Australia: The Land Where Time Began
Supercontinent Cycle &
Icehouse to Greenhouse Cycle
Supercontinents & Superplume Events
A cycle has been proposed that states that the continents come together in a supercontinent, fragment into individual continents, then reform as another supercontinent, only to break up again, on a scale of about 400 million years, (White, 1994). Knowledge of times earlier than about 1.2 billion years ago is so meagre that it can only be seen occurring since that time. The number of times supercontinents formed and fragmented prior to that time can be speculated about, but it could take some time for the evidence to test the theory prior to 1.2 billion years ago to be found, if it still exists.
It has been observed that there are at least 35 cratons of Archean age known at the present (Bleeker, 2003), most of which display rifted margins, an indication that they probably rifted from larger landmasses during the Late Archean-Early Proterozoic. Several possible scenarios have been suggested to explain the global distribution of these cratons. Among these suggestions is one called Kenorland, a single supercontinent (Williams et al,. 1991), named after an orogenic event in the Superior Province of Canada, as well as a few to many possible independent aggregations referred to as supercratons. It was concluded that the degree of geologic similarity among these cratons indicates the presence of a number of supercratons, that were probably transient and more or less independent, or many small landmasses, rather than a single supercontinent, that were dispersed (Bleeker, 2003). According to his definition there were at least 3 supercratons -Sclavia, Superia and Vaalhara, each displaying distinct histories of amalgamation and breakup. At about 2.6 Ga it is thought the Sclavia supercraton stabilised. Detailed chronostratigraphic profiles are required of all 35 Archean cratons before this can be confirmed.
As defined by Bleeker (2003), during the period 2.5-2.0 Ga was the time of dichronous breakup of the defined supercratons into the 35 or more cratons that drifted independently. Palaeomagnetic evidence is claimed to support the proposal of significant differences in palaeolatitudes between at least several fragments in the Early Proterozoic (Cawood et al., 2006). These cratons appear to have amalgamated into several supercontinents following the breakup of the cratons. A Middle Proterozoic supercontinent, Nuna, has been proposed (Hoffman, 1997). This was considered to be the first true supercontinent by Bleeker (2003). It has also been recognised that orogenic events dating to 2.1-1.8 Ga appear to be indicated by evidence from most continents (Zhao et al., 2002). The collisional assembly of Columbia, a supercontinent from the Early-Middle Proterozoic, is proposed by them. Speculative evidence suggests the possibility of at least 1 supercontinent before Rodinia, following the stabilisation of the Archean cratons during the Late Archean.
It is believed, at least by some, that a very large landmass, such as a supercontinent like Pangaea, has an insulating effect on the heat being transported towards the surface from the molten core in the form of convection cells. The heat is trapped in the molten zone beneath the crust, which prevents it from reaching the surface, and the crust rises. The Earth is then in a geocratic state, in which the surface is dominated by land. The single ocean that surrounded Pangaea has been called Panthalassa. At this time the sealevel is low compared to that of the continent.
As the heat is rising beneath the supercontinent, but before it reaches a high enough level to trigger the breakup, the land surface is in the grip of a continent-wide glaciation, the snowball Earth condition or Icehouse condition. This condition is believed to have continued for some time after the beginning of the stretching, as the heat accumulated beneath the continent rises sufficiently to crack the crust along the lines of the future spreading. Once the magma begins to reach the surface at the spreading zones, ocean floor is continually produced that pushes the continental blocks apart, large amounts of CO2 are released into the atmosphere which leads to rising atmospheric temperatures towards greenhouse climatic regimes. As the blocks are separated, the crust thins and sags and this allows heat to escape to the atmosphere, and as oceans open between the continental blocks, still more heat is lost from the mantle. As the temperature drops the spreading of the seafloor slows, eventually stopping and begins being consumed by subduction as the blocks are pushed back together again. When the oceans have closed, the next stage of the supercontinent cycle is reached, a new supercontinent comprising most if not all the continental blocks of the Earth.
As the spreading zones form, basins are produced by the sagging of the thinning crust. These become depocentres for erosion products from the continental rocks, forming thick sedimentary deposits. The production of volcanic rocks along rift lines has been found to coincide with a 400 million year cycle. According to the suggestion of the 400 million year cycles, a Wilson cycle, once a supercontinent forms it takes about 150 million years for sufficient heat to accumulate to tear the continental crust apart where the convection cells in the mantle reach the surface. The next stage, the spreading of the seafloor and its subsequent consumption by subduction, is believed to take about 350 million years, thus totalling about 400 million years.
The latest global glaciation is believed to have occurred in the Late Carboniferous-Early Permian, about 320 million years ago. According to the 400 million year cycle suggestion the next global glaciation is due in about 80 million years time.
Cawood has presented evidence for the existence of Rodinia, (see Rodinia and Terra Australis Orogen) and Guochun Zhao et al. have presented evidence for the existence of Columbia, the name proposed by Rogers & Santosh (2002) for a supercontinent that predated Rodinia.
Geological evidence that Gondwana and Larurtentia were attached, or at least in close proximity, at the end of the Late Proterozoic, is the main basis for models suggesting the existence of Pannotia (Dalziel, 1997). There has also been an alternative configuration proposed that has Laurentia and Gondwana being separated at this time, based on the palaeomagnetic poles for these landmasses that do not overlap (Meert & Torsvik, 2003).
It is suggested by most models that Pannotia began breaking up with the opening of the Iapetus Ocean, when Laurentia rifted from South America and Baltica. Along the margins of Gondwana and Laurentia, subduction zones formed on the edge of the Iapetus Ocean, resulting in the formation of a series of volcanic arcs, with associated backarc basins as well as rifted terranes, a complex assemblage of which accreted to the margins of Gondwana and Laurentia. Prior to the assembly of Pangaea in the Permo-Carboniferous, some control of the relative longitudes and palaeogeography of Gondwana and Laurentia is provided by the provenance of these terranes (Dalziel, 1997).
The terranes that accreted to Gondwana and Laurentia during the Early Palaeozoic have been separated into native and exotic groups, whether or not they formed adjacent to the cratons they accreted to (Keppie & Ramos, 1999; Cawood, 2005). The Notre Dame-Shelburne Falls (Taconic) and Lough-Nafooey volcanic arcs, native to Laurentia, accreting to it during the Early-Middle Ordovician. These collisions were associated with the Taconic Orogeny in the Appalachians (Karabinos et al., 1998), and in Britain, the Grampian Orogeny, as well as the Finnmarkian Orogeny of Scandinavia. At this time, near the western margin of South America, the Gondwanan Famatina arc terrane formed, accreting onto South America (Conti et al., 1996).
Avalonia, Meguma, Carolina and Cadomia are all terranes exotic to Laurentia, having rifted from northwestern Gondwana during the Early Ordovician, subsequently accreting to the margin of Laurentia, to become part of the Acadian and Salinic orogens in the northern Appalachians and the Caledonides of Baltica and Greenland during the Silurian-Devonian. An exotic terrane that is presently located in Argentina, Cuyania, was rifted from Laurentia during the Early Cambrian, accreting to the margin of Gondwana (Dalziel, 1997). It has been suggested that during the Palaeozoic at least 2 different plate regimes existed in the eastern and western Iapetus, subduction zones forming along parts of Gondwana and Laurentia, based on these tectonic exchanges. The piecemeal manner of growth of both continents before the formation of Pangaea can be explained by the interpretation of distinctive plate regimes, though the actual geometry of plate boundaries is presently highly speculative.
Mechanisms involved in supercontinent formation
According to Murphy, Nance & Cawood, the evidence is increasing that the continental crust of the Earth has repeatedly undergone amalgamation into supercontinents followed by their breakup since the end of the Archean, many researchers concluding there is a cycle of supercontinent formation and dispersal (Worsley et al., 1984; Nance et al., 1986), though the mechanisms involved are not yet certain. There are 2 types of ocean that form as supercontinents are breaking up, that are geodynamically distinct, interior oceans that form between the dispersing former components of the supercontinent, beneath which the lithosphere is younger than the time of breakup, and exterior oceans, in which the associated lithosphere is older than the breakup, as these oceans were present around the margins of the former supercontinent.
To be able to evaluate the models of supercontinent formation it is necessary to determine which of the 2 forms of ocean is consumed during amalgamation of the component blocks of the supercontinent. It is necessary to study mafic complexes that have accreted to the margins of continents before the collision of the continents, as much of the evidence is destroyed by the process of subduction. Sm-Nd isotope systematics is used to determine the age constraints of the mantle lithospheric sources from which the accreted complexes were derived. In the termination of the formation of the supercontinent Pannotia in the Late Neoproterozoic, the mafic terranes accreted in orogens during the termination have been dated to 1.7-0.71 Ga by Sm-Nd Tdm (depleted mantel) model ages. The implication of these ages is that much of the subducted and recycled lithosphere that formed these complexes was formed earlier than about 755 Ma, the breakup time of the former supercontinent, which was Rodinia, Tdm>Tr. This indicates that the lithosphere of the mafic complexes formed beneath the peri-Mirovoi Ocean that surrounded Rodinia, thus Pannotia was formed by the closure of an exterior ocean (extroversion).
The origin and evolution of the oceans of the Palaeozoic, Iapetus, Rheic and Paleotethys, between Laurentia (ancestral North America), Baltica (Northern Europe) and Gondwana (South America-West Africa), the closing of which formed Pangaea, holds the primary record of the formation of the supercontinent. These oceans formed in the Palaeozoic, after about 550 Ma, following the breakup of Pannotia. Samples of uncontaminated mafic rocks from both oceans have epsilonnd values close to depleted mantel values at a time close to their respective emplacement times. The crystallisation and depleted mantle model ages from the both sites are similar, not exceeding the rifting age (Tdm=/<Tr), indicating that the oceanic lithosphere sources of these suites were both generated following the time of rifting of Pannotia, with the result that Pangaea formed as the result of interior ocean closure (introversion).
This suggests that Pannotia and Pangaea were formed by distinctly different geodynamic processes. There has been a debate over the mechanisms of plate tectonics, either the top-down model, in which the cooling of the plates leads to their subduction, and this drags the plates across the ocean, stimulating the convection of the mantle. The other suggested mechanism is based on the convection cells in the mantle, the plate motions being a surface manifestation of the mantle convection (Zhong & Gurnis, 1997; Anderson, 2001). The formation of Pannotia is believed to be broadly consistent with the top-down geodynamic models, but the formation of Pangaea, the better documented of the 2, is not consistent with such top-down models.
About 550 Ma, as the Iapetus Ocean formed, separating Baltica, Laurentia and Gondwana, at the leading edges of the dispersing continents subduction zones were already well established, such as the 18,000 km long Terra Australis Orogen, which preserved a continuous record of subduction from 580 to 230 Ma (e.g. Cawood, 2005; Cawood & Buchan, 2007). The conventional top-down models suggest that the force of the slab being subducted should pull the dispersing continents to the subduction zones, eventually resulting in an extroverted supercontinent.
Instead of this happening, the initiation of subduction in the relatively newly formed Iapetus and Rheic Oceans, that separated Baltica, Laurentia and Gondwana, the motion of the dispersing continents reversed. At present, the oldest, coldest and negatively buoyant oceanic lithosphere is selectively removed by subduction. Such selectivity is generally believed to be fundamental to the operation of plate tectonics, as well as being applicable to plate tectonics operating at earlier periods of Earth history. It has been found that this scenario apparently didn't apply in the formation of Pangaea. About 90 Ma the Rheic Ocean opened, and was soon followed by the initiation of subduction in the Iapetus Ocean with the associated convergence of Laurentia and Baltica. Subduction began in the Rheic ocean about 460 Ma, leading to the convergence of Gondwana with Laurentia and Baltica, ultimately forming Pangaea. Associated with the assembly of Pangaea, subduction was initiated of the relatively young lithosphere in the new Palaeozoic oceans, that reached such a rate that it overcame the pull of the previously well established subduction zones of the exterior ocean.
According to Murphy, Nance and Cawood:
'The mechanisms responsible for the formation of Pangaea are enigmatic. To a first order, we know where and when, but not why. At the heart of this debate is a lack of understanding of the forces that initiate the subduction process. Likewise, the documented evolution of Pangaea highlights fundamental gaps in our understanding of the processes responsible for its amalgamation. To understand the processes leading to the formation of Pangaea, we need to investigate the geodynamic linkages between the evolution of the interior Rheic Ocean and the penecontemporaneous evolution of the exterior Paleopacific ocean.' See source 2 below.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|