OR/15/058 Early impressions of South Georgia as part of 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.|
After his misidentification of South Georgia as a volcanic island, Klutschak (1881, English translation p.88) continued, “[a]s regards its relation to other continents, South Georgia is part of a submarine range of high mountains connecting the uplands of South America with the Antarctic continent. The highest peaks of this range between Cape Horn and the Cape of Good Hope are represented by the Falkland Islands, South Georgia, the South Orkneys [and] the South Sandwich Islands. With the exception of the Falkland Islands … they are all of the same volcanic nature.”
The concept of a geological link between South America and the Antarctic Peninsula via the islands of the Scotia Arc was not a new idea. After sailing to the South Sandwich Islands from South Georgia at the beginning of 1820, the Russian explorer Thaddeus von Bellingshausen (1831) had proposed that they were “the summits of a mountain range” now submerged between them and continuing westward as a submarine ridge to the South American mainland via the Falkland Islands (1945 English translation, p.110). At about the same time, Sir John Barrow (1831, p.62), noted that the South Shetland Islands “seem to be a continuation of the cordillera of the Andes, and Archipelago of Tierra del Fuego; being for the most part precisely of the same formation with the latter and their strata even inclining in the same way”. Barrow was not writing from first hand experience but, as Second Secretary to the British Admiralty (1804–1845) was well-placed to receive geographical intelligence.
The geological assumptions inherent in Barrow’s account were naïve, inevitably so given the circumstances. A more informed assessment followed the Belgian Antarctic Expedition of 1897–1899, when Arçtowski (1895) concluded, independently, that the mountains of Graham Land were the geological continuation of the Andes. A little later, in 1901, in a discussion of the objectives of the proposed British National Antarctic Expedition, Professor John Gregory of Glasgow University illustrated a continuation of the Andes into the Antarctic Peninsula via an eastward-closing loop, admittedly one that closed far to the west of South Georgia (Gregory 1901). At the time he wrote, Gregory had anticipated leading the National Antarctic Expedition’s science programme, but in the event he did not take part and the 1901–1904 Discovery expedition was led by Captain R F Scott, with Dr Edward Wilson as chief scientist. The proposal that the Andes continued southwards to Graham Land via an eastward, horseshoe- shaped bend was then discussed in some detail by Eduard Suess (1909, 488–497). One possibility that he considered was that a submerged geological link followed the arcuate submarine ridge recently discovered during the Scotia expedition (Bruce 1905). This feature — the Scotia Arc (Herdman 1932) — extends through South Georgia, around the South Sandwich Islands and on into the South Orkney Islands (Figure 1) but Suess, whilst supporting the broad idea, felt it more likely that the eastward bend closed to the west of these islands, linking Burdwood Bank with Elephant Island and the South Shetlands. This left the more easterly, poorly known islands as geological enigmas.
Like most important scientific concepts, continental drift had a long history of maverick (and in some cases bizarre) manifestations, but it only attracted the attention of the geological mainstream once promoted by Alfred Wegener from 1912 onwards (Wegener 1912, 1915, 1929). In a specific reference, Wegener (1929) insisted that the development of the Scotia Arc was best explained by ‘drift theory’, noting palaeontological evidence that supported a terrestrial link between the Antarctic Peninsula and Tierra del Fuego until as recently as the Pliocene. He citied bathymetry to conclude (1929, p.94) that “[s]ince then they have drifted from there [the vicinity of the South Sandwich Islands] westward … In the depth chart one can clearly see how the echeloned chains were torn off seriatim from the drifting blocks and then left behind.”
Despite the best efforts of Wegener and his supporters — notably Alexander Du Toit and Lester King in South Africa, and Arthur Holmes in Britain — continental drift was at first generally dismissed as implausible (Newman 1995; Oreskes 1999). In the prevailing view, the necessary intercontinental connections required by palaeobiogeography were provided by land bridges that had now foundered. An extensive review of these putative structures was given by Schuchert (1932) who reproduced as a starting point (1932, figure 1) Melchior Neumayr’s palaeogeographical map from 1887 which featured a vast ‘Brazil-Ethiopian Continent’ occupying the Jurassic South Atlantic as far south as the Falkland Islands. This putative landmass had been extended by Gregory (1922, 1929) to include South Georgia and its Mesozic fossils, but it was reduced in Schuchert’s (1932) Permian palaeogeography to a much more restricted ‘bridge’ linking Brazil and West Africa. Farther south, Schuchert proposed another land bridge running from Tierra del Fuego to Graham Land via South Georgia and the South Orkney Islands, but cutting obliquely across the Scotia Sea to avoid the actively volcanic South Sandwich Islands for which there was no evidence of a continental basement.
In his preview of the British National Antarctic Expedition, Gregory (1901) had assumed that there was no geological similarity between South Georgia and Tierra del Feugo. This notion was subsequently challenged by both Wegener (1929) and Holtedahl (1929), but the two men had very different regional perspectives. Wegener (1929, p.94) cited the geological relationships as “a model for demonstrating drift theory”. In contrast, Holtedahl favoured predominantly vertical tectonic movements and was not convinced of the reality of continental drift, deciding (1929, p.110) that “I can see nothing in the geological character of the whole area here discussed [the Scotia Arc] that speaks decidedly against the old theory of a connection through an old folding range. On the contrary, there are a number of important features in favour of it” — italics as in the original. So, whilst there might have been agreement that some sort of ‘Scotia Arc’ existed, there was no agreement as to its origins.
The majority view undoubtedly favoured Holtedahl, and was prominently articulated by Gregory (1929) in his Presidential Address to the Geological Society of London. Therein, the Falkland Islands, South Georgia, and the other islands of the Scotia Arc were regarded as vestiges of a foundered continent that had once occupied the South Atlantic region. But there was also an informed alternative view. Arthur Holmes had established an absolute time-scale based on the decay rates of radioactive elements in igneous rocks and realised that the thermal effects of radioactive decay had implications for the continental drift debate. In a remarkably prescient paper published in the Transactions of the Geological Society of Glasgow, Holmes (1929) described a mechanism for continental drift based on deep thermal convection currents and amongst the detailed examples cited, referred to the Scotia Arc as arising from the westward drift of South America and Antarctica (1929, p.595): “The island arc here is probably a lag effect due to a strung-out belt of sial having been dragged against part of the Pacific floor left between the two continental blocks as they advanced.” This interpretation, similar in most respects to Wegener’s earlier proposal, was clearly illustrated by Holmes (1929, figure 7). There has been much speculation that Holmes’ choice of an ‘obscure’ journal in which to publish his ideas arose from an uncertainty as to their validity (e.g. Wood, 1985, p.94). Be that as it may, there is some irony in the fact that publication was on Gregory’s ‘home turf’; he retired as Professor of Geology at the University of Glasgow at the end of September 1929.
Gregory’s opinion of the Scotia Arc and its geology had been much influenced by the work of Ferguson on South Georgia. In particular, the ultimately erroneous evidence for early Palaeozoic rocks there had encouraged interpretations of the island as forming a relic of a foundered ancient continent; Gregory’s (1915) palaeontology was certainly instrumental in fostering that view. Nevertheless, in his assessment of Ferguson’s South Georgia work, Gregory (1915, p.822) clung to his earlier (1901) view of the Scotia Arc and felt that the evidence presented [by Ferguson] was insufficient to determine whether the island formed part of the ‘mountain loop that must once have connected Patagonia [South America] with Graham Land [Antarctic Peninsula]’, or whether that ‘loop’ passed west of the island leaving South Georgia as the vestige of a foundered Atlantic continent, Neumayr’s ‘Brazil-Ethiopian Continent’ or ‘Flabellite Land’ as it had been designated by Schwarz (1906) based on the distribution of a small Devonian brachiopod then known as Leptocoelia flabellites (Southern Hemisphere examples are now known as Australocoelia palmata). Tyrrell (1915), in an accompanying description of the petrography of Ferguson’s South Georgia specimens, stressed their similarity to rocks from the South Orkney Islands — as had Pirie (1913) in his unpublished Scotia report — but was equally non-commital with regard to the overall regional structure. However, after reviewing the subsequently collected specimen suites Tyrrell (1918) came down against Suess’s ‘great loop’, preferring that “South Georgia and the South Orkneys are remnants of an ancient continental land which once occupied the South Atlantic”. Pirie (1913, p.8) had also debated the various possible lines of a structural link between South America and the Antarctic Peninsula and found the Falkland Islands particularly problematical. One possibility, he concluded, was that “they are, along with South Georgia and the South Orkneys, fragments of an ancient ‘Flabellites land’.”
Gregory (1922) illustrated the ‘Brazilio-Ethiopian Continent’ spanning the South Atlantic, and such thinking carried through to his (1929) Presidential Address to the Geological Society of London, wherein he included South Georgia (but not the South Orkney Islands) within a Devonian ‘Flabellites Land’ and both South Georgia and the Falkland Islands within a late Palaeozoic ‘Gondwanaland’ that included the entire area of the South Atlantic Ocean. Clearly Gregory was not convinced by Du Toit’s (1927) reconstruction in which the South Atlantic was eliminated and geological links between Africa and South America were emphasised; these had first been detailed by the Argentine geologist Juan Keidal (1916). In his reconstruction Du Toit moved the Falkland Islands a considerable distance north to align their geology with the structural and stratigraphical trends seen in the two, now contiguous continents. Du Toit (1937) developed his continental drift interpretations and reconstruction of Gondwana in the seminal geological classic Our Wandering Continents. Therein, (1937, figure 7) the Scotia Arc is shown looping into the Palaeozoic ‘Samfrau Geosyncline’, a situation influenced by the presumption that Lower Palaeozoic rocks formed at least part of South Georgia. In the accompanying text (pp.195–196) Du Toit linked the origin of the Scotia Arc islands with that of the Falkland Islands, writing:
“Our interpretation views the near-by Falkland Islands as having drifted from some position off the south-western Cape [South Africa]. South Georgia and the South Orkneys could in similar fashion have been derived from the north-east and inferentially from the Samfrau geosyncline.”
It is worth noting that Du Toit’s assessment of the Falkland Islands was soundly based. The geological association of the Falklands with South Africa had been established from palaeontological evidence in the late 19th and early 20th centuries, as described by Stone and Rushton (2012). Broader geological links had been researched by Halle (1911) and, particularly, by Baker (1924) who emphatically adopted a continental drift solution in principle, but did not propose any specific palaeogeography.
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