Australia: The Land Where Time Began
Australia – Time and Space
Of the 14 large and about 40 small tectonic plates, that range in size
from the Pacific Plate, comprising 20.5 % of the surface of the Earth,
to the Manus Microplate in the Bismarck Sea, comprising 0.016% of the
surface area of the Earth. These tectonic plates, which are believed to
be almost rigid blocks of lithosphere, are defined by their boundaries
and their movement across the surface of the Earth. The boundaries of
the plates can be convergent, divergent or transform. Increasingly it
has become evident that between some plates there are also regions where
seismicity is diffuse that have boundaries that are deforming slowly, as
opposed to sharp, discrete boundaries.
In the past it was believe that the Australian Plate was only part of
the Indo-Australian plate, but more recently it has been considered to
be a plate in its own right. From Cocos Island in the west to Noumea in
the east and incorporates the entire Australian continent. To the
northwest of the Australian Plate is the Capricorn Plate, which is
smaller than the Australian Plate, located between the Indian Plate and
the Australian Plate. A further division of the former Indo-Australian
Plate is the Macquarie Microplate that appears to have existed for about
6 million years, which is situated to the east of the Tasman Fracture
Zone.
The Australian Plate is subjected to boundary forces that vary from
extension to the south and southwest, to compression in the east and
north. The Southeast Indian Ridge, an active spreading centre to the
south, separates the Antarctic Plate from the Australian Plate. The
southern boundary of the Australian Plate, which is a mid-ocean ridge,
took about 100 million years to form developed with the breakup of
Gondwana. Initially Australia was moving to the northwest, but following
a major reorganisation that occurred between 53 and 50 Ma in the Pacific
Ocean, which is suggested to have possibly been caused by the subduction
of the Pacific-Izanagi spreading ridge, and subsequently, the initiation
of Marianas/Tonga-Kermadec subduction. The drifting direction of
Australia changed to the north-northeast direction that it has at the
present, whatever the cause of the change. By 34 Ma the full separation
of Australia from Antarctica was achieved. Up to the present Australia
has drifted more than 3,000 km to the north-northeast at a pace of 6-7
cm/yr, so is the continent that is moving most rapidly. Australia
emerged as the island continent of the present following the final
separation from Gondwana, which marked a significant point in the
evolutionary history of Australia’s geology and landscapes, as well as
the distinctive flora and fauna of the Australian continent.
The eastern boundary between the Australian Plate and the Pacific Plate
is a collisional zone through New Zealand and a clear subduction
boundary along the Tonga-Kermadec Trench to the north of New Zealand.
Along the northeastern margin of the Australian Plate it is being
subducted beneath the Pacific Plate at the New Hebrides and Solomon
Trenches, as well as beneath the South Island of New Zealand at the
Puysegur Trench. At the tectonically complex northern boundary of the
Australian Plate it is colliding with the Pacific Plate through the
island of New Guinea. Further to the west the interaction between the
Australian Plate and the Eurasian Plate is collision that is occurring
in the region of the Banda Arc and at the Java and Sumatra Trenches,
subduction beneath Indonesia. The western boundary of the Australian
Plate is formed by a diffuse zone of seismicity with the Capricorn
Plate.
State of stress
At the plate boundaries forces are generated that are transmitted across
the plates. These forces that are generated have been found to originate
from at least 3 types of settings: subduction of oceanic lithosphere,
collisional zones between continental lithosphere, and seafloor
spreading, which is known as ridge push. Analysis of earthquakes that
have been generated at these boundaries and within the plate can
determine the magnitude, type and direction of the stress. Mapping of
the structure of the lithosphere and asthenosphere can be achieved by
the use of energy originating from these earthquakes. The gravitational
potential that is inherent in the mass distribution within the plate and
within the mantle, which is reflected in the geoid, i.e., the shape of
the gravitational equipotential surface, generates other sources of
stress. The geoid slopes downwards to the southwest beneath continental
Australia, with a fall totalling about 125 m.
Among the continents of the Earth the Australian continent is unique is
that the stress field is not parallel to the direction, to the
north-northwest, of plate motion at the present. A horizontal
compressional stress is present across much of Australia. There are 3
sources at main collisional boundaries of that compressional that stress
control most of that stress state. They are located in New Zealand,
Indonesia and New Guinea, and the Himalaya, the latter being transmitted
through the Indian and Capricorn Plates. The stress trajectories are
oriented east-west to northwest-southeast in the part of the Australian
continent that lies south of -30o. To the north of this
latitude the stress trajectories are closer to those of the plate motion
of the present, i.e., oriented east-northeast-west-southwest. According
to Blewett it is notable that the location of the divergence of the main
stress trajectories from each other is in north-central New South Wales.
Australia experiences a relatively high level of seismicity, for what is
generally considered to be a ‘stable’ intraplate region. For events of
magnitude M ≥ 5.5 the distribution is not uniform across the continent,
with the largest earthquakes being clustered into 4 main regions which
have basement ages that are contrasting (Archaean,
Proterozoic and
Palaeozoic). Seismic activity in these seismic zones, that are
characterised by normally low seismic activity, but punctuated by
periodic enhanced seismicity, that are associated with 1 or more large
earthquakes over decadal time-scales. Some spatial overlap has been
found between such clusters of seismicity and neotectonic reactivation
of ancient fault zones and/or regions of crustal heat flow that are
elevated that has been mapped. It is suggested by these spatial
relationships that prior tectonic structures and local to regional and
lithospheric thermal weakening are most likely responsible for guiding
the active intraplate deformation in Australia.
The Flinders Seismic Zone, South Australia, has had earthquakes up to M
6.5, making it one of the most seismically active areas in Australia.
Located within the South Australian Heat Flow Anomaly, this area has one
of the world’s highest concentrations of heat-producing elements.
According to Blewett, testaments to the tectonic activity of the region
are major fault displacements, and the uplift of the Mt Lofty Ranges to
more than 700 m above sea level. In a number of localities, including
the Wilkatana Fault, central
Flinders Ranges, thrust faults that are
east- and west directed, that have caused basement rocks from the
Precambrian to be above Pleistocene sediments that have been dated to
30,000 years ago. Also, strike-slip and reverse mechanisms, that have a
broadly east-west maximum horizontal stress orientation, indicate
earthquake focal-mechanism solutions for this zone. The damming of the
Murray River and the formation of a large inland lake that lasted for
500,000 years were the result of tectonism in the Quaternary in
southeastern South Australia. The earthquake mechanisms in central
Australia are consistent with north-south compression that is associated
with the ongoing collision between Australia and Indonesia-New Guinea.
In continental Australia the recurrence intervals for active faults are
not well constrained, as is the case for intraplate regions around the
world, with estimates of large earthquakes of M ≥6.0 ranging between
10,000 years and 100,000 years. The infrequency of large events in a
region makes seismic hazard assessment difficult. The most significant
earthquake Australia has experienced, in terms of human impacts,
occurred in Newcastle, New South Wales in 1989, where there were 13
fatalities and 160 injured. In spite of this the Newcastle area has not
been identified as being particularly significant in the hazard maps of
the present (about 1979). In regions where the more prevailing status is
quiescence hazard appraisal is difficult, as is evidenced by the example
of the Newcastle earthquake. Neotectonic fault movements attest to large
earthquakes of M <7.0 in proximity to some of the most populated centres
in the SE Seismic Zone in the recent past. Therefore, it is important
for understanding the seismic hazard, to determine the likely recurrence
rate for these earthquakes
Active volcanism of the ‘Pacific Rim of fire” is associated with the
northern and eastern margins of the Australian Plate. One of the most
seismically active regions of the world is located on the subduction
zones, as well as their overlying environments, of the Pacific Rim, of
which the northern and eastern plate margins of the Australian Plate is
a part, where about ⅓ of all earthquakes throughout the world occur. The
tsunamis generated by these earthquakes impact on Australia within 2-4
hours of the events occurring. The Joint Australian Tsunami Warning
Centre (JATWC) is operated by Geoscience Australia and the Australian
Bureau of Meteorology (BoM). Potential impacts of these tsunamis on the
coast of continental Australia and external territories are monitored,
detected, verified, and warns of potential tsunamis from JATWC.
Earthquakes are located using real-time data collected from 60 seismic
stations in Australia, as well as more than 130 international seismic
stations, and sonobuoys and tide gauges, and calculated the tsunami risk
to near-shore regions. This system is integrated with national emergency
agencies which issue appropriate warnings within minutes of the
earthquake event. It has been found by modelling hypothetical tsunamis
and measured tsunamis that the northwest coast of Australia, the Pilbara
and Kimberley regions in particular, and the east coast, which is more
densely populated, as having the highest risk of tsunamis.
Volcanic activity
In Australian territory there are 2 active and emergent volcanoes, both
of which are on the Kerguelen Plateau, on the Antarctic Plate. The
Kerguelen Plateau, a total area of about 2.2 M km2, is
comprised of basalts that have geochemical characteristics that are
distinct from those of mid-ocean ridges. About 110 Ma mafic volcanism
began that was associated with a hotspot. An active volcano is on the
Big Ben massif of Heard Island. The most recent lava flow from this
volcano is 2 km long by 50-90 m wide (Box 1.4). McDonald Islands, 44 km
west of Heard Island, is another volcano that was dormant for 75 years,
then in 1992 it erupted, and activity has continued several times since
that time.
In the North island of New Zealand, which is on the Australian Plate,
the main belt of volcanoes was the result of the westward subduction of
the Pacific Plate beneath the Australian Plate. On this island there are
a number of active centres with widespread geothermal activity, that
sometimes have dangerous consequences for the human population. Ruapehu
and Tarawera, that are strato-volcanoes, have both claimed lives in the
past 200 years through phreatic eruptions and lahars. The caldera of a
super volcano, which erupted about 10 thousand years ago, leaving a
shroud of tephra across the entire North Island, is occupied by Lake
Taupo.
Another active region affecting the edge of the Australian Plate is the
mid-ocean ridge between Antarctica and Australia. As well having a
significant physiographic influence on marine life, this ridge also has
an influence on the climate of the Earth. About 20-25 % of atmospheric
carbon dioxide is sequestered by the growth of oceanic phytoplankton. As
the waters of the Southern Ocean have insufficient iron to support
vigorous plant growth with the result that carbon storage is below full
capacity. The iron content of ocean water that is derived from
hydrothermal vents is about 5-15 % in some areas, while in others it is
up to 30 %. Over millennial time-scales a relatively constant source of
iron is provided by the volcanoes of the mid-ocean ridge. Short-term
iron fluctuations derived from climate-driven sources such as wind-blown
continental dust and coastal sediments that are re-suspended are also
buffered by these volcanoes.
Along the eastern margin of Australia there are also many dormant
volcanoes, forming the Newer Volcanic Group in a chain, of mostly mafic
volcanic rocks, that erupted along the Great Divide. They Extend from
Cape York Peninsula to southern Victoria and South Australia. They occur
as extensive lava fields (e.g., McBride in north Queensland) and shield
volcanoes, such as Mt Canobolas in New South Wales that has been dated
to 12 Ma. Some of the most fertile soils in Australia were derived from
these volcanic rocks, especially where they occur in areas of higher
rainfall. Gemstones such as diamonds, sapphires, rubies, garnets and
zircons have been carried by diatremes that are associated with the
Newer Volcanic Group. A series of subparallel chains of seamounts that
lie to the east of the continent, seamounts being present in the Tasman
Sea, in the Lord Howe Rise, and Norfolk Island. They represent volcanoes
that have been eroded, such as the Gifford Seamount that has been dated
to 15.6 Ma, which mark the track northward of the Australian Plate above
1 or more mantle plumes.
The East Australian Plume System was initiated about 65 Ma with the
rifting of the Coral Sea. Younger eruptions occurred increasingly to the
south as the continent drifted to the north across the plume system. The
youngest of the dormant volcanoes are located in the intraplate region
of southeast South Australia to western Victoria. They consist of a
variety of basalt known as nepheline hawaiites that have been dated to
4.6 ka, derived from partial melts of mafic rocks from the lower crust
to upper mantle. In the Gambier Basin drill holes have produced
commercial quantities of CO 2 gas that was previously thought
to be volcanic gas, though now it is believed to be sourced from the
mantle, as the gas includes some of the most primitive neon and xenon
gas signatures on Earth. The common presences of mantle xenoliths in the
rocks of the Newer Volcanic Group also imply a connection with the
mantle. The next volcano to erupt on the Australian Plate is suggested
to be in or around Bass Strait, as suggested by the age progression of
volcanism.
Surface relief
The core of the Australian continent is formed of large cratonic rocks
of Precambrian age that comprise the western ⅔ of the continent. Blewett
says the ochre-red colours of the ‘outback’ are a testament to the
aridity, driven by the plate position of Australia with respect to the
monsoonal belts in the north, and in the Southern Ocean, the westerly
winds. Australia is the lowest and flattest of the continents, and for a
very long time it has been the most slowly eroding and deeply weathered
on Earth.
About 330 m is the average elevation of the continent, and the maximum
local topographical relief is typically less than 1,500 m. Bedrock
erosion rates are typically less than 1-10 m/yr, with the exception of
the upland areas. Most uplift in Australia has been restricted to the
eastern margin, mountain building being largely absent from the interior
for the past 200 Myr. There are local areas where there are areas of
elevation that are relatively young compared to the age of the most
ancient rocks of Australia. There has been intermittent uplift in some
areas such as the Flinders Ranges at recurrence rates of 10-50 m/Myr.
There are also impact craters dotting the old landscape, some of which
are above the low plains.
According to Blewett most of the jurisdiction of Australia is submerged
beneath the ocean and it is here that most of the surface relief is
located. The active spreading centre, which is formed by the east-west
spine of submarine ranges, of the Southeastern Indian Ridge is a legacy
of the separation of Australia from Gondwana. Swarms of north-south
trending transform faults dissect the ridge, with a marked zone of
anomalous depth at the Australian-Antarctic Discordance. The Gondwanan
breakup and marine bathymetry that resulted, was influenced by the older
geology of continental Australia. Along strike from the Gawler Craton of
Archaean age pronounced gaps in transform faults mark the influence of
the old, strong lithosphere. Along the Great Australian Bight and the
coast of Antarctica the conjugate southern margins of Australia that
resulted from the opening of the Southern Ocean, show continental
shelves that are slightly wider. The interior of Australia was drained
by huge deltas into the extending basins along this evolving southern
margin. The end of the long track of some of the earliest and farthest
travelled humans on their journey from Africa to Tasmania is now covered
by the shallow waters of Bass Strait, which at the time they crossed it
to Tasmania was a series of freshwater lakes at periods of low sea
level.
Rifted fragment of continental Australia to the east provide
opportunities for energy resources in the Lord Howe Rise basins, which
becomes part of New Zealand at its southern end. Seamounts remaining
from hotspot activity as the Australian Plate moved northwards lie
between here and the coast. The broad continental shelf off Queensland
remains from the opening of the Coral Sea is a foundation for the
largest single living entity – the Great Barrier Reef, which extends
more than 2,300 km and is comprised of more than 2,900 individual reefs.
-
Blewett RS (ed.) 2012. Shaping a Nation: A Geology of Australia, Geoscience Australia and ANU E Press, Canberra.
| Author: M. H. Monroe |
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| Last Updated 05/10/2015 |
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