Gondwana began to separate from northern Pangea (Laurasia) during theTriassic, and started to fragment during the EarlyJurassic (around 180 million years ago). The final stages of break-up saw the fragmentation of theAntarctic land bridge (involving the separation of Antarctica from South America and Australia, forming theDrake andTasmanian Passages), which occurred during thePaleogene (from around66 to 23 million years ago (Ma)). Gondwana was not considered a supercontinent by the earliest definition, since the landmasses ofBaltica,Laurentia, andSiberia were separated from it.[5] To differentiate it from the Indian region of the same name (see§ Name), it is also commonly calledGondwanaland.[6]
Regions that were part of Gondwana sharedfloral andfaunal elements that persist to the present day.
Eastern Gondwana.620 to 550 Ma post-collisional extension of the East African Orogeny in blue and570 to 530 Ma collisional metamorphism of the Kuunga orogeny in red.[10]
The assembly of Gondwana was a protracted process during theNeoproterozoic andPaleozoic, which remains incompletely understood because of the lack of paleo-magnetic data. Severalorogenies, collectively known as thePan-African orogeny, caused the continental fragments of a much older supercontinent,Rodinia, to amalgamate. One of those orogenic belts, theMozambique Belt, formed800 to 650 Ma and was originally interpreted as thesuture between East (India, Madagascar, Antarctica, Australia) and West Gondwana (Africa and South America). Three orogenies were recognised during the 1990s as a result of data sets compiled on behalf of oil and mining companies:[11] theEast African Orogeny (650 to 800 Ma) andKuunga orogeny (including theMalagasy orogeny in southern Madagascar) (550 Ma), the collision between East Gondwana and East Africa in two steps, and theBrasiliano orogeny (660 to 530 Ma), the successive collision between South American and Africancratons.[12]
The last stages of Gondwanan assembly overlapped with the opening of theIapetus Ocean betweenLaurentia and western Gondwana.[13] During this interval, theCambrian explosion occurred. Laurentia was docked against the western shores of a united Gondwana for a brief period near the Precambrian and Cambrian boundary, forming the short-lived and still disputed supercontinentPannotia.[14]
As the rest of Gondwana formed, a complex series of orogenic events assembled the eastern parts of Gondwana (eastern Africa, Arabian-Nubian Shield, Seychelles, Madagascar, India, Sri Lanka, East Antarctica, Australia)c.750 to 530 Ma. First, the Arabian-Nubian Shield collided with eastern Africa (in the Kenya-Tanzania region) in the East African Orogenyc.750 to 620 Ma. Then Australia and East Antarctica were merged with the remaining Gondwanac.570 to 530 Ma in the Kuunga Orogeny.[17]
The later Malagasy orogeny at about 550–515 Mya affected Madagascar, eastern East Africa and southern India. In it, Neoproterozoic India collided with the already combined Azania and Congo–Tanzania–Bangweulu Block, suturing along the Mozambique Belt.[18]
The 18,000 km-long (11,000 mi)Terra Australis Orogen developed along Gondwana's western, southern, and eastern margins.[19] Proto-Gondwanan Cambrian arc belts from this margin have been found in eastern Australia, Tasmania, New Zealand, and Antarctica. Though these belts formed a continuous arc chain, the direction of subduction was different between the Australian-Tasmanian and New Zealand-Antarctica arc segments.[20]
Peri-Gondwana development: Paleozoic rifts and accretions
Many terranes were accreted to Eurasia during Gondwana's existence, but the Cambrian or Precambrian origin of many of these terranes remains uncertain. For example, some Paleozoic terranes and microcontinents that now make up Central Asia, often called the "Kazakh" and "Mongolian terranes", were progressively amalgamated into the continentKazakhstania in the lateSilurian. Whether these blocks originated on the shores of Gondwana is not known.[21]
In the Early Paleozoic, theArmorican terrane, which today form large parts of France, was part of Peri-Gondwana; the Rheic Ocean closed in front of it and the Paleo-Tethys Ocean opened behind it. Precambrian rocks from theIberian Peninsula suggest that it, too, formed part of core Gondwana before its detachment as anorocline in theVariscan orogeny close to the Carboniferous–Permian boundary.[22]
Journey of the Asian blocks from Gondwana to Laurasia,Late Ordovician toEarly Jurassic (450, 350, 300, and 200 Mya). View centred on 0°S,105°E.
South-east Asia was made of Gondwanan andCathaysian continental fragments that were assembled during the Mid-Paleozoic and Cenozoic. This process can be divided into three phases of rifting along Gondwana's northern margin: first, in the Devonian,North andSouth China, together withTarim and Quidam (north-western China) rifted, opening the Paleo-Tethys behind them. These terranes accreted to Asia during Late Devonian and Permian. Second, in the Late Carboniferous to Early Permian,Cimmerian terranes opened Meso-Tethys Ocean;Sibumasu andQiangtang were added to south-east Asia duringLate Permian and Early Jurassic. Third, in the Late Triassic to Late Jurassic,Lhasa,Burma,Woyla terranes opened the Neo-Tethys Ocean; Lhasa collided with Asia during the Early Cretaceous, and Burma and Woyla during the Late Cretaceous.[23]
Gondwana's long, northern margin remained a mostly passive margin throughout the Paleozoic. The Early Permian opening of the Neo-Tethys Ocean along this margin produced a long series of terranes, many of which were and still are being deformed in theHimalayan orogeny. These terranes are, from Turkey to north-eastern India: the Taurides in southern Turkey; the Lesser Caucasus Terrane in Georgia; the Sanand, Alborz, and Lut terranes in Iran; the Mangysglak Terrane in the Caspian Sea; the Afghan Terrane; the Karakorum Terrane in northern Pakistan; and the Lhasa and Qiangtang terranes in Tibet. The Permian–Triassic widening of the Neo-Tethys pushed all these terranes across the Equator and over to Eurasia.[24]
During the Neoproterozoic to Paleozoic phase of theTerra Australis Orogen, a series of terranes were rafted from the proto-Andean margin when the Iapetus Ocean opened, to be added back to Gondwana during the closure of that ocean.[25] During the Paleozoic, some blocks which helped to form parts of theSouthern Cone of South America, include a piece transferred from Laurentia when the west edge of Gondwana scraped against southeast Laurentia in theOrdovician.[26] This is theCuyania or Precordilleraterrane of theFamatinian orogeny in northwest Argentina which may have continued the line of theAppalachians southwards.[27]Chilenia terrane accreted later against Cuyania.[28] The collision of the Patagonian terrane with the southwestern Gondwanan occurred in the late Paleozoic. Subduction-related igneous rocks from beneath theNorth Patagonian Massif have been dated at 320–330 million years old, indicating that the subduction process initiated in the early Carboniferous.[29] This was relatively short-lived (lasting about 20 million years), and initial contact of the two landmasses occurred in the mid-Carboniferous,[29][30] with broader collision during the early Permian.[30] In the Devonian, anisland arc namedChaitenia accreted to Patagonia in what is now south-central Chile.[31]
Gondwana as part of Pangaea: Late Paleozoic to Early Mesozoic
In the western end of Pangaea, the collision between Gondwana and Laurasia closed theRheic andPaleo-Tethys oceans. The obliquity of this closure resulted in the docking of some northern terranes in theMarathon,Ouachita,Alleghanian, andVariscan orogenies, respectively. Southern terranes, such asChortis andOaxaca, on the other hand, remained largely unaffected by the collision along the southern shores of Laurentia. Some Peri-Gondwanan terranes, such asYucatán andFlorida, were buffered from collisions by major promontories. Other terranes, such asCarolina andMeguma, were directly involved in the collision. The final collision resulted in the Variscan-Appalachian Mountains, stretching from present-day Mexico to southern Europe. Meanwhile,Baltica collided withSiberia andKazakhstania which resulted in theUralian orogeny andLaurasia. Pangaea was finally amalgamated in the Late Carboniferous-Early Permian, but the oblique forces continued until Pangaea began to rift in the Triassic.[34]
In the eastern end, collisions occurred slightly later. TheNorth China,South China, andIndochina blocks rifted from Gondwana during the middle Paleozoic and opened theProto-Tethys Ocean. North China docked with Mongolia and Siberia during the Carboniferous–Permian, followed by South China. TheCimmerian blocks then rifted from Gondwana to form thePaleo-Tethys andNeo-Tethys oceans in the Late Carboniferous, and docked with Asia during the Triassic and Jurassic. Western Pangaea began to rift while the eastern end was still being assembled.[35]
The formation ofPangaea and its mountains had a tremendous impact on global climate and sea levels, which resulted in glaciations and continent-wide sedimentation. In North America, the base of theAbsaroka sequence coincides with the Alleghanian and Ouachita orogenies and are indicative of a large-scale change in the mode of deposition far away from the Pangaean orogenies. Ultimately, these changes contributed to thePermian–Triassic extinction event and left large deposits of hydrocarbons, coal, evaporite, and metals.[36]
The breakup of Pangaea began with theCentral Atlantic magmatic province (CAMP) between South America, Africa, North America, and Europe. CAMP covered more than seven million square kilometres over a few million years, reached its peak atc.200 Ma, and coincided with theTriassic–Jurassic extinction event.[37] The reformed Gondwanan continent was not precisely the same as that which had existed before Pangaea formed; for example, most ofFlorida and southernGeorgia andAlabama is underlain by rocks that were originally part of Gondwana, but this region stayed attached to North America when theCentral Atlantic opened.[38]
Antarctica, the centre of the supercontinent, shared boundaries with all other Gondwana continents and the fragmentation of Gondwana propagated clockwise around it. The break-up was the result of the eruption of theKaroo-Ferrar igneous province, one of the Earth's most extensivelarge igneous provinces (LIP)c.200 to 170 Ma, but the oldestmagnetic anomalies between South America, Africa, and Antarctica are found in what is now the southernWeddell Sea where initial break-up occurred during the Jurassicc.180 to 160 Ma.[39]
The oldest eastern Indian ocean floor formed between India and Antarctica c. 120 Ma (left). The Kerguelen LIP began to form the Ninety East ridge c. 80 Ma (centre). The Indian and Australian plates merged c. 40 Ma (right).
At c. 126 Ma (left) the Falkland Plateau began to slide past southern Africa and the Paraná-Etendeka LIP had opened the Mid-Atlantic Ridge. At c. 83 Ma (right) the South Atlantic was fully opened and the Romanche Fracture Zone was forming near the Equator.
Theopening of the South Atlantic Ocean divided West Gondwana (South America and Africa), but there is considerable debate over the exact timing of this break-up. Rifting propagated from south to north along Triassic–Early Jurassic lineaments, but intra-continental rifts also began to develop within both continents in Jurassic–Cretaceous sedimentary basins, subdividing each continent into three sub-plates. Rifting beganc.190 Ma at Falkland latitudes, forcing Patagonia to move relative to the still static remainder of South America and Africa, and this westward movement lasted until the Early Cretaceous126.7 Ma. From there rifting propagated northward during the Late Jurassicc.150 Ma or Early Cretaceousc.140 Ma most likely forcing dextral movements between sub-plates on either side. South of theWalvis Ridge andRio Grande Rise theParaná and Etendeka magmatics resulted in further ocean-floor spreadingc.130 to 135 Ma and the development of rifts systems on both continents, including theCentral African Rift System and theCentral African Shear Zone which lasted untilc.85 Ma. At Brazilian latitudes spreading is more difficult to assess because of the lack of palaeo-magnetic data, but rifting occurred in Nigeria at theBenue Troughc.118 Ma. North of the Equator the rifting began after120.4 Ma and continued untilc.100 to 96 Ma.[49] Dinosaur footprints representing identical species assemblages are known from opposite sides of the South Atlantic (Brazil andCameroon) dating to around120 million years ago, suggesting that some form of land connection still existed between Africa and South America as recently as the earlyAptian.[50]
Insular India began to collide with Asia circa70 Ma, forming theIndian subcontinent, since which more than 1,400 km (870 mi) of crust has been absorbed by theHimalayan-Tibetan orogen. During the Cenozoic, the orogen resulted in the construction of theTibetan Plateau between the Tethyan Himalayas in the south and theKunlun andQilian mountains in the north.[54]
Australia was warm and wet during the Paleocene and dominated by rainforests. The opening of the Tasman Gateway at the Eocene-Oligocene boundary (33 Ma) resulted in abrupt cooling but the Oligocene became a period of high rainfall with swamps in southeastern Australia. During the Miocene, a warm and humid climate developed with pockets of rainforests in central Australia, but before the end of the period, colder and drier climate severely reduced this rainforest. A brief period of increased rainfall in thePliocene was followed by drier climate which favoured grassland. Since then, the fluctuation between wet interglacial periods and dry glacial periods has developed into the present arid regime. Australia has thus experienced variousclimate changes over a 15-million-year period with a gradual decrease in precipitation.[59]
The Tasman Gateway between Australia and Antarctica began to openc.40 to 30 Ma. Palaeontological evidence indicates theAntarctic Circumpolar Current (ACC) was established in the Late Oligocenec.23 Ma with the full opening of theDrake Passage and the deepening of the Tasman Gateway. The oldest oceanic crust in the Drake Passage, however, is34 to 29 Ma-old which indicates that the spreading between the Antarctic and South American plates began near the Eocene-Oligocene boundary.[60] Deep sea environments inTierra del Fuego and theNorth Scotia Ridge during the Eocene and Oligocene indicate a "Proto-ACC" opened during this period. Later,26 to 14 Ma, a series of events severally restricted the Proto-ACC: change to shallow marine conditions along the North Scotia Ridge; closure of the Fuegan Seaway, the deep sea that existed in Tierra del Fuego; and uplift of the Patagonian Cordillera. This, together with the reactivatedIceland plume, contributed to global warming. During the Miocene, the Drake Passage began to widen, and as water flow between South America and theAntarctic Peninsula increased, the renewed ACC resulted in cooler global climate.[61]
Since the Eocene, the northward movement of the Australian Plate has resulted in anarc-continent collision with thePhilippine andCaroline plates and the uplift of theNew Guinea Highlands.[62] From the Oligocene to the late Miocene, the climate in Australia, dominated by warm and humid rainforests before this collision, began to alternate between open forest and rainforest before the continent became the arid or semiarid landscape it is today.[63]
Banksia, agrevilleoid Proteaceae, is an example of a plant from a family with a Gondwanan distribution
The adjective "Gondwanan" is in common use inbiogeography 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 theAntarctic flora.[9] For example, the plant familyProteaceae, known from all continents in the Southern Hemisphere, has a "Gondwanan distribution" and is often described as an archaic, orrelict, lineage. The distributions in the Proteaceae is, nevertheless, the result of both Gondwanan rafting and later oceanic dispersal.[64]
During the Silurian, Gondwana extended from the Equator (Australia) to the South Pole (North Africa and South America) whilst Laurasia was located on the Equator opposite to Australia. A short-livedLate Ordovician glaciation was followed by a SilurianHot House period.[65] TheEnd-Ordovician extinction, which resulted in 27% of marine invertebrate families and 57% of genera going extinct, occurred during this shift from Ice House to Hot House.[66]
Reconstructions of (left) a late SilurianCooksonia, the first known land plant, and (right) a Late DevonianArchaeopteris, the first large tree
By the end of the Ordovician,Cooksonia, a slender, ground-covering plant, became the first known vascular plant to establish itself on land. This first colonisation occurred exclusively around the Equator on landmasses then limited to Laurasia and, in Gondwana, to Australia. In the late Silurian, two distinctive lineages,zosterophylls andrhyniophytes, had colonised the tropics. The former evolved into thelycopods that were to dominate the Gondwanan vegetation over a long period, whilst the latter evolved intohorsetails andgymnosperms. Most of Gondwana was located far from the Equator during this period and remained a lifeless and barren landscape.[67]
West Gondwana drifted north during theDevonian, bringing Gondwana and Laurasia close together. Global cooling contributed to theLate Devonian extinction (19% of marine families and 50% of genera went extinct) and glaciation occurred in South America. Before Pangaea had formed, terrestrial plants, such aspteridophytes, began to diversify rapidly resulting in the colonisation of Gondwana. TheBaragwanathia Flora, found only in theYea Beds of Victoria, Australia, occurs in two strata separated by 1,700 m (5,600 ft) or 30 Ma; the upper assemblage is more diverse and includes Baragwanathia, the first primitiveherbaceous lycopod to evolve from the zosterophylls. During the Devonian,giant club mosses replaced the Baragwanathia Flora, introducing the first trees, and by the Late Devonian this first forest was accompanied by theprogymnosperms, including the first large treesArchaeopteris.[68] The Late Devonian extinction probably also resulted inosteolepiform fishesevolving into the amphibian tetrapods, the earliest land vertebrates, in Greenland and Russia. The only traces of this evolution in Gondwana are amphibian footprints and a single jaw from Australia.[69]
The closure of the Rheic Ocean and the formation of Pangaea in the Carboniferous resulted in the rerouting of ocean currents that initiated an Ice House period. As Gondwana began to rotate clockwise, Australia shifted south to more temperate latitudes. An ice cap initially covered most of southern Africa and South America but spread to eventually cover most of the supercontinent, except northernmost Africa-South America. Giant lycopod and horsetail forests continued to evolve in tropical Laurasia together with a diversified assemblage of true insects. In Gondwana, in contrast, ice and, in Australia, volcanism decimated the Devonian flora to a low-diversity seed fern flora – the pteridophytes were increasingly replaced by the gymnosperms which were to dominate until the Mid-Cretaceous. Australia, however, was still located near the Equator during the Early Carboniferous, and during this period,temnospondyl andlepospondyl amphibians and the firstamniote reptilians evolved, all closely related to the Laurasian fauna, but spreading ice eventually drove these animals away from Gondwana entirely.[70]
The Gondwana ice sheet melted, and sea levels dropped during the Permian and Triassic global warming. During this period, the extinctglossopterids colonised Gondwana and reached peak diversity in the Late Permian when coal-forming forests covered all of Gondwana. The period also saw the evolution ofVoltziales, one of the few plant orders to survive thePermian–Triassic extinction (57% of marine families and 83% of genera went extinct) and which came to dominate in the Late Permian and from whom trueconifers evolved. Tall lycopods andhorsetails dominated the wetlands of Gondwana in the Early Permian. Insects co-evolved with glossopterids across Gondwana and diversified with more than 200 species in 21 orders by the Late Permian, many known from South Africa and Australia. Beetles and cockroaches remained minor elements in this fauna.Tetrapod fossils from the Early Permian have only been found in Laurasia but they became common in Gondwana later during the Permian. The arrival of thetherapsids resulted in the first plant-vertebrate-insect ecosystem.[71]
During the Mid- to Late Triassic, hot-house conditions coincided with a peak in biodiversity – the end-Permian extinction was enormous and so was the radiation that followed. Two families of conifers,Podocarpaceae andAraucariaceae, dominated Gondwana in the Early Triassic, butDicroidium, an extinct genus of fork-leaved seed ferns, dominated woodlands and forests of Gondwana during most of the Triassic. Conifers evolved and radiated during the period, with six of eight extant families already present before the end of it.Bennettitales andPentoxylales, two now extinct orders of gymnospermous plants, evolved in the Late Triassic and became important in the Jurassic and Cretaceous. It is possible that gymnosperm biodiversity surpassed later angiosperm biodiversity and that the evolution of angiosperms began during the Triassic but, if so, in Laurasia rather than in Gondwana. Two Gondwanan classes,lycophytes andsphenophytes, saw a gradual decline during the Triassic while ferns, though never dominant, managed to diversify.[72]
The brief period of icehouse conditions during theTriassic–Jurassic extinction event had a dramatic impact on dinosaurs but left plants largely unaffected. The Jurassic was mostly one of hot-house conditions and, while vertebrates managed to diversify in this environment, plants have left little evidence of such development, apart fromCheiroleidiacean conifers andCaytoniales and other groups of seed ferns. In terms of biomass, the Jurassic flora was dominated by conifer families and other gymnosperms that had evolved during the Triassic. ThePteridophytes that had dominated during the Paleozoic were now marginalised, except for ferns. In contrast to Laurentia, very few insect fossils have been found in Gondwana, to a considerable extent because of widespread deserts and volcanism. While plants had a cosmopolitan distribution, dinosaurs evolved and diversified in a pattern that reflects the Jurassic break-up of Pangaea.[73]
The Cretaceous saw the arrival of theangiosperms, or flowering plants, a group that probably evolved in western Gondwana (South America–Africa). From there the angiosperms diversified in two stages: themonocots andmagnoliids evolved in the Early Cretaceous, followed by thehammameliddicots. By the Mid-Cretaceous, angiosperms constituted half of the flora in northeastern Australia. There is, however, no obvious connection between this spectacular angiosperm radiation and any known extinction event nor with vertebrate/insect evolution. Insect orders associated with pollination, such asbeetles,flies,butterflies and moths,wasps, bees, ants, radiated continuously from the Permian-Triassic, long before the arrival of the angiosperms. Well-preserved insect fossils have been found in the lake deposits of theSantana Formation in Brazil, theKoonwarra Lake fauna in Australia, and theOrapa diamond mine in Botswana.[74]
Dinosaurs continued to prosper but, as the angiosperm diversified, conifers, bennettitaleans and pentoxylaleans disappeared from Gondwanac. 115 Ma together with the specialised herbivorousornithischians, whilst generalist browsers, such as several families ofsauropodomorphSaurischia, prevailed. TheCretaceous–Paleogene extinction event killed off all dinosaurs except birds, but plant evolution in Gondwana was hardly affected.[74]Gondwanatheria is an extinct group of non-therian mammals with a Gondwanan distribution (South America, Africa, Madagascar, India, Zealandia and Antarctica) during the Late Cretaceous and Palaeogene.[75]Xenarthra andAfrotheria, two placental clades, are of Gondwanan origin and probably began to evolve separatelyc.105 Ma when Africa and South America separated.[76]
The plant genusNothofagus provides a good example of a taxon with a Gondwanan distribution, having originated in the supercontinent and existing in present-day Australia, New Zealand, New Caledonia, and South America'sSouthern Cone. Fossils have also been found in Antarctica.[77]
Thelaurel forests of Australia, New Caledonia, and New Zealand have a number of species related to those of thelaurissilva of Valdivia, through the connection of theAntarctic flora. These include gymnosperms and the deciduous species ofNothofagus, as well as the New Zealand laurel,Corynocarpus laevigatus, andLaurelia novae-zelandiae. New Caledonia and New Zealand became separated from Australia bycontinental drift 85 million years ago. The islands still retain plants that originated in Gondwana and spread to the Southern Hemisphere continents later.
Plate tectonics, a theory which describes the large-scale motions of Earth's lithosphere
South Polar dinosaurs, which proliferated during the Early Cretaceous (145–100 Mya) while Australia was still linked to Antarctica to form East Gondwana
Gondwana Research, a scholarly journal including Gondwana among its emphases
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