Australia: The Land Where Time Began
Australian Tectonic and Metallogenic Evolution Pt.3
700-0 Ma - Pangaea-Gondwana assembly and breakup
In Australia extensive tectonic activity occurred in the Phanerozoic, particularly in the Palaeozoic, especially in the Tasman Element, that comprises most of the eastern third of the Australian continent. The assembly and breakup of Pangaea, the assembly of which began at about 600 Ma and completed by about 250 Ma, is related to this tectonic activity, with Pangaea beginning to breakup soon after its formation was complete. During the assembly of Pangaea Australia remained along its active margin. Pangaea began to breakup after about 180 Ma, then Gondwana began to breakup, which culminated at about 34 Ma in the final separation of Australia from Antarctica (Veevers et al., 1991). Australia has remained relatively stable tectonically since 65 Ma., though according to the authors1 this stability will last a relatively short time, in geological terms, as the Australian continent is currently colliding with South East Asia, which has been called a prelude to the assembly of a new supercontinent, as interpreted by some, that has been called Amasia (Santosh et al., 2009).
Evidence of major global ice ages has been recorded during the evolution of the Central Australian Basin System, that continued into the Palaeozoic, in widespread locations from the Yeneena Basin, West Australia, to the Adelaide Rift System, South Australia and King Island in Bass Strait between Tasmania and the mainland. It has been suggested (Hoffman et al., 1998) that these glacial events are representative of "Snowball Earth". The complexity of life increased during the Late Neoproterozoic to the extent that the first known evidence of multicellular life is found towards the end of the era, including fossils that were delicately preserved in the upper unit of the Pound Subgroup in the Ediacara Hills, in the Adelaide Rift System (Glaessner and Wade, 1966). The authors1 suggest it is notable that these animals began to flourish following the final glacial event of the Neoproterozoic.
The earliest activity related to the amalgamation of Gondwana is in West Australia. From 650-600 Ma deformation was accompanied by the emplacement of Granite and related copper-gold and W-copper-zinc mineralisation in the Paterson Province (Goellnicht et al., 1989; Huston et al., 2010) Between 560-525 Ma deformation that was more extensive occurred during the King Leopold and Petermann Orogenies that were coeval, affecting the Kimberley Province and the Petermann Province respectively.(Shaw et al., 1992; Aitken et al., 2011; and references therein). In the Tanami Province zircon uranium-lead discordia have lower intercept ages that are similar (Maidment, 2006), which suggests the King Leopold Orogeny and the Petermann Orogeny, that were linked, extended from the northwest into central Australia. It has been suggested (Aitken et al., 2011) that the Peterman Orogeny is an intraplate response to the Kuunga Orogeny, involving oblique collision between Australia and India during the assembly of Gondwana.
Along the southern part of the Australian west coast more direct evidence has been found of the assembly of Gondwana, where the Pinjarra Orogen (Pinjarra Element) comprises elements of eastern Gondwana, notably the Australo-Antarctic Domain and the Indo-Antarctic Domain. According to the authors1 these represent distinct continental fragments that have different histories during the Proterozoic that were juxtaposed by by oblique collision by about 522 Ma (Collins, 2003a; Collins & Pisarevsky, 2005). These domains have been re-separated by the Breakup of Gondwana, though the fragments are still present in the Pinjarra Element (Boger, 2011). At about 510 Ma, immediately following the King Leopold, Petermann and Pinjarra Orogenies, the Kalkarindji Large Igneous Province, that is one of the most extensive known, was emplaced through much of western and north-central Australia, possibly as response to a mantle plume (Glass & Phillips, 2006).
The Tasman Element of eastern Australia, extending from Tasmania to northern Queensland, was built on a passive margin that was formed by the breakup of Rodinia. This orogen is part of a global scale orogenic system, the Terra Australis Orogen that extends for more than 18,000 km, and extended along the Gondwana margin (Cawood, 2005). The Tasman Element developed as a result of a sequence of tectonic cycles (Glen, 2005). Arc magmatism and/or backarc extension, related in most case to west-dipping subduction, initiated each of these cycles that lasted for between 30-130 My, ending with a contractional orogenesis as accretion of continental slivers and island arcs back onto the Australian continent occurred (Gray & Foster, 2004; Collins and Richards, 2008). 5 of these cycles have been recognised (Glen, 2005; Champion et al., 2009):
Though older cycles are overprinted by younger cycles, especially in North Queensland, the cycles broadly are younger to the east. It has been suggested (Collins, 2003b; Collins & Richards, 2008) that this cycling may related to subduction retreat, that is long-lived, and associated extension, and subduction advance, that is short-lived, and associated contraction.
There is a progression of the type of mineral deposits formed within individual cycles and there is an eastward progression of the activity of the mineral systems over time. Deposits that are formed in backarcs (e.g., massive sulphide deposits that are volcanic-hosted) formed in the early part of cycles, while deposits forming mostly during contractional deformation, such as lode gold and base metal deposits that are structurally hosted, formed late in the cycles. Through most of the Tasman Element a highly complex metallogeny has resulted from juxtaposition of these mineral systems, that is possibly exemplified in western Tasmania, one of the richest and most diverse metallogenic provinces in the world, that has major zinc-lead-silver-copper-gold, copper-gold, gold, tin, wolfram, and iron deposits, and all within a few 10s of kilometres of each other. In the Tasman Element other important districts include lode gold deposits in the goldfields of Victoria and Charters Towers in north Queensland, and in the Macquarie Arc, New South Wales, porphyry copper-gold deposits, e.g., Cadia,
During the Palaeozoic a series of intraplate deformational events occurred in central Australia, many of which are time equivalent to events in the Tasman Element, which suggests that activity related to convergence on the eastern seaboard also had an influence on the interior of the continent. Included among these events in central Australia was the development of the Larapinta Seaway and the Alice Springs Orogeny. The marine sediments of the Amadeus and Georgina Basins from the Ordovician were linked by the Larapinta Seaway to the margin of Gondwana in eastern Australia. The Larapintine petroleum system, which was sourced from organic rich rocks that had been deposited in a warm, shallow seas of the Early Palaeozoic, is hosted in these basins (Bradshaw, 1993).
The Larapinta metamorphic event, 480-460 Ma, is characterised by granulite facies metamorphism, is an extensional event restricted to the Iridina Province (Hand, et al., 1999; Maidment, 2005, 2006). This province has been interpreted to be a deep extensional sub-basin in the Larapintine Seaway, possibly up to 30 km deep, with tholeiitic basaltic rocks which host semi-massive copper-cobalt deposits, present near the base. It has been suggested (Hoatson et al., 2005) that the formation of these basaltic rocks took place in an extensional environment, and they have been correlated to the Antrim Plateau Volcanics of the Kalkarindji Large Igneous Province.
The Alice Springs Orogeny was comprised of a succession of 4 contractional events (or movements) the:
by which large parts of the Larapinta Seaway were inverted (Gray & Foster, 2004; Scrimgeour & Close, 2011). In the Irindina Province deep crustal rocks were exhumed from 30 km during orogenesis and juxtaposed with time-equivalent rocks of sub-greenschist grade in the Georgina Basin (Hand et al., 1999; Maidment, 2005; Korsch et al., 2011b). The south-directed Shear Zone, as part of this orogeny, offset the Moho to form one of the largest anomalies (about 150 mgal) from continental interiors on Earth (Mather, 1976). Formation of a foreland basin resulted from uplift and erosion associated with the Brewer Movement (Haines et al., 2001) and it has been suggested it may have driven hydrothermal flow that formed the Mississippi Valley-type lead-zinc deposits in the Canning Basin, Western Australia, about 357 Ma (Christensen et al., 1995).
In the Sydney-Bowen-Gunnedah basin system of eastern Australia, most of the vast coal deposits formed in the Permian in a foreland basin behind a continental arc system that developed in the New England Orogen. It is believed a basin that was slowly subsiding that included thick coal measures was formed by counter flow in the mantle wedge above a subduction zone (Waschbusch, et al., 2009). At this time Australia was situated at high latitudes with the result that it had an environment that was similar to the Russian Taiga at the present, and these conditions allowed the accumulation of vast amounts of peat from Glossopteris and other species (Veevers, 2006). After accumulating for hundreds of thousands of years these peat seams were compacted into black coal, the thickest seams forming near Blair Athol that are more than 31 m thick (Hobday, 1987).
250-160 Ma - Stabilisation and breakup of Pangaea
At about 250 Ma the final amalgamation of Pangaea was complete, after which the supercontinent remained stable for less than 100 My before it began breaking up, which began about 180 Ma with the initiation of the Atlantic Ocean and the Tethys Sea, which separated Gondwana, including Australia, South America, India, Africa and Antarctica from Laurasia, which included North America and Eurasia (Dietz & Holden, 1970). Before the breakup the emplacement of tholeiitic dolerite sills about 175 Ma had taken place in Tasmania (Brauns et al., 2000), that is part of the Karoo-Ferrar Large Igneous Province.
160-65 Ma - Breakup of Gondwana and the formation of Australia
A triple point extension that was situated just southwest of Perth, Western Australia, began the separation of Australia, Antarctica and India in the Late Jurassic (Veevers, 2006). By 155 Ma the northwest margin of Australia had formed after the last of a series of continental slivers, had rifted away, Argo Land, that is now Burma. The formation of the major gas fields of the Westralian petroleum system on the North West Shelf are directly related to the Gondwana breakup. The rifting that occurred in the Late Jurassic along the northwest-southeast Westralian trend produced deep marine depocentres into which organic-rich source sediments were deposited (Bradshaw, 1993). In the Late Jurassic-Early Cretaceous clastic rocks of the Barrow Delta, which formed as extension continued, buried the rocks into the hydrocarbon-generating window and provided reservoir rocks. Extensive marine shales were deposited during the Cretaceous following continental breakup to provide the seal (Bradshaw, 1993).
A rift system that was easterly propagating separated Australia from Antarctica. At 155 Ma rift basins began to form as far east as the Polda Basin, South Australia, and rifting was occurring in the Gippsland Basin, Victoria, by 145 Ma (Norvick & Smith, 2001). At 110 Ma the submarine Kerguelen Large Igneous Province in the Southern Ocean was emplaced (Duncan, 2002). Central and eastern Australia was covered by a large inland sea during the Jurassic and Cretaceous which formed the Eromanga Basin and related basins, which are up to 1,600 m thick. These basins have been suggested to have been formed by dynamic changes to the topography, that have been suggested to have possibly formed by corner flow in the asthenosphere beneath eastern Australia during subduction that was west-directed (Waschbusch et al., 2009).
Corner flow was stopped at 65 Ma when subduction jumped eastward and uplift of the Australian eastern highlands was triggered (Waschbusch et al., 2009). The Eromanga Basin was inverted and erosion of it began and offshore from the Ceduna region, South Australia, a large delta system was deposited (Bradshaw, 1993; Norvick and Smith, 2001). A significant seaway had developed between Australia and Antarctica by this time, though there was still a connection through Tasmania. A rift basin system formed, that included the Otway, Gippsland and Bass Basins, that cumulatively host one of Australia's main hydrocarbon provinces (Bradshaw, 1993) as extension that was associated with the separation of Australia from Antarctica occurred between present day Tasmania and Victoria.
At about 84 Ma in the south seafloor spreading began along the east coast and propagated northwards, resulting in the opening of the Tasman Sea and rifting off of the Lord Howe Rise and New Zealand.
At about 56 Ma the Tasman Sea had stopped opening (Gaina et al., 1998). Further to the north seafloor spreading began as the Coral Sea opened and the Queensland Plateau rifted from the northeast coast of Queensland (Wiesel & Watts, 1979).
Compared to the continents of the Northern Hemisphere one of the distinguishing features is the long-term, deep weathering, with much of this weathering occurring as Gondwana broke apart. It is indicated by oxygen isotope data that clays produced during the weathering during the Mesozoic had formed at low latitudes as Australian began to drift north from Antarctica (Chivas & Althopheng, 2010). The fundamental physiography and the palaeovalley architecture of the arid regions of Australia of the present was established (Fujioka & Chappell, 2010). It has been suggested by fission track dating that some of these palaeo-land surfaces are of Cambrian age though some of these very ancient landscapes had been buried then exhumed more recently (Pillans, 2007).
65-2.6 Ma - Australia is surrounded by sea
The past 65 My have left 80 % of the surface geology of Australia comprised of Cenozoic deposits. The cover is comprised mostly of thin aeolian, lacustrine and fluvial deposits, though there are many discrete basin entities that developed, including the Eucla, Murray, Lake Eyre and the Karumba Basins. At abut 45 Ma seafloor spreading in the Southern Ocean accelerated but it was not until 34 Ma that Australia was completely separated from Antarctica (Zachos et al., 2001; Livermore et al., 2005).
About 45-10 Ma
Sediments of Cenozoic age were deposited into the Lake Eyre, Eucla, Karumba, and Murray Basins. The climate was warmer and wetter than at present, and there were extensive rainforests (Fujioka & Chappell, 2010). There were several marine transgressions at this time, examples being shorelines at about 40 Ma in the Eocene and 15-5 Ma in the Miocene that are preserved along the northern margins of the Eucla and Murray Basins (Benbow et al., 1995). Palaeoplacer deposits of rutile and other heavy minerals have been left by these marine incursions (Hou et al., 2003).
About 45-5 Ma
At this time there were episodes of magmatism, that was mainly mafic, that occurred in Australia producing a series of volcanic provinces in the Tasman Element (O'Reilly & Zhang, 1995). Many of these volcanic provinces define what has been interpreted to be a series of tracks of a hot spot getting younger from north to south that are suggested to track the movement of the Australian continent as it moves to the north (Johnson et al., 1989). It has been suggested that others may have resulted from the upwelling of deep mantle (O'Reilly & Zhang, 1995).
Other resources, such as iron ore, coal, oil shale, uranium and groundwater are hosted by Cenozoic basins, regoliths and palaeovalleys. A significant proportion of iron ore from the Pilbara Craton is from pisolitic iron that formed palaeochannels active about 30 Ma. These ores were originally derived from banded-iron formation and iron ore deposits that have been dated to 28-5 Ma (Morris & Ramanaidou, 2007).
In the La Trobe Valley in southeast Victoria is a site of deposits of brown coal from which most of the state's electricity is generated are up to 130 m thick, having formed between 30-20 Ma, at a time when the climate was humid (Fujioka & Chappell, 2010). Oil shale from pullapart, lacustrine basins near Gladstone and Proserpine, Queensland were also formed at this time (Henstridge & Missen, 1982).
By 25 Ma the separation between Australia and Antarctica was enough to allow full circulation to occur between the Southern Ocean and the Pacific Ocean leading to major changes to the Australian climate as well as the Antarctic climate (Fujioka & Chappell, 2010). About 15-10 Ma the Antarctic ice sheet began to develop in the east and about 10-6 Ma in the west, which fundamentally changed the marine and terrestrial climate of the Earth as well as the levels of the ocean. There were large impacts on Australia, with the steepening of meridional temperature gradients, the contraction of the subtropical monsoon to the north, and the drying of the continent. Climatic zone boundaries strengthened, leading to the increased aridification of continental regions of Australia at mid-latitudes, as well as in Africa and North and South America. The evolution of grazing animals was stimulated by the spread of grasslands and the contraction of rainforests in Australia (White, 1994; Fujioka & Chappell, 2010).
At about 40 Ma active convergence began and has continued to the present along the northern margin of Australia. Timor is now part of the Australian plate and is moving to the north with Australia (Keep et al., 2002). According to the authors1 New Guinea is the best place to see the results of the movement to the north of the Australian continent, as it is on the northern margin of the Australian plate. The emplacement of the Irian Ophiolite and the uplift of the mountainous topography forming the spine of New Guinea occurred as a result of the collision with proto-Indonesia and related fragments. An active fold-thrust belt has formed in southern New Guinea, and the foreland basins that resulted host major accumulations of hydrocarbon at Tangguh, and Hides (Van Ufford & Cloos, 2005). The location of the giant gold deposits of Irian Jaya and PNG, such as Grasberg and Ok Tedi (Hill et al., 2002; Davies, 2012) are also controlled by the collision and the underlying structure of the Australian Plate.
At about 5 Ma convergence and major strike-slip faulting began across the plate boundary of the Australian Plate in New Zealand. This resulted in far-field shortening in Australia and it also had an impact on the state of stress, causing uplift in the Flinders Ranges, and contributed to the earthquake state of the present in Australia.
2.6-0 Ma Formation of modern Australia
By 2.6 Ma, at the beginning of the Quaternary, the ecology of Australia of the present had been mostly established, with Australia separated from Antarctica and moving north towards Southeast Asia, and setting up the stress fields that are prevalent at the present. The most significant that occurred in Australia in the Quaternary are related to climate and the arrival of the first humans onto the continent rather than to tectonics. In Australia the Quaternary is characterised by repeated episodes of aridity that are linked to glacial-interglacial cycles. The maximum extent of ice in Tasmania occurred at about 1 Ma, and over the last 100,000 years with less extensive advances. Periods of aridity accompanied these glacial periods, with atmospheric circulation being intensified, atmospheric moisture being reduced, and a climate that was becoming more continental, that led to more arid conditions (Fujioka & Chappell, 2010). A consequence of these conditions was the repeated deflation and formation of vast sand dune areas, culminating at the present in 40 % of the land surface of the continent being covered by aeolian sand dunes and sand plains (Haberlah et al., 2010).
At 130-110 ka In South Australia, Lake Eyre reached its maximum water level, the levels being particularly low at 95-80 ka and at 65-62 ka. Lake Eyre dried up completely during the Last Glacial Maximum which exposed its bed to the wind which scoured its base and deepened the Lake floor (Hesse, 2010).
At 22 ka continental Australia was 10oC cooler than at than at present, and it was much drier and windier, and with the sea level 120 m lower (Fujioka & Chappell, 2010), which resulted in a land bridge from Tasmania to the mainland and between the mainland and PNG. By 6 ka the land bridges had been flooded by the sea as sea levels rose. The return to warmer and wetter conditions are recorded by palynological data, tropical rainforests expanded between 8-6 ka and again at 3 ka (Fujioka & Chappell, 2010).
At least 60-45 ka (Roberts et al., 1990; Pope & Terrell, 2008) the continent was occupied by Aboriginal and Torres Strait Islander people, being present through many of the major changes of climate that have affected Australia during the latter part of the Quaternary. These changes greatly impacted the Aboriginal people, with climate changes causing episodic occupation of specific sites as the result of the availability of water and food, as well as changing sea levels. Shark Bay, 800 km to the north of Perth, is an example, being occupied during 3 distinct periods, 30-18 ka, 7-6 ka and since about 2.3 ka (Bowdler, 1999). The isolation of Aboriginal people in Tasmania was a far greater consequence of climate change. About 40 ka Tasmania was first inhabited by Aboriginal people when the land bridge existed between Tasmania and the mainland. At about 8 ka the sea level rose in association with warming of the interglacial the land bridge was severed (Pardoe et a., 1991). Until European settlement just before the turn of the 19th century, Tasmania was remained isolated.
Amasia, the next supercontinent, is expected to be fully formed by 250 My in the future (Santosh et al., 2009). The Australian continent, which included Papua New Guinea, is colliding with the Indonesian Archipelago at the present. Natural resources were produced in the past by this process of convergence and will be in the future, as is occurring in Papua New Guinea and Indonesia at the present. Hydrocarbon deposits will be formed in foreland sedimentary basins related to fold-thrust belts.
Conclusion of Huston et al.
The geological evolution of the Australian continent is a microcosm of the evolution of the Earth, that is closely linked to the supercontinent cycle, as most geological and metallogenic events being related to the assembly and breakup of the supercontinents of Vaalbara, Kenorland, Nuna, Rodinia and Pangaea-Gondwana. Broadly, the oldest part of the continent is the West Australian Element comprised of the 2 major Archaean cratons, Yilgarn and Pilbara Cratons, which formed the basis of the continent as it grew from west to east. The North and South Australian Elements, from the Palaeoproterozoic-Mesoproterozoic mainly dominate the centre of the continent, and the east of the continent is dominated by the Tasman Element, from the Phanerozoic. It appears that during the Palaeoproterozoic amalgamation of Nuna the West, North and South Australian Elements appear to have been initially assembled, though there was a partial breakup of proto-Australian that had been formed, only to be reassembled during the amalgamation of Rodinia. During the Palaeozoic the eastern margin of Australia was an accretionary margin as Gondwana was assembled, and later Pangaea. At 34 Ma the last links were severed between Antarctica and Australia as it broke from Gondwana which resulted in the present position of Australia as a continent that was relatively stable with 3 sides being passive margins. At present it is moving north towards South East Asia, the authors suggesting it is probably in the earliest stages of the assembly of the next supercontinent, Amasia.
The mineral and energy resources of Australia are linked to its tectonic evolution and the supercontinent cycles. Australian resources cluster strongly in time and space, the clusters being associated with the supercontinent cycle of the Earth, though both mineral and energy resources are found throughout Australia and their formation spans the history of the continent. The Eastern Goldfields in Western Australia, that is the most important gold province on the continent was produced by the assembly of Kenorland, while the major zinc-lead-silver deposits from the Palaeoproterozoic-Mesoproterozoic and the iron oxide-copper-gold deposits of Olympic Dam that formed as the breakup of Nuna was taking place. In the Tasman Element the diverse metallogeny was produced during the assembly of Pangaea-Gondwana and most of the hydrocarbon resources in Australia that resulted from the breakup of this supercontinent. In Papua New Guinea and Indonesia to the north of Australia gold and copper-gold deposits are suggested by the authors1 to possibly be the very early products of the assembly of Amasia.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|