Australia: The Land Where Time Began
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 90o 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 90o 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.
Rodinia connections with Australia and America³
According to the authors3 the basis for various reconstructions of Rodinia during the Neoproterozoic has been comparisons between Australia and North America, though there has been a lack of evidence for this from palaeomagnetic poles. The authors report palaeomagnetic data and U-Pb ages from 2 mafic sill suites from within the intracratonic Bangemall Basin, Western Australia, 1 dated to 1070 ± 6 Ma and that carries a high stability palaeomagnetic remanence. The authors say that comparison of the Bangemall palaeopole with data from Laurentia suggests that at 1070 Ma previous reconstructions of eastern Australia against Canada (SWEAT) or the western United States (AUSWUS) are not viable. The implication of this is that the separation of Antarctica-Australia from Laurentia did not involve the formation of the Pacific Ocean, and that within any Rodinia supercontinent that is proposed it is necessary to have up to 10,000 km of passive margins, of Late Neoproterozoic age, to match with other continental blocks within the Rodinia supercontinent. AUSMEX, a reconstruction that closely aligns orogenic belts of Mesoproterozoic age in northeast Australia and the southernmost margin of Laurentia is permitted by their results. Terra Nova, 14, 121-128, 2002
About 1.3-1.0 Ga during the Mesoproterozoic most of the continental crustal blocks of the earth have been proposed to have fused together to for the supercontinent of Rodinia (Hoffman, 1991). The authors3 suggest it is essential to have knowledge of the configuration of this supercontinent to understand how it assembled and its eventual disintegration about 0.8-0.55 Ga during the Neoproterozoic, and the breakup having been associated with the extreme fluctuations of environmental and biochemical conditions and the explosive evolution of metazoan life that occurred at this time (Valentine & Moores, 1970; Hoffman et al., 1998; Karlstrom et al., 2000). According to the authors3 there is not much consensus among workers in the field with regard to the relative positions taken by the fragments that gathered together to form Rodinia. They say that according to most reconstructions Australia, East Antarctica and India were adjacent to either western Canada, the SWEAT hypothesis (e.g. Moores, 1991; Dalziel, 1991; Hoffman, 1991; Powell et al., 1993) or the western part of the United States, the AUSWUS hypothesis (Brookfield, 1993; Karlstrom et al., 1999; Burrett & Berry, 2000). These models are based on geological and tectonic features that are matching and age provinces, the so-called 'piercing points'. It is difficult to discriminate between the AUSWUS and SWEAT alternatives as the available palaeomagnetic data are presently inadequate. In this report the authors3 describe a study using integrated geochronological and palaeomagnetic data, carried out on diorite (diabase) sills, of Mesoproterozoic age, in the Bangemall Basin, Western Australia. The result was a palaeopole, BBS, that was precisely dated at 1070 ± 6 Ma. In this study they compared the BBS pole with poles in Laurentia of Mesoproterozoic age, and explored the implication for reconstructions between the 2 continents.
Bangemall Basin geology
In the Bangemall Basin, that is of Mesoproterozoic age, there are more than 6 kilometres of unmetamorphosed, fine-grained carbonate and siliciclastic marine sedimentary rocks, the Bangemall Subgroup (Muhling & Brakel, 1985, Martin et al., 1999a). The basin developed on the site of the Capricorn Orogen, which had been dated to 1.83-1.78 Ga, the orogen forming during the collision of the Pilbara and Yilgarn Cratons (Tyler & Thorne, 1990; Sheppard & Occhipinti, 2000). The lower Edmund Group and the overlying Collier Group comprise the Bangemall Supergroup (Martin et al., 1999a). According to the authors3 there is about 4 km of stromatolitic dolomite and clastic sediments that are fine-grained, overlying unconformably the deformed rocks of Palaeoproterozoic age of the Ashburton and Bresnahan Basins in the north, and in the southwest, they overly the Gascoyne Complex, that is igneous and metamorphic. In the Collier Group are about 3 km of siltstone and sandstone that overly unconformably, and in the west occupies a regional synclinorium extending along the basin axis. Throughout the Bangemall succession there are extensive quartz dolerite sills, that have been geochemically classified as high-Ti continental tholeiites (Muhling & Brakel, 1985) that are mainly concordant with bedding, though locally the stratigraphy is transgressed by them. The authors3 say sills are typically of more than 100 m in thickness, are mainly medium-grained, with chilled margins and coarse-grained phases that are both locally exposed.
The northern Bangemall Basin is relatively undeformed where it overlies the Ashburton Basin, which was a stable shelf during deposition. The southern parts of the basin were compressed northwards against the Ashburton shelf following the intrusion of dolerite sills, the result being an arcuate region, the Edmund Fold Belt, consisting of elongate, tight to open folds (Muhling & Brakel, 1985). The authors3 suggest that folding may be related to tectonothermal events occurring from 1090-1060 Ma (Bruguier et al., 1999) that occurred in the Darling Mobile Belt that is adjacent, and certainly occurred before the intrusion of dolerite dykes that were N- to NE-trending, of the Mundine Wells swarm, 755 Ma (Wingate & Giddings, 2000), essentially undeformed and cut across all fabrics and rocks.
An older limit for the Bangemall Basin sedimentation is provided by
SHRIMP (sensitive high-resolution ion microprobe) zircon ages of 1679
An older limit for the Bangemall Basin sedimentation is provided by SHRIMP (sensitive high-resolution ion microprobe) zircon ages of 1679± 6 Ma and 1619 ± 15 Ma that have been found for underlying intrusions (Pearson et al., 1995; Nelson, 1998). A maximum age for eruption of 1638 ± 14 Ma has been found by SHRIMP U-Pb analyses of xenocrystic zircons that were obtained from altered rhyolite near the base of the Edmund Group (Nelson, 1995). Ages have been produced of between 1050 and 1075 Ma by several K-Ar and Rb-Sr studies of dolerites and baked sedimentary rocks (Compston & Arriens, 1968; Gee et al., 1976; Goode & Hall, 1981). Dolerite sills were indicated to have been emplaced in 2 distinct events by SHRIMP U-Pb results of the authors3. All the baddeleyite and zircon 207Pb/206Pb ratios for 3 sites, 1, 7 and 10, agree to within analytical precision, yielding statistically identical ages of 1071 ± 8 Ma , 1067 ± 14 Ma and 1068 ± 22 Ma. A mean age of 1070 ± 6 Ma results from the combination of the 3 ages (95 % confidence interval), the age the authors regard as the time of crystallisation of the younger sill suite. A mean age for the older sills of 1465 ± 3 Ma was obtained from baddeleyite and zircon from 2 sites, 11 and 21).
Throughout the western section of the Bangemall Basin samples of dolerite sills and sedimentary rocks were of collected and low-coercivity overprints were removed by alternating field AF demagnetisation to 10 or 20 mT and 2 main types of magnetic behaviour were observed. A remanance that was directed inconsistently of low thermal stability (referred to as type L) was detected in the majority of samples, the authors3 interpreting them as chemical remanant magnetisation (CRM) that was mainly carried by maghemite. In 79 samples from 15 sites, including 3 that had been dated to about 1070 Ma, they isolated a magnetisation that was directed consistently, that is the only stable remanance present at 5 sites. The remanance was shown by unblocking temperatures between 500oC and 580oC to be single component and the dominant carrier was magnetite that was relatively pure. The authors3 demonstrated that most specimens were stable to AF treatment to 100-160 mT, variable proportions of MD and SD grains being indicated by decay curves. A magnetisations, that had directions similar to that of adjacent dolerite, was obtained from shale at 5 sites. Following correction for the tilt of the bedding the site mean directions converge, the concentration parameter, k, increasing from 6-30. With the exception of site 25, where the direction of SSE and upward was detected, corrected directions are NNW with a downwards inclination that is moderate. Following tectonic correction, and inversion of data from site 25, the mean direction is D, I=339.3o, 46.5o (α95 = 8.4o, N = 11 sites).
A primary origin for A magnetisation is indicated by several lines of evidence.
An undated NNE-trending dyke that was found to be carrying the Bangemall A direction has been found to yield a baked-contact test within its rock, the 2.45 Ga Woongarra Rhyolite, though stable resonance was isolated in unbaked rocks. The authors3 suggest that the dyke is of a similar age to that of the sills, and possibly comagmatic, and that the A magnetisation of the sills is also original. The authors3 concluded that the A component is a primary TRM that was acquired by the sill as it was emplaced 1070 Ma. At site 25 the implication of directions of opposite polarity is that the intrusive event spanned at least 1 reversal of the Earth's magnetic field, and the A magnetisations collectively adequately average palaeosecular variation
In the Glenayle area of the eastern Bangemall Basin similar palaeomagnetic directions obtained from undeformed sills, together with the lack of significant deformation of the Mundine Well Dyke swarm are an indication that there has been no internal vertical axis rotation after 1070 Ma in the Bangemall Basin. This palaeomagnetic pole, BBS, is situated at 33°N, 95°E (α95 = 8.3°).
Compared with earlier results
Palaeopoles for Australia of Mesoproterozoic age were previously obtained from the Stuart Dykes and Kulgera Sills in central Australia (Idnurm & Giddings, 1988; Camacho et al., 1991). Sm-Nd and Rb-Sr isochron ages of 1076 ± 33 Ma and 897 ± 9 Ma respectively were obtained from the Stuart Dykes (Black et al., 1980; Zhao & McCulloch, 1993). In the southern Arunta Block, which had been deformed strongly and uplifted during the Alice Springs Orogeny of the Carboniferous, the predominantly N-trending dolerite dykes, that have since been locally sheared and altered, intruded into basement granitoids of Palaeoproterozoic age (Collins & Shaw, 1995). As no data or analytical details have been published that authors3 say the reliability of the preliminary SDS is difficult to assess. Sm-Nd and Rb-Sr isochron ages of 1090 ± 32 Ma and 1054 ± 14 Ma respectively were yielded by the shallowly dipping S- to SE-dipping Kulgera Sills (Camacho et al., 1991; Zhao & McCulloch, 1993). In the eastern Musgrave Block the Kulgera Sills were intruded into gneisses of Mesoproterozoic age, the Musgrave Block subsequently underwent tectonothermal events, Petermann Ranges in the Neoproterozoic and Alice Springs in the Carboniferous. Reliable constraints on palaeohorizontal are not available for the Kulgera or Stuart intrusions (no tectonic corrections were applied), and the authors3 say the crustal blocks where both suites were located were deformed and probably re-orientated following emplacement of the dykes. The palaeopoles of the Stuart and Kulgera intrusions are of similar ages, as suggested by isotopic data, their palaeopoles are significantly different. The BBS palaeopole is more reliable than either the KDS or SDS poles, though it does not agree with them. According to the authors3 The BBS pole is inferred to be primary, is precisely dated, and in adjacent sedimentary rocks structural control is well defined. The BBS pole has a perfect score of Q = 7 in Van der Voo's reliability scheme (1990).
Implications for reconstructions of Rodinia
The fit between Australian and Laurentian poles at about 1070 Ma and 750-700 Ma was optimised to constrain the SWEAT reconstruction. (Powell et al., 1993). The authors3 suggest the previous 1070 Ma poles for Australia are unreliable, as described above, and they also suggest the supposedly 700-750 Ma YB dykes pole for Australia (Giddings, 1976) could possibly represent a younger overprint, possibly of Mesozoic age (Halls & Wingate, 2001). The matching of Australian and Laurentian poles between about 1.75 Ga and 0.75 Ga was used as palaeomagnetic support for the AUSWUS reconstruction (Karlstrom et al., 1999; Burrett & Berry, 2000). Most Australian data from the Mesoproterozoic are of low reliability or are inadequately dated. The Mundine Well dykes of Australia that are precisely dated doesn't allow either the SWEAT or AUSWUS at 755 Ma (Wingate & Giddings, 2000), though by itself it allows either of the 2 previous reconstructions to be valid at an earlier time. The authors3 say a direct test of proposed fits is allowed by their new BBS palaeopole, prior to any plausible age for the fragmentation of Rodinia (Hoffman, 1991). The trend of the Laurentian APW path between 1100 Ma and about 1020 Ma is well defined, though at 1070 there is no pole for Laurentia that is well dated. About 30° separates the BBS pole from the Laurentian path in the SWEAT fit and from the AUSWUS fit by at least 40°. According to the authors3 there is no part of the Laurentian path to which the BBS pole can be matched in a fit similar to the SWEAT or AUSWUS fit, indicating that neither reconstruction is viable at 1070 Ma.
By assuming a constant APW between 1087 Ma and about 1050 Ma poles the authors3 approximated a pole position for Laurentia at 1070 Ma to explore possible reconstructions between Australia and Laurentia at 1070 Ma. When they superimposed the BBS palaeopole on the interpolated pole position for Laurentia at the projection axis the continents were at their correct orientations and palaeolatitudes at 1070 Ma. Situated at lower palaeolatitudes than permitted by either the AUSWUS of SWEAT models, the Cape River Province of northeastern Australia is placed at a similar latitude as the southwest end of the Grenville Province of Laurentia at about 1250-980 Ma (Rivers, 1997). In the Cape River Province there are high grade metamorphic and magmatic rocks containing zircon age components of 1240, 1145 and 1105 Ma, and the authors3 suggest they may correlate with rocks of 'Grenville age' in the Musgrave Orogen and the Albany-Fraser Orogen (Blewett et al., 1998). If this is the case the Grenville Province may have continued through Australia. The authors3 suggest the study of older palaeopoles from each block could possibly indicate the time when Australia came into contact with Laurentia. In Fig.5 the tight reconstruction permits the α95 confidence circles of the 1140 Ma IAR and AB poles to overlap, though these poles are not sufficiently precise for a rigorous test. The authors3 suggest it is possible that Australia and Laurentia were not connected at 1070 Ma as a result of the lack of palaeolongitude control. They also suggest that there is the possibility that fits similar to SWEAT and AUSWUS could have been achieved by a collision between a unified Australia-Mawson Craton and some part of the proto-Cordilleran Laurentian margin at some time after 1070 Ma because final 'Grenvillian' assembly of Rodinia could conceivably have occurred after 1070 Ma. They suggest that if this was the case it would appear that a intermediary craton was required to carry the main record of any such collision (e.g., South China; Li et al., 2001), though there is as yet no known evidence of an intermediary craton in either eastern Australia or western North America. Similarities between rocks from the Palaeoproterozoic and Mesoproterozoic in Australia and North America would be fortuitous, removing some of the very foundations for the SWEAT and AUSWUS models.
The authors3 suggest that AUSMEX (Australia-Mexico) fit requires more testing such as the comparison of additional palaeopoles of Palaeoproterozoic age of precisely the same age from both Australia and Laurentia, any subsequent reconstructions needing to be elaborated by the use of geological and other constraints. In the AUSMEX reconstruction the most compelling geological arguments used in the generation of the AUSWUS and SWEAT hypotheses, that includes orogenic belts, Palaeoproterozoic and Mesoproterozoic isotopic age provinces, and rift - passive margin sedimentary successions remain robust. The opening of the Pacific Ocean by the separation of Australia-Antarctica from Laurentia in the Neoproterozoic is implied by the SWEAT and AUSWUS models. However, the authors3 say the results of their study suggest that the conjugate margin to eastern Australia-Antarctica was not Laurentia, the origin of the Pacific therefore being undermined. They suggest that in eastern Australia and western Laurentia up to 10,000 km of passive margins from the Late Neoproterozoic are required to be matched with other continental blocks within any proposed Rodinia supercontinent.
Rodinia reconstructions for Grenville Era(1300-1100 Ma)
Province of Australia, that is polydeformed and poorly exposed, has
been studied using high-resolution aeromagnetic data, the results
revealing the Grenville-aged architecture. According to the authors
4magnetic anomalies relating to an orogenic event in the Late Neoproterozoic were filtered out by a combination of upward continuation and the potential field tilt, emphasising the more subtle magnetic structural grain that formed during the Musgrave Orogeny about 1320-1100 Ma. The images that resulted indicated the distribution of magnetic granitoids, dated to about 1150 Ma, within basement that was less magnetic and dominantly northeast-trending, and are continuous beneath the Amadeus Basin and the Officer Basin, defines the crustal architecture, and defines an orogenic belt that connects provinces of Mesoproterozoic age. The truncation of the orogenic belt within Australia precludes an east-trending direct connection between the Musgrave Province and the orogens in Laurentia that are contemporaneous. The authors4 say their preferred model to explain the architecture of Grenville age they observed in Australia involves a clockwise rotation of the South Australian Craton, with subsequent collision with the North Australian Craton and the West Australian Craton.
According to the authors4 it is typical with palaeocontinental reconstructions to rely on a combination of palaeomagnetic data such as (Meert & Torsvik, 2003; Pisarevsky et al., 2003), and continental stitching points that are correlated, such as orogenic belts (Karlstrom et al., 2001), and blocks of crust (Burrett & Berry, 2000). According to the authors4 there is a failure of these methods to asses unambiguously the connection between stitching points, their relationships often being underpinned by geological inferences that are indirect and non-unique. The construction of the proposed continent Rodinia therefore involves much conjecture. A new way to constrain the architecture and connectivity of the fragments comprising Rodinia is available as a result of the development of merged magnetic data sets on a continental scale and the increasing availability of high-resolution aeromagnetic data on a regional basis.
A continuous orogenic belt of Grenville age that connected Australia with Antarctica and Laurentia is a key constraint in a number of reconstructions of Rodinia such as AUSWUS (Karlstrom et al., 2001) and SWEAT (Moores, 1991). Intercontinental stitching points have been invoked between the Albany-Fraser Province and the Bunger Hills in Antarctica, the Musgrave Province and western Laurentia. The link between the Albany-Fraser Province and the Bunger Hills in Antarctica is well-established (Duebendorfer, 2002), but the authors4 claim that link between the Musgrave Province and western Laurentia is more subjective. According to the SWEAT model the Musgrave Province aligns with the northern Canadian Wopmay Province (Moores, 1991), and in the AUSWUS model the Mexican Oaxaca Terrane connects the Musgrave Province to the Grenville Belt (Karlstrom et al., 2011). These stitching points that have been proposed have magmatic and metamorphic evolutions that differ significantly from those of the Musgrave Province (c.f. Laughton et al., 2005; Solari et al., 2003; White et al., 1999).
Both the AUSWUS and the SWEAT models have been shown by palaeomagnetic studies to not be viable and North America and Australia were moving independent of each other at 1.2 Ga, possibly separated by an ocean (Pisarevsky et al., 2003). A model that is viable at 1070 Ma based on palaeomagnetic data that connects Australia to the Grenville Belt via Mexico of the present, AUSMEX configuration (Wingate et al., 2002).
According to the authors4 in these previous reconstructions connecting the Musgrave Province with western Laurentia it has been assumed that the present east-trending architecture of the Musgrave Province was also valid at the Grenville age, in spite of the Petermann Orogeny having overprinted the Musgrave Province about 550 Ma, which has been interpreted as the principal influence on the crustal architecture of the present (Camacho & MacDougall, 2000).
An analysis of aeromagnetic data the authors4 present in this paper is said to "see through" the overprinting of the Petermann Orogeny to determine what the structural architecture of the Musgrave Province was in Grenvillian times. They say the resolution of regional aeromagnetic data of the Musgrave Province is sufficient, about 200 m line spacing, to define the magnetic signature of rock packages and correlate them with rocks that are exposed, to separate magnetic anomalies related to the architecture of the Grenvillian Era from those of the Late Neoproterozoic architecture.
Magnetic signatures to show inherited crustal structure
Major obstacles to understanding Proterozoic tectonics are polydeformation and surface expression that is discontinuous. The subsurface expression of terranes and constraining of their crustal architecture at depth can be mapped by the use of high-resolution aeromagnetic grids on a regional scale (e.g. Finn & Sims, 2005), allowing the testing of the validity of geological connections between these terranes.
Relating magnetic signatures to rock types or alteration textures of known age can define the preserved architecture. According to the authors4 magnetic signature is described in terms of field intensity, anomaly amplitude and magnetic texture described by its wavelength, orientation and amplitude.
Particular magnetic signatures can be selectively amplified or attenuated to reduce the signal from specific rock units by the application of image processing techniques. An image of a preserved early architecture can be produced as long as the magnetic signatures of younger events differ sufficiently from those of the older event. It is necessary to consider the possibility that significant deformation of the inherited architecture occurred during the younger event(s) when interpreting the results.
In Australia, in the Albany-Fraser Province, the Musgrave Province and the Warumpi Province, Grenville-age igneous and metamorphic rocks have have been preserved. The Officer Basin and the Amadeus Basin (Lindsay, 2002), from the mid-Proterozoic to Devonian age, cover all 3 provinces resulting in poor exposure of the areas between the Musgrave Province and the Albany-Fraser and Warumpi Province making it difficult assess the connectivity of these terranes. High-grade gneiss that was metamorphosed during the 2 stages of the Musgravian Orogeny, stage 1 being from 1324-1296 Ma and stage II 1200-1150Ma, comprise the Musgrave Province basement (White et al., 1999). There are 2 subdomains, separated by the Woodroffe Thrust, both of which preserve Musgravian rocks, with facies of amphibolite and granulite metamorphism to the south, and to the north of which greenschist and amphibolite facies metamorphism can be seen (Camacho & Fanning, 1995). Syn- to Late Musgravian granitoids of the Pitjantjatjara Supersuite from about 1190-1150Ma have been intruded by basement gneiss, as well as by mafic/ultramafic intrusions of the Giles Complex about 1080-1050 Ma (Sun et al., 1996).
An east-trending network of dextral transpositional shear zones on a crustal scale that accommodated rapid burial and exhumation of the Musgrave Province was formed during the Petermann Orogeny of the Late Neoproterozoic (Camacho & MacDougall, 2000). According to the authors4 reorientation of the Grenville structural grain within 10-30 km of these shear zones was caused by the focusing of deformation on discrete crustal boundaries.
Magnetic signatures used to identify Grenvillian architecture4
According to the authors4 Broad magnetic anomalies of low amplitude are caused by Musgravian gneisses and granitic rock packages compared to those caused by shear zones associated with the Petermann Orogeny that are anomalies of relatively short wavelength and high amplitude. There were 5 magnetic anomaly categories that were defined and were interpreted in terms of lithology by correlation with outcrop.
C1 Anomalies that are broad, moderately negative, often having low amplitude, short wavelength of 1-5 km, and sub-linear fabric oriented to the northeast.Where this fabric is present it is cross-cut by anomalies of categories 3,4 and 5. C1 has been interpreted as representing basement gneiss with a Musgravian fabric.
C2 These sub-linear to circular anomalies are moderately positive to weakly negative, and have texture that is similar to C1 anomalies. They have been interpreted as an early phase of the Pitjantjatjara Supersuite with a Musgravian Orogeny fabric
C3 Anomalies that are sub-circular to sub-rectangular, strongly positive and relatively broad, 10-30 km. Many C3 anomalies have a texture that is a secondary feature, and the primary feature can be stippled or smooth. They have been interpreted as a Pitjantjatjara Supersuite late phase.
C4 These anomalies are typically strongly negative with high amplitude, though they can occasionally be strongly positive or even bipolar, and sub-linear to sub-circular and having a texture that is smooth. C4 anomalies correlate with intrusions of the Giles Complex.
C5 These anomalies are high amplitude and are generally strongly negative and are linear and narrow, less than 10 km, with a texture that is smooth. Most of them trend southeast, forming a network transecting the entire Musgrave Province. Anomalies C1 to C4 are cross-cut by C5 anomalies, correlating with outcrops of mylonite zones dated to about 550 Ma (Camacho & Fanning, 1995). These anomalies have been interpreted as representing shear zones of the Petermann Orogeny.
The authors4 applied upward continuation to remove the C5 short wave anomalies and enhance the C1-C3 anomalies in order to produce an image of the Musgravian architecture. The reduced to pole magnetic field was upward continued to 5 km and 15 km. The amplitudes of the C5 anomalies has been greatly reduced while the anomalies that define the overall distribution of the Pitjantjatjara Supersuite, C1 and C3, and basement gneiss, C1) are enhanced relatively, though these anomalies have lost their shortwave texture. Within the Musgrave Province subdomains of amphibolite facies and granulite facies are juxtaposed, resulting in a large anomaly that is east-trending, of low amplitude and long wavelength, within which are poorly defined C1-C4 anomalies. Small amplitude anomalies are allowed to be resolved better within a regional field of large amplitude as the potential field tilt (Miller & Singh, 1994) is relatively independent of the amplitude of the input signal.
A pervasive northeast-trending magnetic grain at moderate wavelengths, of about 10 km, is shown by the tilt of 5 km upward continued data, that is well developed in the central and eastern Musgrave Province that correlates with the distribution and orientation of the Pitjantjatjara Supersuite.(Positive tilt phase) within gneiss basement (negative tilt phase). Extensive Giles Complex intrusions and the Petermann Orogeny shear zones that form a dense network, resulting in the trend being only weakly observed in the western part of the Musgrave Province. Arcuate magnetic highs, that are northeast-trending, on a continental scale, as shown by the tilt of the 15 km upward continued data, have been observed to extend beneath the sedimentary rocks of the Amadeus Basin and Officer Basin.
New Rodinia construction restraints
Chains of magnetic granitoids similar to those of the Pitjantjatjara Supersuite are suggested by the northeast-trending anomalies to extend beneath the Amadeus Basin to connect with the contemporary Teapot Granites (Black & Shaw, 1995) of Warumpi Province. Chains of magnetic granitoids linking the Pitjantjatjara Supersuite to the Nornalup Complex granitoid suites (Clark et al., 2000) of the Albany-Fraser Province, of Grenville age, are similarly suggested by magnetic highs above the Officer Basin.
The authors4 suggest that a lack of systematic geochemical analysis has resulted in the magmatic origins of the Pitjantjatjara Supersuite, Nornalup Complex granites and Teapot Granites, that are compositionally diverse, being enigmatic. Geochemical data have been compiled that suggests they are predominantly I-type to transitional A-type granitoids (Budd et al., 2001). The authors4 suggest syn-to-late orogenic magmatism occurring within the crust of a collisional orogen is probably reflected by most granitoids. Fragments of arc magmatism may also have been incorporated during the process of accretion at the beginning of orogenesis. A continuous granitoid orogenic belt, of Grenville age, and trending northeast from the continental margin to the southern margin of the Arunta Inlier, have been suggested by these granitoid chains to connect the Mesoproterozoic age Albany-Fraser Province, the Musgrave Province and the Warumpi Province. Older terranes surround this orogenic belt on 3 sides and doesn't extend eastwards as far as the Tasman Line. The authors4 suggest it is important that the orogenic belt of Grenville-age terminated abruptly and obliquely against the North Australian Craton from the Palaeoproterozoic. At the Redbank Thrust Zone the Warumpi Province is thrust over by the North Australian Craton (Goleby et al., 1989; Selway et al., 2006), the last activity of this zone occurring about 400-300 Ma during the Alice Springs Orogeny, though it is interpreted to record activity as far back as about 1500-1400 Ma (Biermeier et al., 2003).
The authors4 suggest new constraints are imposed by the architecture that has been imaged within the orogenic belt, and these must be satisfied in any reconstruction models of Australia, the models needing to explain the dominant northeast trending of the architecture, as well as orogenic belts that terminate intracontinentally and granitoid magnetism on a voluminous scale.
According to the authors4 a link between the Musgrave Province and Laurentia (Karlstrom et al., 2001; Moores, 1991; Wingate et al., 2002) cannot explain the architecture that has been observed within the orogenic belt or its termination against the North Australian Craton. A model has been proposed in which a collision between the South Australian Craton and the already joined West Australian Craton and the North Australian Craton, after rotating clockwise by 52° about a pole located 136°E and 25° S (Giles et al., 2004), the rotation being interpreted to be caused by asymmetric rollback of a subduction that initially dipped to the northeast outboard of the South Australian Craton, beginning about 1500 Ma and completing about 1100 Ma. The Musgravian Orogeny and the Albany-Fraser Orogeny are implied to have occurred simultaneously by this model (Giles et al., 2004), being part of an orogenic belt that was northeast-trending and terminating in the interior of the continent. The authors4 suggest that this is a generalist model that failed to consider the internal architecture of the orogenic belt, and assumed that the limits of the Musgrave Province of the present was the limit of extent of the Musgravian Orogeny. According to the authors4 the internal geometry of the Musgravian and Albany Fraser Orogenic belt is defined by their imaging, and illustrates that the orogeny extends past the limits of the Musgravian Province to the Warumpi Province.
The authors4 say anomalies from Grenville-age gneiss and granitoids and the reduction of those of a younger event that were strongly overprinted have been enhanced by image processing of the aeromagnetic data that has proven to be effective. Australian provinces of Mesoproterozoic age that terminate abruptly against the North Australian Craton, of Palaeoproterozoic age, are defined by the aeromagnetic data to have been linked by a continuous northeast-trending orogenic belt that has an internal architecture that is dominantly northeast-trending. The authors4 say that any proposed model of orogenesis in Australia is required by this orogenic belt to explain the northeast-trending architecture as well as termination that is intracontinental. They suggest that any model proposed involving continuation of the Musgrave Province in an eastward direction to connect with orogens in Laurentia that are broadly contemporaneous are unviable. They also suggest that a model proposed that involves the rotation of the South Australian Craton that collided with the North Australian Craton and the West Australian Craton (Giles et al., 2004) is their preferred model to explain the Grenville-era orogenesis in Australia as it is consistent with their results.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|