Australia: The Land Where Time Began

Mawson Continent

The term 'Mawson Continent' was first used for the Gawler Craton of southern Australia, from the Archaean-Mesoproterozoic, and Antarctic terrains that correlated with it (Fanning et al., 1996). Other names used since then are 'Mawson Block' (Oliver & Fanning, 1996; Wingate et al., 2002a; Finn et al., 2006; Mikhalsky et al., 2006) and the 'Mawson Craton' (Condie & Meyers, 1999; Fitzsimons, 2003; Bodorkos & Clarke, 2004b). The authors use 'Mawson Continent' because it has precedence and because it is not a genetic descriptor.

A result of the widespread cover of Neoproterozoic to Phanerozoic rocks in Australia, and a cover of ice and snow in Antarctica, is that the extent of the Mawson Continent is uncertain. Another block from South Australia and Western Australia, the Coompana Block, that is not exposed, is often included as part of the Mawson Continent (Condie & Meyers, 1999; Bodorkos & Clarke, 2004a). In the Transantarctic Mountains, the Miller Range and the Shackleton Range are often included in the Mawson Continent (Fanning et al., 1999; Goodge et al., 2001). It has been suggested, based on a compilation of airborne and satellite magnetic geophysical data (Finn et al., 2006), that there are fundamental differences in petrophysical properties of the crust between the Gawler Craton and the Adélie Craton, and the Miller Range, as well as the remainder of the East Antarctic Shield. Differing geological evolutions of the various terrains, and tectonism at about 1,700 Ma, have been suggested to be evidence of a single Palaeoproterozoic continent (Fanning et al., 1999; Goodge et al., 2001)

The authors include the region of the Gawler Craton, Terre Adélie Craton, the Miller Range and the Coompana Block in the Mawson Continent. During the Proterozoic and Early to Middle Phanerozoic, the Gawler Craton, Adélie Craton, and the Miller Range were presumed to have acted as a coherent crustal fragment, following the initial amalgamation at about 1,700 Ma, the Coompana Block being added at about 1,600-1,550 Ma. At the breakup of Gondwana the Mawson Continent was divided.

The Mawson Continent is comprised of the Gawler Craton in South Australia, and coastal outcrops that have been correlated with it, such as Cape Hunter and Cape Denison, of Terre Adélie and George V Land, Antarctica, as well as a number of other terrains of East Antarctica (Fig. 1, Oliver & Fanning, 1997; Goodge et al., 2001; Fitzsimons, 2003). The lack of exposure of the Mawson Continent is suggested to possibly be its most outstanding feature. The Gawler Craton has been estimated to have <5 % of basement exposed of its total area of about 530,800 km², an area that is slightly less than that of France, and the exposure in Antarctica is even less. Many models have been proposed for the evolution of the Mawson Continent, as well as its interactions with other terrains of the Proterozoic, especially with other parts of the Australian continent (Borg & DePaolo, 1994; Daley et al., 1998; Betts et al., 2002; Dawson et al., 2002; Fitzsimons, 2003; Giles et al., 2004; Betts & Giles, 2006; Wade et al., 2006).

The Authors describe the lack of evidence for magmatism associated with subduction as an intriguing aspect of the Mawson Continent geology. They have summarised the documented instances throughout Australia of subduction-related magmatism from the Late Palaeoproterozoic to the Early Mesoproterozoic:

About 1850 Ma Accretion and associated magmatism of the Kimberley Craton (Sheppard et al., 1999).
1770-1750 Ma Calcalkaline-Trondhjemite (CAT) Suite in the Arunta Region, granites that were volumetrically minor (Foden et al., 1998; Zhao & McCulloch, 1995).
1620-1600 Ma St Peter Suite, Gawler Craton (Swain et al., 2008).
1600-1550 Ma Musgrave Province magmatism (Wade et al., 2006).

In the Warumpi Province, the Arunta Region, on the southern margin, based on reconnaissance geochemical data, has been suggested to possibly represent magmatism related to subduction (Scrimgeour et al., 2005). In Laurentia and Baltica, magmatism related to subduction is often preserved in orogenic belts from the Palaeoproterozoic (eg. Gandhi et al., 2001; Therisult et al., 2001; Ketchum et al., 2002; Mueller et al., 2002; Ansdell, 2005; Whitmeyer & Karlstrom, 2007; Ahall & Connelly, 2008, and references). It is common for these orogenic belts to be quasi-linear belts, and they are often associated with inverted back-arc basins, as well as island arcs and accreted micro-continents (eg. Ketchum et al., 2002; St-Onge et al., 2006; Ahall & Connelly, 2008). Many Australian orogenic belts from the Palaeoproterozoic to the Mesoproterozoic (e.g., summary of Betts & Giles, 2006), that are commonly craton-wide events that lack these elements.

The Strangways Orogeny in the Arunta Region and Kimban Orogeny in the Gawler Craton, are examples of the complex tectonic systems that are preserved in the Mawson Continent and Australia from 1730-1690 Ma. The Calcalkaline-Trondhjemite Suite magmatism, that has been interpreted as subduction-related, preceded the Strangways Orogeny. The authors suggest this is a cornerstone in the argument for the presence on the southern margin of the North Australian Craton (NAC) of a long-lived accretionary system (e.g. Betts & Giles, 2006). From east to west the limited extent of the Strangways Orogeny is not easy to reconcile with an east-west trending margin that is accretionary.

Evidence for subduction-related magmatism is not preserved in the Kimban Orogeny, that is distributed craton-wide, from about the same time (Hand et al., 2007). In the Kimban and Strangways deformation, the (current) aggregate geometry is not easy to reconcile with the linear plate margin setting that is east-west trending.

Pseudo-linear continental margins, that are long-lived, as found in the Andean and Caledonian systems of the Phanerozoic, are not easy to reconcile with the geological record of Australia in the Proterozoic, is suggested by the style of many of the tectonic events in Australia during the Palaeoproterozoic and Mesoproterozoic (McLaren et al., 2005). The authors suggest this appears to be a fundamental difference between Australia and other continents such as Laurentia. They suggest reconstruction models of the Mawson Continent and Australia need to take the nature of Australian tectonic events into account, and be based on available geological constraints, not on a priori proposed plate tectonic models. Many of the phenomena of Proterozoic in Australia have been attributed high geothermal gradients, and predominantly high temperatures together with metamorphism of low to medium pressure, to the nature of the North Australian crust that produced high heat in the Palaeoproterozoic. Many of the issues with the existing reconstruction models of the Mawson Continent and Australia are not resolved by attributing high heat production, though it is considered likely that high heat production was involved in shaping the character of the Mawson Continent and Australia.

Mawson Continent Assembly

The Gawler Craton, Terre Adélie Craton, the Miller Range and the Shackleton Range are regions believed to be part of the Mawson Continent. Based solely on tectonism at about 1700 Ma they share some similar tectono-thermal events. The Gawler Craton and Terre Adélie Craton are exceptions to this generality, having coincident histories in the Late Archaean to Early Proterozoic. The authors suggest they can be considered to have been parts of a continuous bock of crust from the Archaean until rifting occurred in the Cretaceous, based on correlations between them of many tectono-thermal and geographical correlations, that are relatively precise (Oliver & Fanning, 1997). The Gawler-Adélie Craton has been proposed as the name of this terrain by the authors.

1,700 Ma connection of the Gawler-Adélie-Miller Range-Shackleton Range

A related, and possibly contiguous tectono-thermal event that involved both regions is suggested by the timing of the Nimrod Group metamorphism, at 1730-1723 Ma, that is almost identical with the timing of the metamorphism of the early Kimban Orogeny, 1730-1720 Ma. The authors suggest that accretion of the Miller Range terrain to the Gawler-Adélie Craton is recorded by the Kimban-Nimrod Orogeny, 1730-1690 Ma, based in the presence of relict eclogite, about 1730 Ma, and the lack of evidence for the later accretion of terrains. The suture zone accommodating this amalgamation is potentially at or near the Nimrod Group location, suggested by the presence of eclogite-facies metamorphic lithologies within this sequence.

The Shackleton Range is about 3500 km from Gawler-Adélie Craton outcrops, and about 1800 km from the Nimrod Group in the Miller Range. A temporal correlation between the 3 terrains is suggested by tectonism at about 1730-1690 Ma, that occurred in all 3 terrains, based on the geochronology of Zeh et al. (2004). A Palaeoproterozoic age for source regions of granite along the Transantarctic Mountains between Victoria Land and the Beardmore Glacier, T_DM = 2.2-1.6 Ga, as indicated by Nd-isotope date of Borg & DePaolo (1994). Beyond the Beardmore Glacier dating indicates Mesoproterozoic model ages. Fitzsimons (2003) based his suggestion of 3 possible paths for the Pinjarra Orogen, about 550-500 Ma, on this change in crustal evolution. According to Path 3, the Transantarctic Mountains are bisected between the Miller Range and the Shackleton Range, the Shackleton Range being considered to have been added to the proto-East Antarctic Shield in the Neoproterozoic or Cambrian. The authors exclude the Shackleton Range from the Mawson Continent because of the high level of uncertainty regarding its age, though they suggest it may have been part of the continent.

See Source 1 for more detailed information.

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