Cimmeria (continent)

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Cimmeria rifted off Gondwana's north-eastern shores around 250 Ma.[1]
As Cimmeria migrated from Gondwana to Eurasia the Paleo-Tethys closed and the Neo-Tethys opened.[2]
After 150 million years Cimmeria collided with Eurasia and the Cimmerian orogeny closed the Paleo-Tethys. As the break-up of Gondwana began in the south, the opening of the Indian Ocean initiated the closure of the Neo-Tethys.[1]

Cimmeria was an ancient continent, or, rather, a string of terranes, that rifted from Gondwana in the Southern Hemisphere and was accreted to Eurasia in the Northern Hemisphere. It consisted of parts of what is today Turkey, Iran, Afghanistan, Tibet, Shan–Thai, and Malaya.[3] Cimmeria rifted from the Gondwanan shores of the Paleo-Tethys Ocean during the Carboniferous-earliest Permian and as the Neo-Tethys Ocean opened behind it, during the Permian, the Paleo-Tethys closed in front of it.[4] Cimmeria rifted off Gondwana from east to west, from Australia to the eastern Mediterranean.[5] It stretched across several latitudes and longitudes and spanned a wide range of climatic zones.[6]

Origin of concept

First concepts

A "large, ancient Mediterranean Sea" was first proposed by Austrian palaeontologist Melchior Neumayr in 1883.[7] Studying the distribution of Jurassic faunas, he concluded that an equatorial ocean stretching from India to Central America must have separated a large continent in the northern hemisphere from one in the southern hemisphere. Austrian geologist Eduard Suess named this Mesozoic ocean the Tethys, a mythical ocean which separated a mythical continent — Gondwanaland, home of the tongue-shaped flora — from a boreal continent.[8] German geophysicist Alfred Wegener, in contrast, developed a concept of a single, global continent – the supercontinent Pangea — which, in his view, didn't leave any room for an equatorial ocean. A wedge-shaped, east-facing Tethys within Pangea was, nevertheless, proposed by Australian geologist Samuel Warren Carey in 1958.[9] This ocean was later identified as a succession of oceans separated by north-migrating terranes or continental blocks, one of which was Cimmeria.

Iranian microcontinent

In 1974, after extensive field work in the Middle East, Swiss geologist Jovan Stöcklin identified the northern foot of the Alborz Range in northern Iran as the suture which in the Paleozoic was the northern shore of Gondwana and the remains of Paleo-Tethys Ocean. Stöcklin also noted that an early Mesozoic or late Paleozoic rift separated the Iranian Plate from the Arabian Plate, and that another southern suture must be the remains of the Neo-Tethys Ocean. The opening of this later ocean, Stöcklin realized, must have transformed Iran into a microcontinent. Those observations made Stöcklin the first to identify a small part of what would later be known as Cimmeria.[10]

Stöcklin also noted that his proposal resembled the old concept of the world in which there were two continents, Angaraland in the north and Gondwana in the south, separated by an elongated ocean, the Tethys. Iran belonged to neither continent but was part of the realm of Tethys.[10] Stöcklin's southern suture was later confirmed by observations of the evolution of microflora in Iran; which had a Gondwanan affinity during the Carboniferous but an Eurasian affinity during the Late Triassic — Iran had clearly drifted from Gondwana to Laurasia.[11]

Eurasian super-terrane

In the 1980s Turkish geologist Celâl Şengör finally extended Stöcklin's Iranian microcontinent further west to Turkey and further east to Tibet and the Far East.[12] Şengör also reused the name introduced by Suess in 1901,[13] the "Kimmerisches Gebirge" or "Cimmerian Mountains".[11][14] In the mountain range that stretches from the Alps to Indonesia Şengör identified, using a simplified scheme, two distinct but superimposed orogenic systems: the older Cimmerides and the younger Alpides together forming what Şengör called the Tethysides super-orogenic system. Among a large number of anastomosing sutures within this complex system there are two major periods of ocean closure: one earlier, more northern, and much larger – the Cimmerides; and a later, more southern, and much smaller – the Alpides. Cimmeria was the long continent, or the continental archipelago, between them.[14]

This realm of Tethys thus covers most of Eurasia and a large time span (from north to south):[14]

  • Laurasia, Permian to Cretaceous
  • Palaeo-Tethys, Early Carboniferous to Middle Jurassic
  • Cimmeria, Triassic to Middle Jurassic
  • Neo-Tethys, Permian or Triassic to Eocene, locally still extant
  • Gondwana, Ordovician to Jurassic

Tectonic history

During the Early and Middle Palaeozoic Cimmeria was located at an Andean-style active margin. Glacial deposits and paleomagnetic data indicate that Qiang Tang and Shan Thai-Malaya were still located far south adjacent to Gondwana during the Carboniferous. The equatorial fauna and flora of China indicate that it was separated from Gondwana during the Carboniferous. Cimmeria crossed the Paleo-Tethys in the Late Paleozoic and eventually collided with the southern margin of the Siberian continent during the Middle to Late Triassic Cimmerian Orogeny.[3]

It was slab-pull forces in the Paleo-Tethys that detached Cimmeria from Gondwana and opened the Neo-Tethys. The mid-ocean ridge in the Paleo-Tethys subducted under Eurasia (leaving Permian MORB (mid-ocean ridge basalt) in Iran). Slab roll-back in the Paleo-Tethys opened a series of back-arc basins along the Eurasian margin and resulted in the collapse of the Variscan cordillera. As the Paleo-Tethys subducted under the Eurasian southern margin, back-arc oceans formed from Austria to China. Some of these back-arcs closed during the Cimmerian orogeny (e.g. the Karakaya-Küre sequence of back-arc oceans in Turkey), others remained open (e.g. the Meliata-Maliac-Pindos back-arc oceans in the eastern Mediterranean) leading to the formation of younger back-arc oceans.[5]

Iran

The subduction of western Neo-Tethys under Eurasia resulted in extensive magmatism in what is now northern Iran. In the Early Jurassic this magmatism had produced a slab pull force which contributed to the break-up of Pangea and the initial opening of the Atlantic. During the Late Jurassic-Early Cretaceous the subduction of the Neo-Tethys mid-ocean ridge contributed to the break-up of Gondwana, including the detachment of the Argo-Burma terrane from Australia.[5] The Central-East Iranian Microcontinent (CEIM) sutured with Eurasia in the Late Triassic during the regional "Eocimmerian" orogenic event in northern Iran, but Iran is made of several continental blocks and the area must have seen a number of ocean closures in the Late Paleozoic and Early Mesozoic.[15]

Caucasus

The Greater and Lesser Caucasus has a complicated geological history involving the accretion of a series terranes and microcontinents from the Late Precambrian to the Jurassic within the Tethyan framework. These include the Greater Caucasian, Black Sea-Central Transcaucasian, Baiburt-Sevanian, and Iran-Afghanistan terranes and island arcs.[16] In the Caucasus region remnants of the Paleo-Tethys suture can be found in the Dzirula Massif which outcrops Early Jurassic sequences in central Georgia. It consists of Early Cambrian oceanic rocks and the possible remnants of a magmatic arc; their geometry suggests that suturing was followed by strike-slip faulting. Ophiolites also outcrop in the Khrami Massif in southern Georgia and another possible segment of the suture is present in the Svanetia region. The suture is older east of the Caucasus (northern Iran–Turkmenistan) but younger both west of the Caucasus and further east in Afghanistan and the northern Pamirs.[17]

Sibumasu

The eastern-most part of Cimmeria, the Sibumasu terrane, remained attached to north-western Australia until 295–290 Ma when it began to drift northward, as supported by paleomagnetic and biogeographic data. The Qiantang terrane was located west of Sibumasu and contiguous with it. Lower Permian layers in Sibumasu contain glacial-marine diamictites and Gondwanan faunas and floras which then developed independently before Sibumasu docked with Cathaysia. Sibumasu's rapid northern journey is especially evident in the development of brachiopods and fusulinids.[18]

Lhasa

The Lhasa terrane has been interpreted as part of Cimmeria and, if this is the case, must have rifted from Gondwana together with Sibumasu and Qiantang. Sedimentological and stratigraphical evidence, however, suggest that it separated from Gondwana in the Late Triassic when Qiantang was already being accreted to Eurasia. The timing of Lhasa's northward drift is still controversial and paleomagnetic data is extremely scarce.[19]

Economic importance

The present remains of Cimmeria, as a result of the massive uplifting of its continental crust, are unusually rich in a number of rare chalcophile elements. Apart from the Altiplano in Bolivia, almost all the world’s deposits of antimony as stibnite are found in Cimmeria, with the major mines being in Turkey, Yunnan and Thailand. The major deposits of tin are also found in Malaysia and Thailand, whilst Turkey also has major deposits of chromite ore.

See also

References

Notes

  1. ^ a b Reconstruction from Dèzes 1999, p. 16
  2. ^ Reconstruction from Stampfli & Borel 2002, p. 27
  3. ^ a b Scotese & McKerrow 1990, pp. 4, 5, 17
  4. ^ Golonka 2007, p. 182
  5. ^ a b c Stampfli & Borel 2002, pp. 24, 26
  6. ^ Metcalfe 2002, p. 556
  7. ^ Neumayr 1883
  8. ^ Suess 1893; Suess 1901
  9. ^ Hsü & Bernoulli 1978, Paleotethys, pp. 943–944 and references therein including Carey 1958
  10. ^ a b Stöcklin 1974, Introduction, p. 873
  11. ^ a b Stampfli 2000, Some definitions, pp. 1–2
  12. ^ Şengör 1984, Şengör 1987
  13. ^ Suess 1901, p. 22
  14. ^ a b c Şengör et al. 1988, pp. 119–120, 123
  15. ^ Buchs et al. 2013, Introduction, pp. 267–268
  16. ^ Gamkrelidze & Shengelia 2007, Introduction, p. 57
  17. ^ Şengör et al. 1988, pp. 139–140
  18. ^ Metcalfe 2002, p. 556; Position of the Sibumasu Terrane, pp. 562–563; Position of the Qiangtang Terrane, p. 563
  19. ^ Metcalfe 2002, Position of the Lhasa Terrane, p. 563

Sources

External links

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