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Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains.

Tectonic history

1.1 Ga Baltica was located in what is now the South Pacific. (Current location of Australia added for reference.)
550 milllion years ago Baltica (green) was an isolated continent located near the South Pole.

Baltica formed at c. 2.0–1.7 Ga by the collision of three Archaean-Proterozoic continental blocks: Fennoscandia (including the exposed Baltic Shield), Sarmatia (Ukrainian Shield and Voronezh Massif), and Volgo-Uralia (covered by younger deposits). Sarmantia and Volgo-Uralia formed a proto-craton (sometimes called "Proto-Baltica")[1] at c. 2.0 Ga which collided with Fennoscandia c. 1.8–1.7 Ga. The sutures between these three blocks were reactivated during the Mesoproterzoic and Neoproterozoic.[2]

750-600 Ma Baltica and Laurentia rotated clockwise together and drifted away from the Equator towards the South Pole where they were affected by the Cryogenian Varanger glaciations. Initial rifting between the two continents is marked by the c. 650 Ma Egersund dike swarm in southern Norway and from 600 Ma they began to rotate up to 180° relative to each other, thus opening the Iapetus Ocean between them. Laurentia quickly moved northward but Baltica remained an isolated continent in the Southern Hemisphere closer to Gondwana on which endemic trilobites evolved in the Early Ordovician.[3]

During the Ordovician Baltica moved northward approaching Laurentia again allowing trilobites and brachiopods to cross the Iapetus Ocean and form the Caradoc Series. In the Silurian, c. 425 Ma, the final collision between Scotland-Greenland and Norway resulted in the Scandian Orogeny.[3]

Margins

Baltica is a very old continent and its core is a very well-preserved and thick craton. Its current margins, however, are the sutures that are the result of mergers with other, much younger continental blocks. These often deformed sutures do not represent the original, Precambrian–Early Palaeozoic extent of Baltica; for example, the curved margin north of the Urals running parallel to Novaya Zemlya was probably deformed during the eruption of the End-Permian Siberian Traps.[4]

Baltica's western margin is the Caledonide orogen which stretches northward from the Scandinavian Mountains across Barents Sea to Svalbard. Its eastern margin is the Timanide orogen which stretches north to the Novaya Zemlya archipelago.[5] The extent of the Proterozoic continent are defined by the Iapetus Suture to the west; the Trollfjorden-Komagelva Fault Zone in the north; the Variscan-Hercynian suture to the south; the Tornquist Zone to the southwest; and the Ural Mountains to the east.[6]

Northern margin

Terranes of the North American Cordillera, including Alaska-Chukotka, Alexander, Northern Sierra, and Eastern Klamath, share a rift history with Baltica and most likely were part of Baltica from the Caledonian orogeny to the formation of the Ural Mountains.[7] These terranes can be linked to either northeastern Laurentia, Baltica, or Siberia because of a similar sequence of fossils; detrital zircon from 2-1 Ga-old sources and evidence of Grenvillian magmatism; and magmatism and island arcs from the Late Neoproterozoic and Ordovician-Silurian.[8]

Southern margin

From at least 1.8 Ga to at least 0.8 Ga the southwestern margin of Baltica was connected to Amazonia while the southeast margin was connected to the West African Craton. Baltica, Amazonia, and West Africa rotated 75° clockwise relative to Laurentia until Baltica and Amazonia collided with Laurentia in the 1.1-0.9 Ga Grenville-Sveconorwegian-Sunsás orogenies to form the supercontinent Rodinia. When the break-up of Rodinia was complete c. 0.6 Ga Baltica became an isolated continent — a 200 million year period when Baltica was truly a separate continent.[9] Laurentia and Baltica formed a single continent until 1.265 Ga which broke up some time before 0.99 Ga. After the subsequent closure of the Mirovoi Ocean Laurentia, Baltica and Amazonia remained merged until the opening of the Iapetus Ocean in the Neoproterozoic.[10]

Western Margin

The Western Gneiss Region in northern Norway is composed of 1650-950 Ma-old gneisses overlain by continental and oceanic allochtons that were transferred from Laurentia to Baltica during the Scandian orogeny. The allochtons were accreted to Baltica during the closure of the Iapetus Ocean c. 430-410 Ma; Baltica's basement and the allochtons were then subducted to UHP depth c. 425-400 Ma; and they were finnaly exhumed to their present location c. 400-385 Ma.[11] The presence of micro-diamonds in two islands in northern Norway, Otrøya and Flemsøya, indicate that this margin of Baltica was buried c. 120 km (75 mi) for at least 25 million years around 429 Ma shortly after the Baltica-Laurentia collision.[12]

Eastern margin

The eastern margin, the Uralide orogen, extends 2,500 km (1,600 mi) from the Arctic Novaya Zemlya archipelago to the Aral Sea. The orogen contains the record of at least two collisions between Baltica and intra-oceanic island arcs before the final collision between Baltica and Kazakhstania-Siberia during the formation of Pangaea. The Silurian-Devonian island arcs were accreted to Baltica along the Main Uralian Fault, east of which are metamorphosed fragments of volcanic arc mixed with small amounts of Precambrian and Paleozoic continental rocks. No Rocks, however, unambiguously originating from either Kazakhstania or Siberia have been found in the Urals.[13] The basement of the eastern margin is composed of an Archaean craton, metamorphosed rocks at least 1.6 Ga-old, which is surrounded by the fold belt of the Timanide orogeny and overlain by Mesoproterozoic sediments. The margin became a passive margin facing an Ural Ocean in the Cambrian-Ordovician.[14]

See also

References

Notes

  1. ^ E.g. Torsvik & Cocks 2005, Norway’s earliest times, pp. 74–75
  2. ^ Bogdanova et al. 2008, The crustal segments of the East European Craton, pp. 2–3
  3. ^ a b Torsvik et al. 1996, Abstract
  4. ^ Cocks & Torsvik 2005, The margins of Baltica, p. 41
  5. ^ Gee, Bogolepova & Lorenz 2006, p. 507
  6. ^ Torsvik et al. 1992, Introduction, pp. 133–137
  7. ^ Miller et al. 2011, Abstract
  8. ^ Colpron & Nelson 2009, Terranes of Siberian, Baltican and Caledonian affinities, pp. 280–281
  9. ^ Johansson 2009, Abstract
  10. ^ Cawood & Pisarevsky 2017, Abstract
  11. ^ Hacker et al. 2010, Western Gneiss Region, pp. 150–151
  12. ^ Spengler et al. 2009, Abstract; Conclusion and implications, pp. 33–34
  13. ^ Brown et al. 2006, Geological framework of the Uralides, p. 262; For a model of the arc-continent collsion see Puchkov 2009, Fig. 12, p. 178; Fig. 14, p. 180
  14. ^ Brown et al. 2006, The Paleozoic continental margin of Baltica in the Southern Urals, pp. 262–266

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