Australia: The Land Where Time Began
Australian Geological History See also
Although the Australian landmass of today is much different from the original one that emerged from the sea, parts of it have remained unchanged, or mostly so, from that first dry land, possibly the first dry land in the world. Parts of Australia emerged from the sea at least 3000 million years ago. In the southern part of Western Australia is an ancient block of rock called Yilgarnia by geologists. This block has not been covered by the sea since it first rose from the water. The granite rock of the Yilgarn Craton crystallised more than 2700 million years ago, detrital quartz crystals having been found at Jack Hills on the Yilgarn Craton that has been dated to 4.4 Ga.
More than half of the surface rocks of Australia formed in the Precambrian, more than 600 million years ago. It is widely believed by scientists that rocks of a similar age underlie the younger rocks over much of the remainder of the continent.
Between 2.3 and 1.9 billion years ago the original Pilbara and Yilgarn blocks were joined by a number of other blocks of granite that rose above sea level. During that time period, the foundations of the Australian continent are believed to have drifted from the North Pole to the South Pole and to be on their way north yet again. Between 1.8 and 1.4 billion years ago the aggregation of granite blocks that comprised the Australian land mass, the western 2/3 of the present continent, were welded together by crustal movement and massive intrusion of granite between the original blocks to form 3 large blocks that now comprise a rigid base for the remainder of the continent to be built from. These blocks were separated by mobile belts of thin crust that acted like shock absorbers as blocks were moved around and they absorbed some of the shock of the collision with Antarctica by being pushed up to form mountain ranges. By about 900 million years ago all the blocks had been welded together to form the single land mass.
A mosaic of crustal fragments, with a range of ages, as well as the degree of deformation, comprise the complex ancient Australian Craton. Beneath the basins of central Australia the crust is generally between about 40-50 km, thicker than normal (Lindsay & Levin, 1996; Lambeck & Penny, 1984; Lambeck et al., 1988; Korsch et al., 1998). The craton is comprised of 8 crustal mega-elements (Shaw et al., 1996), 4 beneath the basins of central Australia - the Southern Australian, the Western Australian, Central Australian and Northern Australian Mega-elements, that developed largely in the Palaeoproterozoic, though their amalgamation into the Australian Craton continued into the Mesoproterozoic (Source 1).
This amalgamation occurred in 2 stages, both of which appear to have been associated with the assembly of supercontinents. Much of the evidence suggests Columbia had assembled by about 2.0 Ga (Hoffman, 1988, 1989, 1991; Rogers & Santosh, 2002). At about 2.1 Ga crustal accretion began in west Africa, culminating at 1.9 Ga in a series of major tectonic episodes (Abounchami et al., 1990; Boher et al., 1992; Davies, 1995). In Laurentia and Baltica, equivalent accretionary events have been identified (Gower, 1985; Gaal & Gorbatchev, 1987), rapid assembly of crustal blocks from the Archaean apparently occurring in the formation of northern Laurentia, 1.95-1.8 Ga (Hoffman, 1988). It has been suggested, based on regional data, that assembly of the supercontinent began around 2.0 Ga, with subsequent dispersal beginning at about 1.8 Ga, probably resulting from instability of the mantle (cf. Gurnis, 1988).
As part of the assembly of this putative supercontinent, the Northern Australian Mega-element, during the Barramundi Orogeny, evolved together with the Western Australian Mega-element, during the Capricorn Orogeny. Across wide areas of northern Australia the Barramundi Orogeny was a significant event (Williams, 1988; Le Messurieer et al., 1990; Needham & De Ross, 1990; Plumb et al., 1990; O'Dea et al., 1997), it is suggested to have possibly been associated with the final phase of assembly of the supercontinent from the Palaeoproterozoic. Basement rocks beneath much of northern Australia (Plumb et al., 1980) were produced during this event by crustal shortening, large-scale igneous activity (Wyborn, 1988) and low pressure metamorphism (Etheridge et al., 1988). The crust, that is at least 43-53 km thick (Collins, 1983), is believed to have evolved on earlier continental crust from the Archaean, and is thought to have possibly been thicker in the past.
Subsidence of large areas of the North Australian Mega-element began about 1.8 Ga, that is believed to have possibly resulted from mantle instability and anorogenic granite intrusion that occurred during the breakeup of the supercontinent (cf Gurnis, 1988; Idnurn & Giddings, 1988; Wyborn, 1988; Pysklywec & Mitrovica, 1998). A series of intracratonic basins, that included the McArthur Basin, Mount Isa Basin, Victoria Basin and the Kimberley Basin, that cover the North Australian Mega-element (Lindsay, 1998), resulted from this subsidence. The subsidence of these basins, that were complex polyphase structures, continued for in excess of 200 million years, preserving more than 10 km of sediment, all with similar basin-fill architectures (Lindsay & Brasier, 2000).
The Yilgarn Craton and Pilbara Craton, 2 well-defined Archaean blocks, comprise the Western Australian Mega-element. They were sutured together along the Capricorn Orogen when the North Australian Mega-element was evolving. The Pilbara and Yilgarn cratonic margins became active by about 2.3 Ga, as ocean closure was underway. It has been suggested that at this time ocean floor may have been subducted beneath the Yilgarn Craton, leading to the suturing of the 2 cratons between 2.0 and 1.8 Ga (Tyler & Thorne, 1990; Thorne & Seymour, 1991; Occhipinti et al., 1998). The convergence of the 2 cratons is recorded in a series of basins along the cratonic suture, the Yeridda, Bryah, Padbury and Earaheedy Basins. On the newly-formed Western Australian Mega-element, the Bangemall Basin developed around the time the northern Australian basins were developing on the Northern Australian Mega-element (Muhling & Brakel, 1985).
The amalgamation of the Central Australian Mega-element and the Southern Australian Mega-element is believed to have been complete by about 1.1 Ga in the Late Mesoproterozoic, later than the northern and western mega-elements (Myers et al., 1994, 1996; Camancho & Fanning, 1995; Clarke et al., 1995). During the aggregation of the supercontinent Rodinia the final amalgamation occurred. Beginning at about 1 Ga, the aggregation and fragmentation of Rodinia (McMenamin & McMenamin, 1990) has been broadly outlined by a number of authors (e.g. Bond et al., 1984; Dalziel, 1991, 1992; Li et al., 1996). The connection between Rodinia and these Australian basins has been discussed by several authors (Lindsay et al., 1987; Powell et al., 1994).
This thick older craton (Haddad et al., 2001) lies beneath Neoproterozoic basins of central Australia, apart from the north, where part of the Georgina Basin covers younger rocks, 1.7-1.5 Ga, that have been gently deformed, of the Mount Isa Basin and the McArthur Basin (Lodwick & Lindsay, 1990; Lindsay, 1998), that overlie the Northern Australian Mega-element. The Officer Basin of the Western Australian Mega-element covers sediments of the Bentley Group, of Mesoproterozoic age, and locally overlapping the Bangemall and Earaheedy Basins from the Palaeoproterozoic.
Earlier models proposed of the formation of the Australian continent during the Proterozoic suggested that the continent evolved as an essentially intact block of lithosphere. According to more recent models, Australia was formed by the assembly in the Proterozoic by tectonic processes that included horizontal motions on a large-scale. It has been found necessary to use palaeomagnetic data to determine whether the Australian continent was assembled from major cratons that were close together or more widely separated during the Proterozoic. To determine relative motions of plates during the Precambrian, Palaeomagnetic data is the only data that can provide quantitative constraints. Although there are deficiencies in the palaeomagnetic records of Australia during the Proterozoic, groups of overlapping palaeopoles for 1.8-1.7 Ga and 1.6-1.5 Ga have allowed the North Australian Cratonic assemblage and the West Australian Cratonic assemblage to be present in the same relative positions as at present since about 1.7 Ga, with the addition of the South Australian Cratonic assemblage since about 1.5 Ga. Other evidence, from geology, geochronology, as well as palaeomagnetism, is required to determine if there had been large oceans between any of the blocks that needed to close to form the continent (Source 2).
Tectonothermal events associated with the Capricorn Orogen, Late Barramundi Orogen and the Halls Creek Orogen, occurred at roughly similar times, about 1.8 Ga. Widely developed low-P/high-T conditions of metamorphism, as well as widespread magmatism with interplate geochemical signatures, and a number of other similarities occurred across the North Australian Cratonic assemblage (e.g. Etheridge et al., 1987; Wyborn, 1988; Mortimer et al., 1988a). Between about 1.8-1.6 Ga, evolution across the North Australian Cratonic assemblage (Scott et al., 2000) was contemporaneous with deformation occurring in the Gawler Craton and central Australia. In the Gawler Craton, the Kimban Orogeny, 1.74-1.7 Ga, was contemporaneous with the Late Strangways Orogenic event in the Arunta Inlier, though in the intervening Musgrave Block, no rocks have been observed (or preserved) that are more than 1.6 Ga (White et al., 1999).
Amalgamation of the North Australian Cratonic assemblage, the West Australian Cratonic assemblage and the South Australian Cratonic assemblage before 1.3 Ga is suggested by a number of geological observations. The presence of a combined West Australian and North Australian block by 1.3 Ga is suggested by orogenic events in the Albany-Fraser Belt and the Musgrave Belt at 1.3 and 1.2 Ga, though activity continued in the Miles Belt, between them, between 1.3 and 1.1 Ga. The eastern Gawler Craton, Curnamona Province, the Georgetown Inlier and the eastern Mt Isa Inlier display striking similarities in age, about 1.8-1.55 Ga, of a number of features such as basin formation, magmatism, mineralisation, deformation and alteration styles. These similarities have led to suggestions that in the Late Palaeoproterozoic-Early Mesoproterozoic all these blocks were part of a mobile belt that was regionally extensive, the Diamantina Orogen (Page & Laing, 1992; Connors & Page, 1995; Laing, 1996; Black et al., 1998; Robertson et al., 1998). The North Australian Cratonic assemblage and in the South Australian Cratonic assemblage, the Curnamona Province, were joined since at least 1.7 Ga, has been implied by this hypothesis. It has been suggested that between the Musgrave Block and the Cape River Block, there was an extension of a collisional orogen (Blewett et al., 1998). This would preclude the linking of the Mt Isa and Curnamona Province by an orogen in the Late Palaeoproterozoic with these blocks in their present relative positions.
An anticlockwise rotation of 55o has been proposed, and a slight eastward translation of the South Australian Cratonic assemblage relative to the North Australian Cratonic assemblage, based on a detailed tectonostratigraphic comparison (Giles & Betts, 2000), to improve the alignment of what may have been continuous and linear tectonic elements in the Palaeoproterozoic-Mesoproterozoic. A continuous accretionary margin would have been formed between 1.88 and 1.67 Ga by the mobile belts of the Arunta and Gawler-Curnamona Blocks in their suggested reconstruction. A suggestion has been made that between the South Australian Cratonic assemblage and the North Australian Cratonic assemblage there was a cycle of extension and breakup at about 1.45 Ga, reamalgamation occurring subsequently between 1.3 and 1.1 Ga, resulting in the present cratonic configuration (Giles & Betts, 2000). According to the authors, this reconstruction has received independent support from palaeomagnetic data to first order. see Source 2.
According to the authors, it is likely that collisional orogeny occurred by interactions between the Australian crustal blocks, as well as intraplate processes that can occur within a context of plate tectonics. They suggest there is no compelling evidence for the presence of large ocean basins that could have closed between any of the Australian blocks, at least later than 1.8 Ga.
See Source 2 for more detailed information
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