Antarctica in the Upper Palaeozoic Mesozoic c 450180 Ma Evolution of Gondwana

In this time window, the main geological events so far known in the Antarctic geological record include the formation of a continental basin where a dominantly fluvial succession, the Beacon Supergroup, was deposited in Devonian to Triassic times, and the development of two main deformational events generally known as the middle Palaeozoic Borchgre-vink Orogeny and the Permo-Triassic Gondwanian Orogeny.

7.4.2.1. Borchgrevink Orogeny

The occurrence in northern Victoria Land of a calc-alkaline magmatic suite of middle Palaeozoic (c. 350 Ma) age is considered as evidence of an orogenic event which is temporally well separated from the waning phase of the Ross Orogeny (Craddock, 1970). This suite includes dominant granitoids (Admiralty Intrusives) and minor felsic volcanic (Gallipoli Volcanics). The granitoids form several plutons which are mainly concentrated in the

Robertson Bay Terrane, although some cut across the tectonic boundary between the Bowers and Wilson Terrane, where the coeval volcanics are also concentrated. All the intrusions are characterized by isotropic fabrics and discordant contacts with respect to the surrounding metasediments suggesting a post-tectonic emplacement. These features and meagre evidence of radiometric data for concomitant metamorphism and deformation have so far limited a comprehensive reconstruction of the tectonic setting and development of the Borchgrevink Orogeny in northern Victoria Land. Nevertheless, the continental arc geochemical signature of the Devonian-Carboniferous magmatic suite indicates that the plutonism most likely occurred as the result of a renewed period of subduction activity along the palaeo-Pacific margin (Kleinschmidt and Tessensohn, 1987; Borg and DePaolo, 1991). Similar suites are known in Tasmania, New Zealand and the Campbell Plateau (Gibson and Ireland, 1996; Bradshaw et al., 1997) and in the Ford Range in Marie Byrd Land. A correlation between the Borchgrevink Orogeny of northern Victoria Land with the Tasmanian Orogeny was put forward by Findlay (1987). Elsewhere in Antarctica, marine sediments from the Ellsworth Mountains and Pensacola Mountains are considered to have been deposited in the same time window, within a basin that according to Elliot (1975) could have extended from the Weddell Sea to the Ross Sea.

7.4.2.2. Beacon Supergroup

The Beacon Supergroup consists of dominantly continental sedimentary deposits which constitute a generally flat-lying, 2.5-3.5km thick cover developed over a marked unconformity (Kukri Peneplane) above the Ross orogenic belt throughout most part of the Transantarctic Mountains (Fig. 7.2). Similar sequences are known in limited outcrops in East Antarctica (Prince Charles Mountains, Dronning Maud Land and Ellsworth Mountains) (Barrett, 1991) and as bedrock of the Cenozoic glaciomarine sediments in the Victoria Land Basin (VLB) in the Ross Sea as documented by the CRP-3 drill-hole (Cape Roberts Science Team, 2000). Outside Antarctica, similar sedimentary rocks, collectively called Gondwanian sequences, occur in South Africa, Australia and South America. The deposition of these sediments in Antarctica started in Devonian time with the Taylor Group, consisting dominantly of quartz-arenites and conglomerates. Deposited as the result of erosion and fluvial processes under arid and semiarid conditions (Campbell and Claridge, 1987), the Taylor Group accumulated in a series of basins along the palaeo-Pacific margin of

Gondwana. After the deposition of fossiliferous siltites, the subsequent stage corresponds to an erosional phase, probably related to a glacial event which led to the deposition of glaciogenic sediments in late Carboniferous-Early Permian time. In the Transantarctic Mountains, all the Carboniferous to Triassic sediments are formally known as the Victoria Group, which includes carbonaceous layers and feldspathic sandstones.

The low sedimentation rate (c. 12.5 m/My) and absence of concomitant compressional deformation suggest that deposition occurred over a thick continental crust (Barrett, 1991) but different tectonic models have been proposed, including a passive margin (Isbell, 1999), intra-cratonic (Barrett, 1991; Woolfe and Barrett, 1995) or marginal/back-arc (Bradshaw and Webers, 1988).

7.4.2.3. Ellsworth or Weddell Orogeny

The Ellsworth or Weddell Orogeny (or - in a larger context - also known as the Gondwanide Orogeny including components in southern Africa - Cape Fold Belt - and South America - Sierra de la Ventana Fold Belt) occurred in Permo-Triassic time ca. 250-200 Ma. As noted by Cawood (2005), this orogeny overlaps with the end Palaeozoic assembly of Pangea (Li and Powell, 2001), through ocean closure and accretion of Gondwana, Laurussia (Laurentia+Baltica) and Siberia, as well as completion of terrane accretion in the Altaids. Stratigraphic and geochronological data (Dalziel, 1982; Dalziel and Elliot, 1982; Storey et al., 1987; Trouw and De Wit, 1999; Johnston, 2000) indicate that Permo-Triassic deformation of variable intensity and distribution occurs throughout West Antarctica and the adjoining Cape Fold Belt of southern Africa. In Antarctica, this orogenic event is well documented in the Ellsworth-Whitmore Mountains and in the Pensacola Mountains, where upright to inclined folds with axial planar cleavage are inferred to have formed in a dextral transpressive environment (Curtis, 1998) (Fig. 7.2). The Ellsworth-Pensacola Mountains chain represents the fold belt, while the Whitmore Mountains represent the magmatic arc of an Andean-type orogen. It merges with the Ross Orogen in the Pensacola Mountains, where it partly overprints the older Ross-aged structures. Elsewhere, deformation is heterogeneously distributed, with Storey et al. (1987) noting that in the Antarctic Peninsula, unconformities previously ascribed to the Gondwanide Orogeny are younger and that the only event related to Gondwanide deformation s.s. is regional metamorph-ism at 245 Ma of parts of the Trinity Peninsula Group.

The Ellsworth Orogen trends noticeably crossways to the palaeo-Pacific margin of Antarctica as indicated by the Ross Orogen. This obliqueness is due to secondary rotation, as proven by palaeomagnetic investigations by Funaki et al. (1991) and confirmed by Randall and MacNiocaill (2004).

7.4.2.4. The Antarctic Andean Orogen

The orogen of the Antarctic Andes occupies the entire Antarctic Peninsula down to the Walgreen Coast (Fig. 7.2). It formed mainly in three episodes: (i) Late Jurassic through Early Cretaceous (150-140 Ma), (ii) mid-Cretaceous (~ 105 Ma) and (iii) ~ Tertiary (~50Ma to recent), and is partly still active (e.g. Birkenmajer, 1994; Vaughan and Storey, 1997). Thus, it represents the youngest growth zone of the continent. The Antarctic Andes are a typical subduction orogen accompanied by orogenic magmatism in the form of granitic plutons and volcanic rocks. In detail, the deformation and metamorphism are very complicated, because they are polyphase. Folding and thrust faulting is reported mainly from the southern portion of the Antarctic Peninsula (Palmer Land and Alexander Island) and from the extreme north (Trinity Peninsula and eastern South Shetland Islands). The distribution of related metamorphism is also heterogeneous, including high-pressure metamorphism with blueschists characteristic of subduction complexes, e.g. on Elephant Island (Trouw et al., 1991).

7.4.2.5. The plate-tectonically active parts of Antarctica

The only plate-tectonically active part of Antarctica is situated north-west of the Antarctic Peninsula, in the South Shetland Islands (from Snow Island in the south-west up to Elephant Island in the north-east) and the Bransfield Strait (Fig. 7.2). North-west of the South Shetland Islands, a small section of the ocean floor, called the Drake Plate (i.e. the remnant of the older, but largely subducted Phoenix Plate), is being subducted at the South Shetland Trench beneath the Antarctic Plate. Related, mainly andesitic, volcanism forms the island arc of the South Shetland Islands. Parts of the South Shetland Islands (part of Livingston Island and Elephant Island) belong - as does the Peninsula itself - to earlier stages of the Antarctic Andean orogeny and consist of strongly deformed Jurassic trench sediments.

The Bransfield Strait is located south of the subduction-related volcanic island arc and forms an active extensional basin accompanied by tholeiitic volcanism, partly submarine, partly as active island volcanoes (e.g. Penguin Island and Deception Island; Smellie and Lopez-Martinez, 2002). The Bransfield Strait often is regarded as a classic example of a back-arc basin, but recently this has been disputed (Gonzales-Casado et al., 2000).

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