Gondwana

Gondwana (pronounced /ɡɒndˈwɑːnə/[1][2]), originally Gondwanaland, is the name given to a southern precursor supercontinent. Its final geological suturing occurred between ca. 570 and 510 million years ago (Ma), joining East Gondwana to West Gondwana.[3] It later separated from Laurasia 180-200 million years ago during the breakup of the Pangaea supercontinent that existed about 500 to 200 Ma into two large segments, nearly equal in area.[4] While the corresponding northern-hemisphere continent Laurasia moved further north, Gondwana drifted south. It included most of the landmasses in today's southern hemisphere, including Antarctica, South America, Africa, Madagascar, Australia-New Guinea, and New Zealand, as well as Arabia and the Indian subcontinent, which have now moved entirely into the Northern Hemisphere.

The continent of Gondwana was named by Austrian scientist, Eduard Suess, after the Gondwana region of central northern India (from Sanskrit gondavana "forest of the Gonds"), from which the Gondwana sedimentary sequences (Permian-Triassic) are also described.

The adjective "Gondwanan" is in common use in biogeography when referring to patterns of distribution of living organisms, typically when the organisms are restricted to two or more of the now-discontinuous regions that were once part of Gondwana, including the Antarctic flora. For example, the Proteaceae, a family of plants that is known only from southern South America, South Africa, and Australia are considered to have a "Gondwanan distribution". This pattern is often considered to indicate an archaic, or relict, lineage.

Formation
Orogens and Kuungan Orogens.

The assembly of Gondwana was a protracted process. Several orogenies led to its final amalgamation 550–500 million years ago at the end of the Ediacaran, and into the Cambrian.[3] These include the Brasiliano Orogeny, the East African Orogeny, the Malagasy Orogeny, and the Kuunga Orogeny. The final stages of Gondwana assembly overlapped with the opening of the Iapetus Ocean between Laurentia and western Gondwana. During this interval the Cambrian Explosion occurred.

Gondwana was formed by these earlier continents and microcontinents, and others, colliding in these orogenies:

* Azania: much of central Madagascar, the Horn of Africa and parts of Yemen and Arabia. (Named by Collins and Pisarevsky (2005): "Azania" was a Greek name for the East African coast.)
* The Congo–Tanzania–Bangweulu Block of central Africa.
* Neoproterozoic India: India, the Antongil Block in far eastern Madagascar, the Seychelles, and the Napier and Rayner Complexes in East Antarctica.
* The Australia/Mawson continent: Australia west of Adelaide and a large extension into East Antarctica.
* Other blocks which helped to form Argentina and around, including a piece transferred from Laurentia when the west edge of Gondwana scraped against southeast Laurentia in the Ordovician.[5][6] This is the Famatinian block, and it formerly continued the line of the Appalachians southwards.

One of the major sites of Gondwana amalgamation was the East African Orogen (Stern, 1994), where these two major orogenies are superimposed on each other:
Reconstruction showing final stages of assembly of Gondwana, 550 Ma ago.

The East African Orogeny (as later defined) at ~650–630 Ma affected a large part of Arabia, north-eastern Africa, East Africa and Madagascar. Collins and Windley (2002) propose that in this orogeny Azania collided with the Congo–Tanzania–Bangweulu Block.[7]

The later Malagasy orogeny at ~550–515 Ma affected Madagascar, eastern East Africa and southern India. In it Neoproterozoic India collided with the already combined Azania and Congo–Tanzania–Bangweulu Block.

At the same time, in the Kuunga Orogeny Neoproterozoic India collided with the Australia/Mawson continent.

Pangaea

Other large continental masses, including the cores of North America (Canadian Shield or Laurentia), Europe (Baltica), and Siberia were added over time to form the supercontinent Pangaea by Permian time. When Pangaea broke up (mostly during the Jurassic), two large masses, Gondwana and Laurasia, were formed.

When Pangaea broke up, the re-formed Gondwana continent was not precisely the same as before Pangaea formed; for example, most of Florida and southern Georgia and Alabama are underlain by rocks that were originally part of Gondwana but that were left attached to North America when Pangaea broke apart.[citation needed]

Climate

During the late Paleozoic, Gondwana extended from a point at or near the south pole to near the equator. Across much of Gondwana, the climate was mild. During the Mesozoic, the world was on average considerably warmer than today. Gondwana was then host to a huge variety of flora and fauna for many millions of years. But there is strong evidence of glaciation during Carboniferous to Permian time, especially in South Africa.

Breakup

Mesozoic
Nothofagus is a plant genus that illustrates Gondwanan distribution, having descended from the supercontinent and existing in current day Australia, New Zealand, New Caledonia and Chile. Fossils have also recently been found in Antarctica[8].

Gondwana began to break up in the mid-Jurassic (about 167 million years ago), when East Gondwana, comprising Antarctica, Madagascar, India and Australia, began to separate from Africa. South America began to drift slowly westward from Africa as the South Atlantic Ocean opened, beginning about 130 million years ago during the Early Cretaceous, and resulting in open marine conditions by 110 million years ago. East Gondwana then began to separate about 120 million years ago when India began to move northward.

The Madagascar block, and a narrow remnant microcontinent presently occupied by the Seychelles Islands, were broken off India; elements of this breakup nearly coincide with the Cretaceous-Tertiary extinction event. The India–Madagascar–Seychelles separations appear to coincide with the eruption of the Deccan basalts, whose eruption site may survive as the Réunion hotspot.

Australia began to separate from Antarctica perhaps 80 million years ago (Late Cretaceous), but sea-floor spreading between them became most active about 40 million years ago during the Eocene epoch of the Tertiary Period.

New Zealand probably separated from Antarctica between 130 and 85 million years ago.

Cenozoic

As the age of mammals got underway, the continent of Australia-New Guinea began gradually to separate and move north (55 million years ago), rotating about its axis to begin with, and thus retaining some connection with the remainder of Gondwana for about 10 million years.

About 45 million years ago, the Indian Plate collided with Asia, buckling the crust and forming the Himalayas. At about the same time, the southernmost part of Australia (modern Tasmania) finally separated from Antarctica, letting ocean currents flow between the two continents for the first time. Cooler and drier climates developed on both continents because ocean currents enveloping Antarctica were no longer directed into the subtropics, where they would have flowed around northern Australia.

The separation of South America from West Antarctica some time during the Oligocene, perhaps 30 million years ago also caused climate changes. Immediately before this, South America and East Antarctica were not connected directly, but the many microplates of the Antarctic Peninsula remained near southern South America acting as "stepping stones" allowing continued biological interchange and stopped oceanic current circulation. But when Drake Passage opened, there was now no barrier to force the cold waters of the Southern Ocean north, to be exchanged with warmer tropical water. Instead, a cold circumpolar current developed and Antarctica became what it is today: a frigid continent that locks up much of the world's fresh water as ice. Sea temperatures dropped by almost 10°C, and the global climate became much colder.

By about 15 million years ago, the collision between New Guinea (on the leading edge of the Australian Plate) and the southwestern part of the Pacific Plate pushed up the New Guinea highlands, causing a rain shadow effect which drastically changed weather patterns in Australia, drying it out.

Later, South America was connected to North America via the Isthmus of Panama, cutting off a circulation of warm water and thereby creating the Arctic[citation needed], as well as allowing a faunal interchange between the two continents.

The Red Sea and East African Rift are modern examples of the continuing dismemberment of Gondwana.

See also

* Alexander du Toit
* Alfred Wegener
* Continental drift
* Plate tectonics
* Polar dinosaurs in Australia
* Geology of the Australasian ecozone

References

* Cattermole, Peter John (2000). Building Planet Earth: Five Billion Years of Earth History. London: Cambridge University Press. ISBN 9780521582780. OCLC 317422973.
* Collins, Alan S; Pisarevsky, Sergei A (August 2005). "Amalgamating eastern Gondwana: The evolution of the Circum-Indian Orogens". Earth-Science Reviews 71 (3–4): pp. 229–270. doi:10.1016/j.earscirev.2005.02.004.
* Cowen, Richard (2000). History of Life (3rd ed.). Malden, MA: Blackwell Science. ISBN 9780632044443. OCLC 41572551.
* Lowrie, William (1997). Fundamentals of Geophysics. Cambridge ; New York: Cambridge University Press. ISBN 9780521461641. OCLC 35651121. Also ISBN 9780521467285.
* Meert, JG (2003-02-06). "A synopsis of events related to the assembly of eastern Gondwana". Tectonophysics 363 (1): pp. 1–40. doi:10.1016/S0040-1951(02)00629-7.
* Stern, RJ (May 1994). "ARC Assembly and Continental Collision in the Neoproterozoic East African Orogen: Implications for the Consolidation of Gondwanaland". Annual Review of Earth and Planetary Sciences 22: pp. 319–351. doi:10.1146/annurev.ea.22.050194.001535.

1. ^ "gondwana". Dictionary.com. Lexico Publishing Group. http://dictionary.reference.com/browse/gondwana. Retrieved 2010-01-18.
2. ^ "Gondwanaland". Merriam-Webster Online Dictionary. http://www.merriam-webster.com/dictionary/gondwana. Retrieved 2010-01-18.
3. ^ a b Buchan, Craig (November 7–10, 2004). "Paper No. 207-8 - Linking Subduction Initiation, Accretionary Orogenesis And Supercontinent Assembly". 2004 Denver Annual Meeting. Geological Society of America. http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_78645.htm. Retrieved 2010-01-18.
4. ^ Houseman, Greg. "Dispersal of Gondwanaland". University of Leeds. http://homepages.see.leeds.ac.uk/~eargah/Gond.html. Retrieved 21 Oct 2008.
5. ^ Rapalini, AE (2001). "The Assembly of Southern South America in the Late Proterozoic and Paleozoic: Some Paleomagnetic Clues". Spring Meeting 2001. American Geophysical Union. http://adsabs.harvard.edu/abs/2001AGUSM..GP32D03R. Retrieved 2010-01-18.
6. ^ Rapalini, AE (1998). "Syntectonic magnetization of the mid-Palaeozoic Sierra Grande Formation: further constraints on the tectonic evolution of Patagonia". Journal of the Geological Society 155 (1): pp. 105–114. doi:10.1144/gsjgs.155.1.0105.
7. ^ Collins, Alan S; Windley, Brian F (May 2002). "The Tectonic Evolution of Central and Northern Madagascar and Its Place in the Final Assembly of Gondwana". The Journal of Geology 110 (3): pp. 325–339. doi:10.1086/339535.
8. ^ H.M. Li and Z.K. Zhou {2007) Fossil nothofagaceous leaves from the Eocene of western Antarctica and their bearing on the origin, dispersal and systematics of Nothofagus. Science in China. 50(10): 1525-1535.