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Australia: The Land Where Time Began |
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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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |