OR/15/058 After the Falklands conflict: geological research since 1982

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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.

As an adjunct to the 1982 Falklands Islands conflict, South Georgia was also briefly occupied by an Argentine military force, and whilst these events were immensely damaging, they did raise the international profile of the island. Perhaps as a result, the first post-war geological research was carried out by members of a private expedition: the 1984 New Zealand South Georgia Expedition. During the 1984–85 austral summer the area inland from Royal Bay, between the Heaney and Weddell glaciers, was examined, in effect extending inland the earlier study by Stone (1980)[1]. Arising from this work, Craw and Turnbull (1986)[2] gave a description of the geology which, to the east of Mount Brooker, included the thrust contact between the Cumberland Bay and Sandebugten formations. Bivalve fossils indicative of an Early Cretaceous age were discovered in Cumberland Bay Formation strata on the south side of the Ross Pass. Two discrete structural domains were defined in the Cumberland Bay Formation, one with a single phase of deformation, the other with two phases, and these were contrasted with a third domain described from the Sandebugten Formation outcrop. The structural interpretation was taken further by Turnbull and Craw (1988)[3] who saw similarities in tectonic style between the Sandebugten Formation and the most deformed parts of the Cumberland Bay Formation. They also challenged the regional model wherein the contrasting Cumberland Bay and Sandebugten sedimentary lithologies were derived from opposite sides of a depositional basin. Instead, Turnbull and Craw cited examples of arc-derived sediment from the Pacific margins in support of derivation of all of the South Georgia sedimentary divisions from one evolved volcanic arc with a sialic basement.

Geological work by the British Antarctic Survey recommenced in the 1988–89 austral summer when T Alabaster revisited the Larsen Harbour Formation, confirmed the chronology of lava eruption and dyke intrusion and carried out detailed geochemical sampling in the Smaaland Cove area. From the geochemical data obtained, five basalt groups were recognized (Alabaster and Storey 1990)[4]: the oldest three derived from magmas generated during the early stages of continental lithospheric extension, the younger two groups were thought similar to some mid-ocean ridge basalts. Overall, Alabaster and Storey favoured eruption of the Larsen Harbour Formation at an oblique-slip continental margin (the Gulf of California provided an analogy) rather than in the supra-subduction, back-arc setting that had been preferred previously.

Alabaster and Storey (1990)[4] were able to cite a U-Pb date of ca 150 Ma from zircon in the Smaaland Cove pluton that had been presented in conference abstracts by Mukasa and others (1988, 1989), a team based at the universities of Michigan and Texas, USA. Full details of this radiometric anlaysis were not published for some years, until Mukasa and Dalziel (1996)[5] provided full results for this and several other dates from the southernmost Andes. The 150±1 Ma age given for the Smaaland Cove pluton provided a minimum age for the Larsen Harbour Formation basalts because the pluton intruded the lowermost part of the lava succession. Radiometric ages for the pluton that had been previously published for the pluton by Tanner and Rex (1979)[6] were younger: 78±3 Ma, K-Ar hornblende; 127±4 Ma, Rb-Sr whole–rock. As had been suspected by Tanner and Rex, these younger ages were regarded by Mukasa and Dalziel as the result of partial resetting by hydrothermal alteration and metamorphism, coupled with sparse original data.

The relationship between the igneous complexes in the south–east of South Georgia and the adjacent metasedimentary Cooper Bay Formation was further investigated by M L Curtis of the British Antarctic Survey during three austral summers between 2005 and 2009. From the Cooper Bay Shear Zone Curtis (2007)[7] described two deformational phases. The earliest displacement was associated with dip-slip reverse shear and resulted in mylonitised granitic rocks along the south-west margin of the shear zone. To the north-east, the mylonitised margin of the Cooper Bay Formation’s outcrop arose from sinistral strike-slip shear. Superimposition of narrow sinistral shear bands on the dip-slip granitic mylonites suggested that the sinistral shear followed the dip-slip shear. The interpretation was carried forward by Curtis and others (2010)[8] in terms of kinematic partitioning with the timing of shear displacement constrained by radiometric dating. Two U-Pb ages from zircon in pre-tectonic granitic rocks caught up in the shear zone were in agreement at about 160 Ma. In contrast, a Rb-Sr age of ca 83 Ma from biotite in a schist at Cooper Bay was interpreted as recording uplift and exhumation shortly after the first episode of shearing and metamorphism to affect the area. Curtis and others (2010)[8] stressed the analogy of their model for the Cooper Bay Shear Zone to interpretations of the main Andean deformation in the Cordillera Darwin, Patagonia.

As an additional part of his South Georgia field programme, Curtis field-tested digital mapping equipment and applications being developed by BAS for more general use (Curtis and others 2011)[9]. This involved visits to a number of localities within the outcrops of the Sandebugten, Cumberland Bay and Annenkov Island formations, where specimens were collected and subsequently utilised for detrital zircon dating and apatite fission-track analysis and thermocronometry. This was a collaborative project between BAS and Birkbeck and University colleges, London, with results from South Georgia and the southern Andes compared and correlated by Carter and others (2014)[10]. The detrital zircon population extracted from two Sandebugten Formation sandstones matched that seen in the Rocas Verdes back-arc basin, southern Andes, making an original close association highly likely. The apatite results were interpreted in terms of the separation of South Georgia from South America at about 45 Ma, followed by a kilometre-scale reburial until uplift and exhumation from about 10 Ma onwards. Carter and others’ (2014)[10] integration of South Georgia geology into models for the overall development of the Scotia Arc was the latest in a long series of such attempts.

References

  1. STONE, P. 1980. The Geology of South Georgia: IV. Barff Peninsula and Royal Bay areas. British Antarctic Survey Scientific Reports, No.96, 45 pp + 8 plates.
  2. CRAW, D, and TURNBULL, I M. 1986. Geological observations in the Ross Glacier area, South Georgia. British Antarctic Survey Bulletin, 71, 1–10.
  3. TURNBULL, I M, and Craw, D. 1988. Relationships between the Cumberland Bay and Sandebugten Formations, South Georgia, and some tectonic implications. Journal of the Geological Society, London, 145, 591–602.
  4. 4.0 4.1 ALABASTER, T, and STOREY, B C. 1990. Modified Gulf of California model for South Georgia, north Scotia Ridge, and implications for the Rocas Verdes back-arc basin, southern Andes. Geology, 18, 497–500.
  5. MUKASA, S B, and DALZIEL, I W D. 1996. Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite 40Ar/39Ar age constraints on tectonic evolution of Southwestern Gondwanaland. Journal of South American Earth Sciences, 9, 349–365.
  6. TANNER, P W G, and REX, D C. 1979. Timing of events in an Early Cretaceous island arc–marginal basin system on South Georgia. Geological Magazine, 116, 167–179.
  7. CURTIS, M L. 2007. Main Andean sinistral shear along the Cooper Bay Dislocation Zone, South Georgia? In: Cooper, A.K., Raymond, C.R. and others (eds). Antarctica: A Keystone in a Changing World — Online Proceedings of the10th ISAES. USGS Open-File Report 2007–1047, Short Research Paper 034, 4 pp.
  8. 8.0 8.1 CURTIS, M L, FLOWEDEW, M J, RILEY, T R, WHITEHOUSE, M J, and DALY, J S. 2010. Andean sinistral transpression and kinematic partitioning in South Georgia. Journal of Structural Geology, 32, 464–477.
  9. CURTIS, M L, and RILEY, T R. 2011. Geological Map of South Georgia (1:250 000 scale). BAS GEOMAP 2 Series, Sheet 4, British Antarctic Survey, Cambridge, UK.
  10. 10.0 10.1 CARTER, A, CURTIS, M, and SCHWANETHAL, J. 2014. Cenozoic tectonic history of the South Georgia microcontient and potential as a barrier to Pacific-Atlantic through flow. Geology, 42, 295–298. doi: 10.1130/G35091.1.