Australia: The Land Where Time Began

A biography of the Australian continent 

This was a supercontinent that is believed to have existed between about 1100 and 750 Ma, when all the continental blocks of the earth were joined together in one massive landmass. There is little definite evidence for the existence of this supercontinent, but some evidence suggests its existence. It is believed by some that the global cooling, often referred to as Snowball Earth or the Cryogenian Period, that occurred about 700 Ma may have been a result of the breakup of Rodinia that began about 750 Ma.

According Peter Cawood, the rifting that broke up Rodinia formed the Pacific Ocean, that never again closed, though since that time there have been periods of plate convergence. The Terra Australis Orogen, from the Neoproterozoic to late Palaeozoic, records the beginning of rifting when the ocean opened and the initiation of subduction associated with ocean closure. Prior to dispersal the orogen was about 18,000 km long and about 1600 km wide, and incorporated in it were the Tasman Orogen of Australia, the Ross Orogen of Antarctica, the Tuhua Orogen of New Zealand and the Andean Cordillera of South America. A series of basement blocks, of either continental or oceanic origin comprise the Terra Australis Orogen, and these blocks can be sub-divided into their geographic affinity to either Laurentia or Gondwana before the initiation of the orogen, and their proximity to the inferred sequences of continental margin, either peri-Gondwanan or intra-oceanic. The tectonic settings before the Orogen is indicated by these divisions, as well as insight into the orogen over time. Elements are inferred to have been outboard of both West and East Laurentia, within Rodinia, are incorporated in the Terra Australis Orogen.

About 570 Ma subduction of the Pacific Ocean began at or near the margin of Gondwana, and at about this time a major reorganisation of plates occurred that was associated with the final assembly of Gondwana and the Iapetus Ocean opened. About 300-250 Ma the Terra Australis Orogen ended, the termination being associated the assembly of Pangaea.

'It is represented by the pan-Pacific Gondwanide Orogeny and is marked by a stepping out in the position of the plate boundary and commencement of the late Palaeozoic to Mesozoic Gondwanide Orogen. The Pacific has been cited as the declining stage of the Wilson cycle of ocean basins. However, its protracted history of ongoing subduction contrasts with the clear evidence of opening and closing of oceans preserved in the Iapetus/Atlantic and Tethyan realms. The Terra Australis Orogen and other orogens that bound the Pacific are accretionary orogens and did not form through the classic Wilson cycle of ocean closure and continental collisions' Peter Cawood.

The life cycle of the 4 main ocean basins and their margins, the Mirovoi, Mozambique,  Pacific and Iapetus Oceans, record the breakup of the supercontinent of Rodinia in the end Mezoproterozoic and its transformation into the Gondwana supercontinent, that existed from the end Neoproterozoic to Palaeozoic.

Rodinia is believed to have been surrounded by a single, pan-Rodinian Mirovoi Ocean at the end of the Mesoproterozoic (Hoffman, 1991; McMenamin, 1990, Meert and Powell, 2001). The opening of the Pacific Ocean along the western margin of Laurentia and of the Iapetus Ocean along the eastern margin, and the closing of the remnants of the Mirovoi Ocean, termed in part the Mozambique Ocean (Dalziel, 1991; 1997), leading to the amalgamation of Gondwana by the close of the Neoproterozoic (Collins & Winfley, 2002). These were all the result of the breakout of Laurentia from the core of Rodinia.

West and East Gondwana and Baltica are examples of major cratonic blocks that broke from Rodinia, they were in turn broken up by the opening of oceans, Braziliano Ocean, Adamastor Ocean and Tornquist's Sea, all of which ultimately closed.

Neoproterozoic rifting of Rodinia formed the Pacific and Iapetus Oceans, the Iapetus Ocean providing the type example of the Wilson cycle - ocean closure and continental collision via ocean closure (Wilson, 1966), leading to the formation of the Appalachian-Caledonian Orogen. The Pacific Ocean has not closed since its initiation, in spite of ongoing cycles of plate convergence, with associated formation of accretionary orogens on its boundaries.

It has been bounded by the continental margins of West Laurentia and East Gondwana throughout its history (Bell and Jefferson, 1987, Dalziel, 1991; Hoffmann, 1991; Moores, 1991). They provide a good record of the development of the ocean from the Neoproterozoic to the Recent, though the original relationship between the continental masses is uncertain (Borg & De Paolo, 1991; Burrnett & Berry, 2000; Karlstrom et al., 2001; Moores, 1991; Wingate et al., 2002).

A continuous orogenic belt of Grenville age is suggested to have connected Australia, Antarctica and Laurentia in a number of proposed reconstructions of Rodinia, such as AUSWUS (Karlstrom et al., 2001) and SWEAT (Moores, 1991). These proposed reconstructions suggest stitching points between the Albany Fraser Province and the Bunger Hills of Antarctica and the Musgrave Province and western Laurentia. Connectivity is well established between the Albany Fraser Province and the Bunger Hills in Antarctica (Duebendorfer, 2002), but the link between the Musgrave Province and western Laurentia is less certain. According to SWEAT, the Musgrave Province aligns with the Wopmay Province of northern Canada (Moores, 1991). Also according to AUSWUS, the Musgrave Province connects to the Grenville Belt via the Mexican Oaxaca Terrane (Karlstrom et al., 2001). The metamorphic and magmatic evolutions of these proposed stitching points differ significantly from that of the Musgrave Province (c.f. Laughton et al., 2005; Solari et al., 2003; White et al., 1999).

According to Aitken & Betts (2008), it is indicated by palaeomagnetic studies that neither AUWUS nor SWEAT are viable, suggesting that Australia was moving independently of North America about 1200 Ma, possibly being separated by an ocean (Pisarevsky et al., 2003). They suggest that a palaeomagnetically viable configuration about 1070 Ma as proposed by the AUSMEX (Wingate et al., 2002), in which the present-day Mexico connects Australia to the Grenville Belt.

Rodinia and glaciation

The authors² suggest a possible causal link between a 90^o rotation of Rodinia in the Neoproterozoic, a superplume, true polar wander and glaciation at low latitudes.

They base their proposal on geochronological and palaeomagnetic data of the Xiaofeng dykes in South China, dated to 810 ± 10 Ma. As well as existing data, these results suggest that at about 800 Ma Rodinia probably extended from the equator to the pole, than after about 800 Ma it rapidly rotated 90^o around an axis located near Greenland, and as a result of this rotation the entire supercontinent was in low latitudes by about 750 Ma. Their proposal is that it was the initiation of a mantle superplume that formed beneath the polar end of the supercontinent triggered an episode of true polar wander (TPW) bringing all of Rodinia into equatorial latitudes. The unusually large area of land above sea level at the equator is then proposed to have increased the drawdown of CO₂ and global albedo, as well as the waning of the plume volcanism, leading directly to the low latitude Sturtian Glaciation at about 750-720 Ma.

Sources & Further reading

1. Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic
2. A 90^o spin on Rodinia: possible causal links between the Neoproterozoic supercontinent, superplume, true polar wander and low latitude glaciation

Links

1. Rodinia - Images
2. Wikipedia
3. High-resolution aeromagnetic data over central Australia assist Grenville-era (1300-1100 Ma) Rodinia reconstructions