ROLE OF ANTARCTICA IN THE ASSEMBLY OF EAST GONDWANA

The assembly of the supercontinent Gondwana was not achieved until sometime in the middle to late Cambrian (Li and Powell, 1993; Powell et al., 1993; Grunow et al., 1996). This was achieved though the gradual closure of a number of ocean basins to form west Gondwana (consisting of Africa and South America) in the Neoproterozoic, followed by Early Cambrian collisions that relate to the formation of east Gondwana (consisting of Australia, Antarctica and India).

Reconstruction of Gondwana centered on Antarctica at ~ 500 Ma. East Antarctica and margins of adjacent continents showing Archean-Paleoproterozoic cratonic blocks and Meso-Neoproterozoic and Paleozoic orogenic belts. Mozambique suture represents proposed boundary between east and west Gondwana. Note position of Cambrian orogenesis in Prydz Bay within east Gondwana. Asterisk shows area described by Boger et al. (2001). D-Dwarhar craton, G-Gawler craton, K-Kalahari craton, P/Y-Pilbara/Yilgarn craton, sPCMs-southern Prince Charles Mountains, V-Vestford Hills, LHB-Lützow-Holm Bay, PB-Prydz Bay, L-Leewin Complex, nPCMs-northern Prince Charles Mountains.

East Gondwana is thought to have formed during the consolidation of Rodinia in the Mesoproterozoic (Hoffman, 1991; Moores, 1991; Rogers, 1996) and remained tectonically stable until the modern continents rifted from Gondwana in the Mesozoic. The internal stability of east Gondwana has been inferred from the perceived absence of younger events within east Antarctica (Fig. B).

Traditional model of east and west Gondwana where east Gondwana is assumed to be tectonically stable from the Mesoproterozoic onwards. Solid line defines the position of the Mozambique suture. Highlighted localities are the same as those illustrated in A.

This model was challenged by Zhao et al. (1992) who obtained Pb-Pb zircon evaporation ages of ca. 550 Ma from Prydz Bay. They argued that at least some of the ductile deformation in Prydz Bay was of Cambrian age and this has been supported by U-Pb crystallization ages obtained from syntectonic granites and leucosomes from syntectonic granites and leucosomes (Carson et al., 1996; Fitzsimons et al., 1997).

Because Prydz Bay lies within east Gondwana a number of studies have interpreted orogenesis in Prydz Bay to form part of a previously unrecognized Pan-African suture, and have attempted to link orogenesis in Prydz Bay with equivalent aged tectonism recognized in Lützow-Holm Bay (west) and in the Leewin Complex (east) (Fig. C).

Unfortunately, there is only limited isotopic data available from inland localities to support this model (Fig. D). Similarly, the significance of this orogenic belt within east Gondwana has yet to be properly established. Some argue for an intraplate setting (Wilson et al., 1997), whereas, others argue for a suture (Fitzsimons, 1997, 2000a and b).

However, Boger et al. (2001) using isotopic results obtained from the southern Prince Charles Mountains, present a new tectonic scenario and discuss the implications that this model has for the reconstruction of both Rodinia and Gondwana.

The isotopic results obtained in the study of Boger et al. (2001) demonstrate that the sPCMs consist of two distinct terranes of different age. The southernmost (Ruker) preserves a Middle Archean tectonic history between ca. 3160 Ma and ca. 2650 Ma. In contrast, the northern (Lambert) terrane was deformed during the Cambrian (~550-490 Ma) and contains detrital zircons of Late Archean to Mesoproterozoic age.

Identifying the continuation of this Cambrian belt beyond the sPCMs and Prydz Bay is problematical. In the sPCMs, the belt trends east-west, parallel to the dominant structural trend in the adjacent basement. Based on the distribution of Archean granitoids and metamorphic isogrades (Tingey, 1991; Kamenev et al., 1993) it can traced across the outcropping regions of the sPCMs. If this trend is maintained the belt will link with the continuation of the Mozambique suture identified in central Dronning Maud Land (Jacobs et al., 1998). Both the nPCMs (Antarctica) and the Eastern Ghats (India) record minor intrusive activity and some evidence of isotopic disturbance during the Cambrian. These events suggest that both were largely unaffected by deformation in the Cambrian, but may well have lain in a position marginal to this belt. Thus, orogenesis in Prydz Bay may have continued into India and the evidence may lie somewhere to the north of the Eastern Ghats. Although no such evidence has been reported, signs of Cambrian orogenesis could be obscured by the deposits of the Ganges and Brahmaputra Rivers. Similar aged rocks recognized in the Denman Glacier - Leewin Complex could equally represent a separate Cambrian belt.

Having defined the probable extension of the Prydz Bay orogenic belt in the sPCMs, the question still remains as to whether this belt defines a suture or an intraplate orogen. If the sPCMs-Prydz Bay belt does indeed define a "Pan-African" suture, then it implies that this belt would define the eastern margin of a distinct Indo-Antarctic craton (Fig. 3). This lithospheric block would have consisted of part of east Antarctica, including the nPCMs, the Rayner and Napier Complexes of Enderby Land, as well as potentially most of cratonic India. Cambrian suturing along the eastern margin of this Indo-Antarctic block implies that it did not form part of Gondwana until the Cambrian. Such an interpretation contradicts most reconstructions of Rodinia and Gondwana presented by many authors up until 2000.

This separate Indo-Antarctic block accreted with west Gondwana along the Mozambique suture between ca. 570 and 520 Ma (Grunow et al., 1996). In contrast, most of the syn-tectonic ages from Prydz Bay and the southern Prince Charles Mountains fall between ca. 550 and ca. 490 Ma. Thus, deformation along the Mozambique suture was partly coeval with, but also predated suturing along the sPCMs-Prydz Bay belt. This difference in age is consistent with the eastward younging of Pan-African orogenesis recognized within west Gondwana (Grunow et al., 1996) and further supports the inference that orogenesis in the sPCMs and Prydz Bay is related to progressive ocean closure and the eastward amalgamation of Gondwana.

LINK BETWEEN GONDWANA AND LAURENTIA

Grunow (1995) has undertake reconstructions of Gondwana and Laurentia in different time windows using palaeomagnetic reference poles. During the Middle to Late Cambrian a large early Palaeozoic Iapetus Ocean may have existed between Laurentia and South America.

At ~515 Ma. In this reconstruction, the palaeomagnetic data allow the possibility of rifting between eastern Laurentia and South America created the Iapetus Ocean. Emplacement of a major granitic batholith associated with subduction along a convergent margin begins along the Pacificward margin of Antarctica and eastern Australia. The continental fragments of the Antarctic Peninsula (e.g. EM, Ellsworth Mountains) and of New Zealand/Marie Byrd Land contain early Palaeozoic rocks and have been speculatively shown in their Mesozoic pre-Gondwana breakup positions relative to East Antarctica. L = Laurentia Pole, G = Gondwana Pole.

At ~470 Ma. Most of Gondwana appears to move equatorward between the Cambrian and Early Ordovician. The palaeomagnetic data can be interpreted to be in agreement with the model by Dalla Salda et al. (1992) that predicts Ordovician collision of Laurentia and South America (the Taconic and Oclyoic deformational events, respectively) associated with closure of the Iapetus Ocean.

The results of Grunow (1995) show that there is good agreement with Early Ordovician poles from elsewhere in Antarctica and with poles from the other Gondwana continents. The similarity of the early Palaeozoic poles around the East Antarctic craton argues for little structural disruption of the rocks in these areas and demonstrates the coherence of East Antarctica since ~475 Ma. The Sør Rondane Antarctic pole is considered to be Middle to Late Cambrian and agrees well with Australian Middle Cambrian poles.

Between ~515 and ~470 Ma, most of Gondwana moved equatorward, especially South America and southern Africa, possible causing changes in the depositional setting and faunal evolution around parts of Gondwana. The Early Ordovician Gondwana and North America poles can be interpreted to indicate that eastern North America and western South America were in near proximity, possibly reflecting closure of the Iapetus Ocean. However, faunal similarities do not necessarily indicate that these continents were at similar latitudes unless the faunas are found in comparable depositional environments, that is, low-latitude, deepwater faunas can be indistinguishable from high-latitude, shallow-water faunas.