In geological terms, Sweden forms part of the Fennoscandian (or Baltic) Shield, which also includes Norway, Finland and the northwestern part of Russia.
The oldest rocks of the Fennoscandian Shield are found in the northeast, in the Kola peninsula, Karelia and northeastern Finland. These are Archean rocks, mainly gneisses and greenstone belts, c. 2500-3100 Ma (million years) old. Within this area, there are also some Paleoproterozoic cover rocks (Karelian rocks), c. 1900-2500 Ma old, and the c. 1900 Ma old collisional Lapland granulite belt (not shown on the map). Some Archean rocks are also found in northernmost Sweden (Norrbotten county), and Archean crust probably underlies much of that area.
Most of northern and central Sweden, however, belongs to the Svecofennian province, together with the southwestern part of Finland. Here, the bedrock formed 1750-1900 Ma ago, during the Svecofennian (also known as Svecokarelian) orogeny. This bedrock includes both metasedimentary and metavolcanic rocks and several generations of granitoids, and hosts important ore deposits of the Bergslagen (iron and sulfide ores), Skellefte (sulfides) and Norrbotten (iron and sulfide ores) districts. This area also contains some younger (c. 1500-1650 Ma) Rapakivi granites as well as Jotnian (c. 1200-1500 Ma) sandstones (not shown on the map).
The Transscandinanavian igneous belt (TIB) consists of largely undeformed granitoids and associated porphyries formed in at least three different episodes between c. 1850 and 1650 Ma ago. It stretches from Småland in southern Sweden through Värmland and western Dalarna (where it is partly covered by Jotnian sandstone) and then continues under much of the Caledonian mountain chain up to northern Scandinavia.
Southwest of the TIB follows the Southwestern gneiss province (also known as the Sveconorwegian province), which has a long and complex evolution ranging from c. 1700 to 900 Ma ago. Much of the bedrock originally formed in the Gothian orogeny 1650-1500 Ma ago, but was later intruded by several generations of granitoids, the youngest in Sweden being the 920 Ma old Bohus granite, and was metamorphosed and deformed again during the Sveconorwegian orogeny c. 1100-900 Ma ago. The Southwestern gneiss province is divided into several north-south-trending segments by Sveconorwe-gian deformation zones. In western Norway, these gneisses were again deformed during the Caledonian orogeny c. 400 Ma ago (diagonally ruled area on map).
The Scandinavian Caledonides, which stretch through most of Norway and include adjacent parts of Sweden, are made up of late Proterozoic to Silurian metasedimentary and metavolcanic rocks, deposited in the Iapetus Ocean (the predecessor of the present-day Atlantic Ocean) c. 700 to 400 Ma ago. Together with slices of older basement, these rocks were thrust several hundred kilometers eastwards over the edge of the Fennoscandian Shield in several large thrust sheets known as nappes, when North America and Greenland collided with Scandinavia during the Caledonian orogeny c. 400 Ma ago. Areas of Caledonian deformation, which also include the Precambrian gneisses of western Norway, have been shown by diagonal ruling on the map.
Remains of Cambro-Silurian sedimentary cover (c. 540-420 Ma old sandstones, shales and limestones) are found in some areas in southern Sweden, while Mesozoic and Tertiary sediments (younger than 250 Ma) are found in southernmost Sweden (Skåne) and in Denmark. Similar Phanerozoic rocks also cover the Baltic republics, Poland and northern Germany. The magmatic rocks of the Permian (c. 250 Ma) Oslo Graben formed in a failed rift system that continues into the Skagerrak and the North Sea.
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Text: Åke Johansson