Australia: The Land Where Time Began
Biogeography of Australia – Distribution in and Around the Tasman Region
The most influential research tradition in Australian ornithology has been the evolutionary synthesis of Mathew, Simpson and Mayr. According to Heads1 using this approach the author studied the colonisation of Australia by birds, finding that evidently ‘it was untenable’ to consider that birds may have been in Australia from the start (Joseph, 2008: 265). Heads1 says the situation is changing, one example being the passerines, an order of birds that comprises more than half all species. The Australasian passerines have been shown by molecular studies to not comprise unrelated minor offshoots of groups from the north, as has been assumed, but instead large groups that are basal in the order and its subclasses. It was concluded (Joseph, 2008) that the Australian passerines are ‘as old as the hills’. Though this was an important result, there is the temptation to simply reverse the model of Mathew, and to suggest instead a new centre of origin in Australasia, with dispersal taking place from south to north rather than of from north to south. Australasia, especially the Tasman-Coral Sea area neither represents a sink nor a centre of origin, but a global centre of differentiation. The distinctive geological history is a possible explanation for this.
Tasman region and island biota evolution – tectonic context
Plate convergence has continued along the southwest Pacific subduction zone beginning in the Palaeozoic. Convergence and terrane accretion at the margins of the plates has dominated tectonic history in the Tasman region, along with accompanying magmatism and orogeny. The immediate precursors of New Zealand, New Caledonia and New Guinea have resulted from this convergence, being formed when different oceanic terranes, formed in the pre-Pacific, fused together with each other and Gondwana. Most of the oceanic terranes included seamounts or material that was arc-related, large igneous plateaus were among the last structures to accrete, e.g., convergence resulted in the accretion of the Hikurangi Plateau at New Zealand during the Late Cretaceous. The subduction geometry in the region was changed as a result of this choking of subduction.
It has at times been assumed that there was no terrestrial biota on these terranes at the time of their collision with Gondwana, and that they were so close to the margins of Gondwana when they emerged from the sea that it was entirely biota from the mainland that colonised them. As Heads1 points out this is consistent with the processes outlined in the ‘island biogeography’ theory (MacArthur & Wilson, 1967); Uplift along the Gondwana margin was the mechanism that formed proto-New Zealand, and ‘the origins of the earliest colonists seem straightforward’ (Winkworth et al., 2002b: 514). According to this model the biota of the Zealandia when it rifted from Gondwana was ‘identical’ to that of Gondwana (Campbell & Hutching, 2007. It has been written ‘early Zealandia ought to have contained a mammalian fauna similar to that of eastern Gondwana at the time of complete separation a. 60 million years ago’ (Lee et al., 2010: 29).
Heads1 points out that, in spite of these suggestions, there is no real need to expect that the arc and seamount terranes that were integrated with New Zealand had a population that only derived from that of the mainland. It is also not necessary to assume that the Gondwana biotas, or those of its daughter fragments, including Zealandia, were homogeneous at any time. It is indicated by many distributions of Tasman-Polynesia that the flora and fauna of the arcs and intraplate volcanics of the Pacific and pre-Pacific plates were just as significant for the biogeography of the Tasman region as those of Gondwana. It is not likely that Gondwana was full of terrestrial life while there was no terrestrial life on the entire Pacific and pre-Pacific regions. Even prior to the formation of the Pacific basin of the present there were always many islands in the Pacific region, and even a small tropical island, as well as a large archipelago, is capable of maintaining a diverse biota. An understanding of the history of life of the central Pacific, together with the life of Gondwana and Tethys, is required for an understanding of the Tasman region biogeography.
The formation and growth of the Pacific Plate, which is demarcated by its active margins, and the emplacement of large igneous plateaus not long after the origin of the plate, are 2 of the most significant events in the history of the Pacific Plate. A metapopulation (group of populations of the same species that are spatially separated) model is supported by this dynamic geological history, this metapopulation model differing from the ‘island biogeography’ model in not requiring a source on the mainland.
The Pacific Plate
The Tongareva triple junction, which was the meeting point where 3 spreading ridges that had developed near the site of the Cook Islands of the present, was the site of origin of the Pacific Plate (Larson et al., 2002). As the Pacific plate expanded from here its dramatic growth occurred at the expense of 3 surround plates, the Phoenix, Farallon and Izanagi plates. The Pacific Plate, which now occupies almost all of the Pacific Basin, reached the half-way point in its growth in the Middle Cretaceous.
Eventually, most of the Phoenix Plate was destroyed by subduction, though some of the old biota would have survived on its islands and would have been transferred to islands in the Tasman region. Heads1 suggests a similar biogeographic redisposition, and a piling up of diversity, probably occurred in the east Pacific/western Americas margin as subduction was destroying the Farallon Plate.
Ontong Java, Manihiki and Hikurangi – large igneous plateaus of the central Pacific
The Pacific Plate was a fraction of its eventual size in the Early Cretaceous and at that time several large igneous plateaus formed in the central Pacific. Also at this time the Ontong Java, Manihiki and Hikurangi Plateaus were emplaced, all 3 probably forming a single plateau that was very large (Taylor, 2006; Ingle et al., 2007; Timm et al., 2011). It was written (Hoernle et al., 2007) that the Hikurangi basement lavas, 118-96 Ma, are surprisingly similar, in major and trace element and isotopic characteristics to the lavas of the Ontong Java Plateau, about 120-90 Ma. They referred to the Greater Ontong Java Event, over a period of time during which ~ 1 % of the surface was covered by volcanism. In total the eruptions covered an area of about 2 million km2, which is the size of Alaska or western Europe, over a period of a few million years, the largest known magmatic event in the entire history of the Earth (Fitton et al., 2004; Hoernle et al., 2004).
The plateaus broke up and drifted apart following their formation the in the central Pacific. At ~ 12 Ma the Ontong Java Plateau collided with the Solomon Islands after it was translated to the west. Spreading at the Osbourn Trough separated the Hikurangi Plateau from the Manihiki Plateau (Worthington et al., 2006), ceasing only when the Hikurangi Plateau arrived at the subduction zone, the Chatham Rise, off the eastern coast of New Zealand during the Cretaceous and then choked the system after it was partially subducted (Whattam et al., 2005), which probably occurred ~86 Ma in the Late Cretaceous. Zealandia rifted from Gondwana (eastern Australia) not long later, 85-80 Ma, so the Hikurangi Plateau remained amalgamated with Gondwana for only 1-6 My of its 120 My history, almost all of which has occurred in the Pacific. Its tectonics and biogeography is a large component of New Zealand that is related to the Pacific rather than Gondwana. An area the size of New Zealand of the present was covered by the exposed portion of the Hikurangi Plateau, which is now completely submerged, while other parts are subducted beneath the North Isand and the Chatham Rise.
Abutting the eastern side of the Solomon Islands, the Ontong Java Plateau is a large structure that is mainly submerged. Of the large igneous provinces of the world the Ontong Java Province is the largest. The immense structure of this plateau has been subjected to geological investigations that have been described as a ‘pinpricking the elephant’ and ‘no simple model appear to account satisfactorily for all the observed first-order features’ (Tejada et al., 2004: 133). A thick succession of basaltic volcaniclastic rocks of the eastern salient of the plateau erupted in a setting that was subaerial is considered to be one of the most interesting features, biogeographically, of the plateau (Thordarson, 2004). Close to the bottom of 4 of the eruptive members fossilised or carbonised fragments of wood have been found (Fitton, 2004). The basement and volcaniclastic layers from the Early Cretaceous that had been formed by subaerial and shallow-water eruptions were discovered on the Manihiki Plateau (Ai et al., 2008).
The Ontong Java Plateau has been suggested by some geologists to have formed in the central Pacific near the Gorgona Plateau, which has since been accreted to western Ecuador, western Colombia and the Caribbean. The Gorgona Plateau drifted to the east, colliding with northwestern South America as the Ontong Java Plateau drifted to the west and collided with the Solomon Islands (Kerr & Tarney, 2005; Chicangana, 2005). The possibility of a single, great igneous province that formed in the central Pacific in the Cretaceous which then broke up, has been discussed by Chicangana. Whatever turns out to be the case, the ideas of these authors would also account for the Melanesia-tropical American connections of many molecular clades, though they based their conclusions on geochemical, geophysical and microfossil evidence. It is indicated by these that the history of the plateaus of the central Pacific, including their drift to the margins of the Pacific, has had a profound effect on evolution in the Pacific. It has been proposed that the tectonic history provides an explanation for the distribution of terrestrial groups, that are trans-tropical (Heads, 2012a), as well as near-shore marine groups such as amphipods (Meyers & Lowry, 2009) and barnacles (O’Riordan et al., 2010). The Ontong Java Plateau, Hikurangi Plateau and the Manihiki Plateau, all from the Cretaceous, were all emplaced at the same time (~ 120 Ma, Aptian; Bryan & Ernst, 2007), as another large igneous province, the Whitsunday Volcanic Province/Median Batholith; extending from northeastern Queensland to Antarctica and, as was the case with the eruptions of the central Pacific, had a profound impact on biological communities.
The Ontong Java event affected marine life, as indicated by fossil evidence, though it has not been found to be associated with a mass extinction event (Hoernle et al., 2004). Heads1 suggests that though some populations and taxa would have been extirpated, plants and animals would have soon occupied the new submarine and terrestrial volcanic strata. At the present it can be seen that vegetation types, such as rainforests, thrive around active volcanoes on islands off the northern coast of New Guinea and the alpine vegetation of New Zealand. Following volcanic eruptions weedy taxa from the region soon colonise pyroclastic deposits and lapilli (“little stones” a size category of tephra), even new lava flows. A spectacular, rifted northeastern margin of the Hikurangi Plateau and in the interior of the plateau, massive seamounts as much as 24 km in diameter, are shown by 3D projection (Hoernle et al., 2004). It is indicated that the seamounts formed as volcanic islands that were later eroded by wave action to sea level after which they subsided to 2,000 m below the surface of the ocean, as they are flat-topped guyots (flat-topped seamounts, found mostly in the Pacific Ocean). There are many seamounts on all the plateaus of the central Pacific. Several low atolls in the Pacific have been found to have fossils such as land snails of groups that inhabit montane moss-forests, which provide further evidence that these atolls were mountainous at an earlier time in their history.
There is a long history of accretion of terranes around the Tasman region and the accretion of the igneous marine plateaus is part of this history. Allochthonous Ophiolite terranes, which are seafloor sections and the seamounts on them, were emplaced in New Guinea, New Caledonia and New Zealand in the Cainozoic after having arrived from the northeast. The Loyalty Ridge-Three Kings Ridge-Northland seamount chain, an island arc terrane associated with the Ophiolites, also accreted to New Caledonia and New Zealand at some stage. During the Eocene and possibly the Cretaceous the arc was active. As the biotas of the Loyalty Islands, as well as the islands off Northland are so distinct from those on the adjacent mainlands, New Caledonia and New Zealand, its history is important for biogeography.
Biogeography of Polynesia – New intraplate magmatism models
According to the traditional interpretations of dispersal of biotas of ocean islands a volcanic island breaks the surface of the ocean at a ransom location and rises high enough above the surface to eventually form an island, the distance from the nearest mainland being the only aspect of its location that is considered to be relevant. It is assumed that islands are colonised from the nearest mainland, endemics being explained by the isolation from the mainland. Heads1 suggests that if the volcanism is not randomly located, but tended to occur in the same general area as previous volcanoes, the colonisation of the volcanic islands in that particular area would probably be by biota from older volcanic islands that are closer than any mainland. Heads1 suggests this probably occurs at subduction zones, in which case new islands would continue to be produced as long as subduction continues. That populations can survive on young strata at the margins of plates, as occurs in Melanesia, might be accepted by biologists, though it would be argued by many that populations on intraplate volcanoes, as occurs in Micronesia and Polynesia, must have resulted from dispersal over long distances, Though long-distance dispersal in modern groups would not be necessary if prior islands remained in these regions. For groups such as the Hawaiian Islands, an example of the conveyor belt, hotspot model for intraplate groups, a possible mechanism for survival of metapopulations in Polynesia, though Heads1 says the entire topic of intraplate volcanism is controversial.
The distribution of most of the volcanism on the surface of the Earth was accounted for by the plate tectonics model that was proposed in the 1960s, as this volcanism occurs at the boundaries of plates. But areas of profuse volcanism within plates were not accounted for by this model, examples being Hawaii and other Pacific Islands. A proposal of hotspots and mantle plumes to power them was a second mechanism, which was independent of plate tectonics to account for this type of volcanism. Though the mantle plume model became popular, more recent explanations have been developed to account for ‘anomalous’ volcanism, that are based on effects of plate tectonics (Foulger & Natland, 2003). According to these new models, great fissures and systems of fissures through the lithosphere have been produced by stress fields, and it is along these that seamount provinces and linear chains form (see paper in Foulger et al., 2005 and Foulger & Jurdy, 2007). These mechanisms would allow survival in the long term of metapopulations, as with the mantle/plume hotspot theory.
The Polynesian and Micronesian islands are among the most isolated in the world, but as a group they maintain many, widespread well-marked taxa in the central Pacific that are endemic there, in spite of their isolation (Heads, 2012a). The reef and subaerial parts of the igneous plateaus (the islands are now seamounts) would have been colonised by many of these taxa in the Cretaceous from other arcs and islands that were in the region at that time. Not long before Zealandia drifted away from Gondwana the Hikurangi Plateau would have transported many islands to Zealandia.
Endemism in western and eastern Polynesia, in the central Pacific, or the area covered by the Ontong Java, Manihiki and Hikurangi Plateaus, is of direct importance to the biogeography of the Tasman Sea region, according to Heads1. Many of the patterns coincide with the igneous plateaus, as well as other centres of volcanism in the Cretaceous in southeastern Polynesia/Line Islands, and with the margin of the Pacific Plate. The long history of volcanism among the islands, plateaus, ridges, and seamounts of Polynesia and that metapopulations have survived on old systems of young islands that are individually ephemeral is suggested by the patterns of allopatry of endemism. Heads1 suggests the biota of Polynesia has been just as important for the Tasman-Coral Sea region as the South Pacific and Gondwana biota of Fagales, Proteaceae, ratites, etc., in the southwest.
Zealandia – pre-breakup geology
The component terranes of Zealandia (including New Caledonia) had come together to form Zealandia prior to the beginning of its drift from Gondwana as a new unit. It is suggested by different models that the component terranes of Zealandia were spread out along an arc that was 6,000 km long before they came together to form Zealandia (Adams et al., 2007), and according to one model (Wandres & Bradshaw, 2005) the arc was even longer. This belt of terranes extended from the Region of New Guinea to some point to the southeast of Tasmania, and the evolution of the region has been critically influenced by the interaction between this terrane belt and the coastal terranes of Australia, and the distortion of many distributions, as well as the formation other distributions, have resulted from the telescoping of the belt in the formation of New Zealand.
The phase of breakup immediately before the breakup of Gondwana was characterised in New Zealand by orogeny, volcanism on a large scale and crustal extension. A block comprised of older and younger terranes that had been accreted separated from Gondwana as Zealandia with the beginning of back-arc basin formation with seafloor spreading in the Tasman Basin. Some of the features from earlier times were cut across by the rifting, such as the Whitsunday Volcanic Province-Median Batholith, and the breakup itself, the seafloor spreading, was of less phylogenetic significance than the events that occurred prior to the breakup of Gondwana, as suggested by biogeographic patterns. A single, vast belt was formed, that extended through Queensland, New Zealand and Antarctica, by the pre-breakup Rangitata orogeny. By the time New Zealand and New Caledonia had been separated from Gondwana by rifting in the Late Cretaceous the biota of the entire sector of New Zealand-New Guinea had already been exposed to repeated phases of accretion, both geological and biological, extension, uplift and magmatism.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|