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.
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.
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.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|