Gothian orogenic complex between 1.75 – 1.55 Ga with several episodes of accretion or as a collision with an unknown exotic terrane.
During these accretions there was several series av plutonic intrusions in interior of the craton, central Sweden och southern Finland. These intrusions were of the well known Rapakivi-granite and there are atleast for events identified between 1.65 Ga and 1.50 Ga. Earlier they were considered anorogenic, but are now probably synorogenic. Several doloritic dyke-swarms are associated with these plutonic intrusions.
The next major event has been named the Denopolonian orogeny by Bogdanova in an earlier paper I really should read. This orogeny happened at 1.5 to 1.4 Ga when the Baltic continents southwest edge collided with another continent, according to Bogdanova this should be Amazonia or another South American subcontinent. The paleoposition of these continents are not described well enough for any surety. Main features of the Danopolonian orogeny are granitoid magmatism between 1.47 – 1.42 Ga in southern Sweden, Lithuania, northern Poland and Bornholm and activation of E-W trending faultzones.. The magmatism are associated with high-grade metamorphism and migmatization.
After the Danopolonian orogeny there was a dramatic change in tectonic in the following 200 million years with bimodal magmatism associated with continental rifting. In the Idrefjord terrane doleritic dykes has been identified from 1.3 Ga and associated granitic dykes in the same terrane from 1.34 – 1.31 Ga. This was followed in 1.27 – 1.24 Ga with several large dolerite sheets in the northern Baltic Sea, further followed in 1.22 – 1.20 Ga by intrusions of syenite, monzosyenite and dolerite magma around the Protogine zone.
These events might, or might not, be associated with a break-up between Baltica and Laurentia in 1.25 Ga.
This part will be about the Sveconorwegian orogeny at the time when Mesoproterozoic becomes Neoproterozoic, 1.14 – 0.9 Ga. This period can be further subdivided into four separate phases. Events just prior to 1.14 Ga indicates extension or transtension followed by accretion of a volcanic arc and then subduction-related oblique collision between western Baltica and Amazonia. The collision resulted in a 500 km wide orogenic N-S trending belt.
The first phase has been named the Arendal phase and has been dated to 1.14 – 1.08 Ga, primarily affected the Telemarkian and Bamble-Kongsberg terranes in Norway and resulted in amphibolite- to granulite-facies metamorphism.
The second phase named the Agder phase at 1.05 – 0.98 Ga represents the main continental collision. Metamorphism are a result of crustal thickening and later exhumation ad the grade are in Norway greenschist- to granulite-facies and in the Idrefjord terrane in Sweden amphibolite- to granulite-facies.
The following Falkenberg phase at 0.98 – 0.96 Ga represents the later phase of the collision. Metamophism continued in Norway and in Sweden propagated to the east and south with amphibolite facies metamorphism in the east segment (Ätran and Klaraälven terranes) and granulite- with some eclogite-facies metamorphism in the south part of the eastern segment. The Mylonite zone was also activated with oblique and extensional faulting, probably due to 60° rotation of Baltica as it was fit into Rodina.
Dalane phase are the last phase at 0.96 – 0.90 Ga and represents orogenic relaxation and collapse following the main collision. Exhumation was centered at the edges of the orogen, Norway and the eastern segment in Sweden. Postcollision magmatism continued further to the east of the Protogine zone in Sweden.
This ends this first installment of what have to be a series of posts, otherwise it would become far to long. This post has described Balticas history from Archean times through several collisions, accretion events, magmatism, rifting and orogens to the formation of Rodina. The next part will pick up from when Rodina start to break up.
Bogdanova, S.V., Bingen, B., Gorbatschev, R., Kheraskova, T.N., Kozlov, V.I., Puchkov, V.N., & Volozh, Yu.A. (2008). The East European Craton (Baltica) before and during the assembly of Rodinia Precambrian Research DOI: 10.1016/j.precamres.2007.04.024