The Main Geological Units of Antarctica Before Gondwana Amalgamation

As with all other continents, Antarctica consists of a number of Archean/ Early Proterozoic cratons (older than 1.5 Ga) surrounded by successively younger belts that formed and/or accreted to the continental margins, as products of convergent plate tectonic events such as subduction of oceanic crust underneath continental crust and/or collision of two former separated continents (Fig. 7.2).

Cratons are confined to East Antarctica and, at least two among several, are generally accepted: the small Grunehogna Craton (ca. 3.0 Ga basement covered by flat-lying and undeformed sediments older than 1 Ga) interpreted as a fragment of the African Kalahari Craton, and the East Antarctic Craton s.s.

The East Antarctic Craton s.s. is exposed only in limited areas including the southern Shackleton Range, the eastern Thiel Mountains (Ford, 1963), the Miller and Geologists Ranges of the central Transantarctic Mountains, near the continental margin in Enderby Land, in the southern Prince Charles Mountains, in the Vestfold Hills, in Wilkes Land, in Terre Adelie and in George V Land. The exposed basement, only occasionally covered by almost flat-lying platform sediments (e.g. Thiel Mountains), consists of high-grade gneisses and granulite facies metamorphic complexes showing radiometric ages of older than 1,500 Ma up to just over 3,500 Ma in Enderby Land. Recently, parts of Enderby, Kemp and Mac. Robertson Lands have been separated from the East Antarctic Craton and attributed to younger orogenic belts, i.e. the Grenvillian and Pan-African belts s.l. (Fitzsimons, 2000a; Boger et al., 2001, 2002; Boger and Miller, 2004), so that northern Enderby Land is now considered a separate craton (Napier Complex). Moreover, the results of more recent investigations suggest that, in the next few years, our appreciation of the present configuration of the East Antarctic Craton could change drastically and be subdivided in much smaller cratons including: (1) the Napier Craton (considered a fragment of Dwarhai Craton in India); (2) a craton comprising the southern Prince Charles Mountains, the southern Shackleton Range and perhaps the eastern Thiel Mountains; (3) the Vestfold Hills Craton; (4) the Mawson Craton (Fitzsimons, 2000a, 2003), probably consisting of the Miller and Geology ranges of the central Transantarctic Mountains, the shield areas of eastern Wilkes Land, Terre Adelie, George V Land, and their Australian counterpart, the Gawler Craton.

Since the relationships between the Mawson Craton, the Vestfold Hills Craton, the southern Shackleton Range and the southern Prince Charles

Figure 7.2: Schematic geological map of Antarctica (modified from Kleinschmidt, 2007). The map shows the possible subice extent of the cratonic areas, the three "Grenvillian" belts, the Pan-African orogenic belts and the Ross Orogen. The map also shows the distribution of Beacon Supergroup and Ferrar Supergroup outcrops and of the two main Mesozoic orogenesis (Ellsworth or Weddell Orogeny and Antarctic Andean Orogen) which formed along the palaeo-pacific margin of Gondwana in West Antarctica, and the three intra-plate fracture zones in East Antarctica (Lambert Graben or Lambert Rift, West Antarctic Rift System, Jutul Penck Graben and the Rennick Graben). Abbreviations - BLM: Bertrab, Little-wood, Moltke nunataks, Bu: Bunger Hills, Ele: Elephant Island, Ge: Geologists Range, JPG: Jutul Penck Graben, KS: Kuunga Suture, LHB: Lutzow-Holmbukta, MF: Matusevich strike-slip fault, Mi: Miller Range, PCM: Prince Charles Mts., Pe: Pensacola Mts., RG: Rennick Graben, Sh: Shackleton Range, Sn: Snow Island, S0: Sor Rondane, Th: Thiel Mts., Wi: Windmill Islands; Lands - DML: Dronning Maud Land, EL: Enderby Land, GVL: George V Land, KL: Kemp Land, KWL: Kaiser-Wilhelm-II-Land, MBL: Marie Byrd Land, MRL: Mac.Robertson Land, OL: Oates Land, PEL: Princess Elizabeth Land, TA: Terre Adelie, VL: Victoria Land, WL:

Wilkes Land.

Mountains are still totally unknown, a tectonic reconstruction is largely speculative and the cratonic areas could be even more reduced and further increased in number if, for example, 1.6-1.7 Ga-old orogens are separated.

The orogenic belts separating the cratons as internal belts in East Antarctica or as elongated peripheral belts at the palaeo-Pacific margin of Gondwana developed during five major orogenic cycles spanning in time from c. 1.3-0.9 Ga (Grenvillian-aged orogens), through 500-600 Ma (Ross and the Pan-African orogens), 250-200 Ma (Ellsworth or Weddell Orogen) and 150-90 Ma (Antarctic Andean Orogen) to the period between 50 Ma and recent, documented only by the active plate-margin processes in the area north-west of the Antarctic Peninsula.

As in eastern North America and other continents, the Grenvillian-aged orogens in Antarctica are considered to record the fundamental orogenic event marking the amalgamation of the Rodinia supercontinent in Meso-/ Neoproterozoic times. Since polyphase deformation, high-grade metamorph-ism and magmatism resembles somewhat those in the cratons, the areas formed during the Grenvillian orogeny have been often undifferentiated from the cratons. Moreover, until the early 1990s, only one very long Antarctic Grenvillian orogen was assumed, following the Antarctic coast as a 250 km wide strip from Coats Land in the west up to George V Land in the east, occasionally specially named the ''Circum East Antarctic Mobile Belt'' (Yoshida, 1992). Such an extension has been demonstrated to be incorrect for Terre Adelie and George V Land and unproven for Coats Land, where the tiny outcrops represented by three little groups of nunataks (Littlewood, Bertrab and Moltke) consist of 1,100 Ma rhyolites and granophyres (Storey et al., 1994), but absolutely undeformed, thus not consistent with the existence of an orogenic belt.

The existence of a Circum East Antarctic Mobile Belt is no more supported by new geological evidence which, instead, indicates the presence of three distinct Grenvillian-aged orogens (Fitzsimons, 2000a) (Fig. 7.2): (1) the Maud Belt, extended from western to central Dronning Maud Land to possibly parts of S0r Rondane, interpreted as the trace of welding the Grundhogna Craton and the Shackleton Range/southern Prince Charles Mountains craton; (2) the Rayner Belt (Yoshida and Kizaki, 1983) that in Enderby, Kemp and Mac. Robertson Lands separates the Napier craton from the southern Prince Charles Mountains craton, and was verified in the northern Prince Charles Mountains and the Rayner Complex (e.g. Boger et al., 2002); (3) the Wilkes province belt (Fitzsimons, 2000a) of Wilkes Land that is exposed in the Bunger Hills and Windmill Islands and merges the Vestfold Hills and the Mawson cratons.

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