Iapetus Ocean

The Iapetus Ocean and surrounding continents

The Iapetus Ocean^[1] was an ocean that existed in the Neoproterozoic and Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia. The ocean disappeared with the Caledonian, Taconic and Acadian orogenies, when these three continents joined to form one big landmass called Euramerica.

Because the Iapetus Ocean was positioned between continental masses that would at a much later time roughly form the opposite shores of the Atlantic Ocean, it can be seen as a sort of precursor of the Atlantic. The Iapetus Ocean was therefore named for the titan Iapetus, who in Greek mythology was the father of Atlas, after whom the Atlantic Ocean was named.^[2]

Research history

Geological fault at Niarbyl. The narrow white diagonal line near centre of picture is the only known remaining visible indication of the Iapetus Ocean

At the start of the 20th century, American paleontologist Charles Walcott noticed differences in early Paleozoic benthic trilobites of Laurentia (such as Olenellidae, the so-called "Pacific fauna"), as found in Scotland and western Newfoundland and those of Baltica (such as Paradoxididae, often called the "Atlantic fauna"), as found in the southern parts of the British Isles and eastern Newfoundland. Geologists of the early 20th century presumed that a large trough, a so-called geosyncline, had existed between Scotland and England in the early Paleozoic, keeping both sides separated.

With the development of plate tectonics in the 1960s, geologists such as Arthur Holmes and John Tuzo Wilson concluded that the Atlantic Ocean must have had a precursor before the time of Pangaea. Wilson also noticed that the Atlantic had opened at roughly the same place where its precursor ocean had closed. This led him to his Wilson cycle hypothesis.

Geodynamic history

Paleozoic

In the early Paleozoic, the Iapetus Ocean formed a wide oceanic basin between the paleocontinents of Laurentia (Scotland, Greenland and North America) to the west and Baltica (Scandinavia and parts of northeastern Europe) to the east.

The ocean was bounded to the south by the large paleocontinent Gondwana (containing the crust of future Africa, South America, southern Eurasia, Australia and Antarctica), later by terranes that broke off Gondwana, like the microcontinent Avalonia (at present lithosphere that is scattered over the east of New England, the south of Newfoundland, parts of New Brunswick and Nova Scotia, southern Ireland, most of England and Wales, the low countries and northern Germany).

Position of the continents after the Caledonian orogeny (Devonian to Permian times). Differences in fossil faunas on both sides of the red line (the Iapetus Suture) are evidence for the existence of an ocean between the two sides in the time before the continents were joined in the supercontinent Pangaea. ^[3]

Southwest of the Iapetus a volcanic island arc evolved from the early Cambrian (540 million years ago) onward. This volcanic arc was formed above a subduction zone where the oceanic lithosphere of the Iapetus Ocean subducted southward under other oceanic lithosphere. From Cambrian times (about 550 million years ago) the western Iapetus Ocean began to grow progressively narrower due to this subduction. The same happened further north and east, where Avalonia and Baltica began to move towards Laurentia from the Ordovician (488-444 million years ago) onward.^[4]

The Finnmarkian phase that is found in the northwest of Scandinavia (560-480 million years old) must have been caused by the development of a new subduction zone along the western edge of Baltica, which was a passive margin before that time.

Trilobite faunas of the continental shelfs of Baltica and Laurentia are still very different in the Ordovician, but Silurian faunas show progressive mixing of species from both sides, because the continents moved closer together.^[5]

In the west, the Iapetus Ocean closed with the Taconic orogeny (480-430 million years ago), when the volcanic island arc collided with Laurentia. Some authors consider the oceanic basin south of the island arc also a part of the Iapetus, this branch closed during the later Acadian orogeny, when Avalonia collided with Laurentia.

Meanwhile, the eastern parts had closed too: the Tornquist Sea between Avalonia and Baltica already during the late Ordovician,^[6] the main branch between Baltica-Avalonia and Laurentia during the Grampian and Scandian phases of the Caledonian orogeny (440-420 million years ago).

At the end of the Silurian period (c. 420 million years ago) the Iapetus Ocean had completely disappeared and the combined mass of the three continents formed the "new" continent of Laurasia,^[7] which would itself be the northern component of the singular supercontinent of Pangaea.

Neoproterozoic origin

Although the way in which the Iapetus Ocean evolved and disappeared in the Paleozoic is reasonably well understood, the geodynamics of its origin are less clear. In the Late Neoproterozoic (around 800 million years ago) Baltica, Laurentia, and the cratons that would later become Gondwana together formed the supercontinent Rodinia.

The exact configuration in which the continents were joined is not clear though.^[8] It is however clear that intracratonic basins had developed in the northern parts of this supercontinent during the Cryogenian period (850-630 million years ago), a first sign of continental break up (other parts of Rodinia probably rifted off even earlier).

In many spots in Scandinavia basaltic dikes are found with ages between 670 and 650 million years. These are interpreted as evidence that by that time, rifting had started that would form the Iapetus Ocean.^[9] In Newfoundland and Labrador, the Long Range dikes are also thought to have formed during the formation of the Iapetus Ocean.^[10]

See also

• Avalonia
• Baltica
• Geologic timescale
• Khanty Ocean
• London-Brabant Massif
• Plate tectonics
• Southern uplands of Scotland

Notes

Literature

• Cocks, L.R.N. & Fortey, R.A.; 1990: Biogeography of Ordovician and Silurian faunas, in: McKerrow, W.S. & Scotese, C.F. (eds.): Palaeozoic Palaeogeography and Biogeography, Geological Society of London Memoirs 12, pp. 97–104.
• Harland, W.B. & Gayer, R.A.; 1972: The Arctic Caledonides and earlier oceans, Geological Magazine 109, pp. 289–314.
• Meert, J.G. & Torsvik, T.H.; 2003: The making and unmaking of a supercontinent: Rodinia revisited, Tectonophysics 375, pp. 261–288.
• Stanley, S.M.; 1999: Earth System History, W.H. Freeman & Co, ISBN 0-7167-2882-6.
• Torsvik, T.H. & Rehnström, E.F.; 2003: The Tornquist Sea and Baltica–Avalonia docking, Tectonophysics 362, pp. 67– 82.
• Torsvik, T.H.; Smethurst, M.A.; Meert, J.G.; Van der Voo, R.; McKerrow, W.S.; Brasier, M.D.; Sturt, B.A. & Walderhaug, H.J.; 1996: Continental break-up and collision in the Neoproterozoic and Palaeozoic - A tale of Baltica and Laurentia, Earth-Science Reviews 40, p. 229-258.
• Windley, B.F.; 1996: The Evolving Continents, John Wiley & Sons (3rd ed.), ISBN 0-471-91739-7.
• Ziegler, P.A.; 1990: Geological Atlas of Western and Central Europe, Shell Internationale Petroleum Maatschappij BV (2nd ed.), ISBN 90-6644-125-9.

External links

• Earth.ox.ac.uk - For more extensive geologic information see Ordovician paleogeography and the evolution of the Iapetus ocean.