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Tethys Ocean

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First phase of the Tethys Ocean's forming: the (first) Tethys Sea starts dividing Pangaea into two supercontinents, Laurasia and Gondwana.

The Tethys Ocean (Greek: Τηθύς) was an ocean that existed between the continents of Gondwana and Laurasia during much of the Mesozoic era, before the opening of the Indian and Atlantic oceans during the Cretaceous period. It is also referred to as the Tethys Sea or Neotethys.

Modern theory

Plate tectonic reconstruction of the Tethys realm at 249 mya.

About 250 million years ago,[1] during the Triassic, a new ocean began forming in the southern end of the Paleo-Tethys Ocean. A rift formed along the northern continental shelf of Southern Pangaea (Gondwana). Over the next 60 million years, that piece of shelf, known as Cimmeria, traveled north, pushing the floor of the Paleo-Tethys Ocean under the eastern end of Northern Pangaea (Laurasia). The Tethys Ocean formed between Cimmeria and Gondwana, directly over where the Paleo-Tethys used to be.

During the Jurassic Period (150 Ma), Cimmeria finally collided with Laurasia. There it stalled, the ocean floor behind it buckling under, forming the Tethysian Trench. Water levels rose, and the western Tethys shallowly covered significant portions of Europe, forming the first Tethys Sea. Around the same time, Laurasia and Gondwana began drifting apart, opening an extension of the Tethys Sea between them that today is the part of the Atlantic Ocean that is between the Mediterranean and Caribbean. As North and South America were still attached to the rest of Laurasia and Gondwana, respectively, the Tethys Ocean in its widest extension was part of a continuous oceanic belt running around the Earth between about latitude 30° N and the Equator. Thus, ocean currents at that time—around the Early Cretaceous—ran very differently from the way they do today.

Plate tectonic reconstruction of the Tethys realm at 100 mya

Between the Jurassic and the Late Cretaceous (which started about 100 Ma), Gondwana began breaking up, pushing Africa and India north across the Tethys and opening up the Indian Ocean. As these land masses crowded in on the Tethys ocean from all sides, to as recently as the Late Miocene (15 Ma), the ocean continued to shrink, becoming the Tethys Seaway or second Tethys Sea. Also, throughout the Cenozoic, global sea levels fell hundreds of meters, and eventually the connections between the Atlantic and the Tethys closed off in what is now the Middle East.

Today, India, Pakistan, Indonesia, and the Indian Ocean cover the area once occupied by the Tethys Ocean, and Turkey, Iraq, and Tibet sit on Cimmeria. What was once the western arm of the Tethys Sea was the ancestor of the present-day Mediterranean Sea. Other remnants are the Black, Caspian, and Aral Seas (via a former inland branch known as the Paratethys). Most of the floor of the Tethys Ocean disappeared under Cimmeria and Laurasia. Geologists such as Eduard Suess and other geologists have found fossils of ocean creatures in rocks in the Himalayas, indicating that those rocks were once underwater, before the Indian continental shelf began pushing upward as it collided with Cimmeria. Similar geologic evidence can be seen in the Alpine orogeny of Europe, where the movement of the African plate raised the Alps. Greece and the Levant also retain many units of limestone and other sedimentary rocks deposited by various stands of the Tethys Ocean.

Paleontologists also find the Tethys Ocean particularly important because much of the world's sea shelves were found around its margins for such an extensive length of time. Marine, marsh-dwelling, and estuarian fossils from these shelves are of considerable paleontological interest. The Solnhofen limestone in Bavaria, originally a coastal lagoon mud of the Tethys Ocean, yielded the famous Archaeopteryx fossil.

Historical theory

Geologist Eduard Suess in 1869

In 1885 Melchior Neumayr deduced the existence of the Tethys Ocean from Mesozoic marine sediments and their distribution, calling his concept 'Zentrales Mittlemeer' and describing it as a Jurassic seaway that extended from the Caribbean to the Himalayas.[2] However, Eduard Suess is generally seen as the first person to provide evidence for the existence of this ancient and extinct sea. In 1893, using fossil records from the Alps and Africa, Suess proposed the theory that an inland sea had once existed between Laurasia and the continents which formed Gondwana II. He named it the 'Tethys Sea' after the Greek sea goddess Tethys.[3] Suess first proposed his concept of Tethys in his four-volume work Das Antlitz der Erde (The Face of the Earth).[4]

In the decades that followed "mobilist" geologists regarded Tethys as a large trough between two supercontinents that lasted from the late Palaeozoic until continental fragments derived from Gondwana obliterated it. This concept evolved during the 20th century and after WW2 Tethys was described as a triangular ocean with a wide eastern end. "Fixist" geologists, however, regarded Tethys as a composite trough that evolved through a series of orogenic cycles and from 1920s to the 1960s they used the terms 'Paleothethys', 'Mesotethys', and 'Neotethys' for the Caledonian, Variscan, and Alpine orogenies respectively. In the 1970s and 1980s these terms, and 'Proto-Tethys', were used in different senses by various authors but the concept of a single ocean wedging into Pangea from the east, roughly where Suess first proposed it, remained.[5]

When the theory of plate tectonics became established in the 1960s, it became clear Suess's "sea" had in fact been an ocean. Plate tectonics also provided the mechanism by which the former ocean disappeared: oceanic crust can subduct under continental crust.

Terminology and subdivisions

Over 400 million years continental terranes intermittently separated from Gondwana in the Southern Hemisphere to migrate northward to form Asia in the Northern Hemisphere. These terranes were separated by three intervening Tethys oceans: the Paleo-Tethys (Devonian–Triassic), Meso-Tethys (late Early Permian–Late Cretaceous) and Ceno-Tethys (Late-Triassic–Late Cretaceous).[6]

The western part of the Tethys Ocean is called Tethys Sea, Western Tethys Ocean or Alpine Tethys Ocean. The Black, Caspian[dubious – discuss] and Aral Seas are thought to be its crustal remains (though the Black Sea may in fact be a remnant of the older Paleo-Tethys Ocean).[7] However, this "Western Tethys" was not simply a single open ocean. It covered many small plates, Cretaceous island arcs and microcontinents. Many small oceanic basins (Valais Ocean, Piemont-Liguria Ocean, Meliata Ocean) were separated from each other by continental terranes on the Alboran, Iberian, and Apulian plates. The high sea level in the Mesozoic era flooded most of these continental domains, forming shallow seas.

During the Oligocene, large parts of central and eastern Europe were covered by a northern branch of the Tethys Ocean, called the Paratethys. The Paratethys was separated from the Tethys by the formation of the Alps, Carpathians, Dinarides, Taurus and Elburz mountains during the Alpine orogeny. It gradually disappeared during the late Miocene, becoming an isolated inland sea.

The eastern part of the Tethys Ocean is likewise sometimes referred to as Eastern Tethys.

As theories have improved, scientists have extended the "Tethys" name to refer to similar oceans that preceded it. The Paleo-Tethys Ocean, mentioned above, existed from the Silurian (440 Ma) through the Jurassic periods, between the Hunic terranes and Gondwana. Before that, the Proto-Tethys Ocean existed from the Ediacaran (600 Ma) into the Devonian (360 Ma), and was situated between Baltica and Laurentia to the north and Gondwana to the south. Neither Tethys oceans should be confused with the Rheic Ocean, which existed to the west of them in the Silurian period.[8] To the north of Tethys sea, the then land mass was called Angaraland and to the south of it, it was called Gondwanaland.

The name is taken from the Greek mythological figure Tethys, sister and consort of Oceanus and mother of the great rivers, lakes and fountains of the world and of the Oceanid sea nymphs.

See also

References

Notes

  1. ^ Palaeos Mesozoic: Triassic: Middle Triassic Archived 2008-05-16 at the Wayback Machine
  2. ^ Kollmann 1992
  3. ^ Suess 1893, p. 183: "This ocean we designate by the name "Tethys," after the sister and consort of Oceanus. The latest successor of the Tethysian Sea is the present Mediterranean."
  4. ^ Suess 1901, Gondwana-Land und Tethys, p. 25: "Dasselbe wurde von Neumayr das 'centrale Mittelmeer' genannt und wird hier mit dem Namen Tethys bezeichnet werden. Das heutige europäische Mittelmeer ist ein Rest der Tethys."
  5. ^ Metcalfe 1999, How many Tethys Oceans?, pp. 1–3
  6. ^ Metcalfe 2013, Introduction, p. 2
  7. ^ Van der Voo, Rob (1993). Paleomagnetism of the Atlantic, Tethys and Iapetus Oceans. Cambridge University Press. doi:10.2277/0521612098. ISBN 978-0-521-61209-8.
  8. ^ Stampfli & Borel 2002, Figs. 3–9

Sources

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