OR/15/058 Towards consensus on South Georgia and the Scotia Arc

Stone, P. 2015. The geological exploration of the sub-Antarctic island of South Georgia: a review and bibliography, 1871–2015. British Geological Survey Internal Report, OR/15/058.

The uncertainties imposed on the early attempts at ‘pre-drift’ reconstructions of the Scotia Arc are well exemplified by the fact that a geological description of in situ specimens from Shag Rocks (Figure 1) was not published until 1982, although the greenschist-facies metamorphic rocks were collected in 1974 (Tanner 1982b). Prior to that, the only geological information available for the Shag Rocks continental block derived from similar schistose rocks dredged from the sea floor in 1930 and described by Tyrrell (1945). Progress elsewhere saw confirmation of the correlations between South Georgia and parts of Tierra del Fuego (Katz and Watters 1966; Frakes 1966; Skidmore 1972, Winn 1978) that had been previously suggested by Wilckens (1933) and Kranck (1934), yet the details of the regional picture remained unclear.

The Barker and Griffiths (1972) pre-drift reconstruction had been based largely on geophysical evidence but was disputed, in advance of its publication and on geological grounds, by Dalziel and Elliot (1971) who referred to it as being ‘in press’. No submission date is recorded in the published Barker and Griffiths paper but the authors, and many others including Dalziel and Elliot, had contributed Scotia Arc papers trailing their positions at a 1970 symposium on Antarctic Geology and Geophysics held in Oslo, Norway (Adie 1972). Dalziel and Elliot’s objection lay primarily in the position of the South Georgia continental block. Barker and Griffiths had placed it at the easternmost edge of their re-assembly forming the ‘Atlantic’ margin. In contrast, and arguing from the evidence of geological comparisons and correlations, Dalziel and Elliot modelled the original South America — Antarctic Peninsula connection as narrow and rectilinear rather than the broader zone extending, cusp-shaped, into the proto-Atlantic, as favoured by Barker and Griffiths. Furthermore, Dalziel and Elliot placed the South Georgia block on the Pacific side of that originally rectilinear continental strip. In their model, the rectilinear strip was intact until the end of the Mesozoic, and was disrupted by oroclinal bending towards the east early in the Cenozoic; only at that stage was South Georgia brought to a position to the south of the Burdwood Bank. Subsequent eastward drift of the Scotia Arc continental blocks was accompanied by a convergence of South America and the Antarctic Peninsula to accentuate the arc physiography.

Bringing together the marine geophysical evidence and new interpretations from South America, Dalziel and others (1975) described the geology of South Georgia in terms of an Early Cretaceous marginal basin that formed “between a calc-alkaline volcanic arc built on a sliver of old South American continental crust and the main part of the South American continent from which the sliver moved away” (1975, abstract). The continental foundation to the arc is now represented by the Drygalski Fjord Complex, an arc-proximal shelf facies by the Annenkov Island Formation, the oceanic crust of the marginal basin by the Larsen Harbour Complex, and the turbiditic basin-fill sediment by the Cumberland Bay and Sandebugten formations (Figure 2) (Curtis and Riley 2011 and references therein). Asymmetrical deformation accompanied closure of the marginal basin in the middle to Late Cretaceous, as the volcanic arc complex moved back to rejoin the continental margin (Dalziel and others 1975).

The collision between the volcanic arc complex and the continental margin initiated its tectonic break-up and the subsequent development of the Scotia Arc, as has been recently reviewed and illustrated in detail by Dalziel and others (2013). These authors note that the origins of the Scotia Arc can be ultimately traced back to the Jurassic fragmentation of Gondwana. Thereafter, they conclude (2013, p.786):

“The North and South Scotia Ridges resulted from the eastward motion of fragments of the Pacific margin cordillera of both South America and Antarctica that was initiated in the early Cenozoic as a result of left-lateral transtensional relative movement between the two continents. This resulted in seafloor spreading that generated the northwestern and southeastern portions of today’s Scotia plate.”

The currently accepted configuration of the region is summarised in Figure 1. However, the route by which that configuration was achieved is still a matter of some controversy. In the view summarised by Dalziel and others (2013). South Georgia originated at the Pacific margin of Gondwana and thence drifted a considerable distance eastward relative to South America following disruption of an original continental link between that continent and the Antarctic Peninsula. There are alternative views. For example, concerned by the length of eastward drift required, Eagles (2010) has proposed a reconstruction wherein the Antarctic Peninsula is removed to the west of South America, and South Georgia becomes part of a Gondwana margin that swings eastward towards South Africa. Debate at this level of geotectonics seems likely to continue.