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Cenozoic Tectonic History of the South Georgia Microcontinent and Potential As a Barrier to Pacifi C-Atlantic Through fl Ow

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Cenozoic Tectonic History of the South Georgia Microcontinent and Potential As a Barrier to Pacifi C-Atlantic Through fl Ow Cenozoic tectonic history of the South Georgia microcontinent and potential as a barrier to Pacifi c-Atlantic through fl ow Andrew Carter1, Mike Curtis2, and James Schwanethal3 1Department of Earth and Planetary Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK, and CASP, West Building 181A, Huntingdon, Cambridge CB3 ODH, UK 3Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK ABSTRACT seafl oor spreading location was at the eastern Cenozoic opening of the central Scotia Sea involved the tectonic translation of crustal blocks end of the North Scotia Ridge and that South to form the North Scotia Ridge, which today is a major topographic constriction to the fl ow of Georgia once belonged to part of an extended the deep Antarctic Circumpolar Current that keeps Antarctica thermally isolated from warmer continental margin along the Falkland Plateau ocean waters. How this ridge developed and whether it was a topographic barrier in the past are that formed as Gondwana broke up in Jurassic unknown. To address this we investigated the Cenozoic history of the South Georgia microcon- time (Eagles, 2010a). The motivation for this tinental block, the exposed part of the ridge. Detrital zircon U-Pb geochronology data confi rm model was driven by the need to explain an ap- that the Cretaceous succession of turbidites exposed on South Georgia was stratigraphically parent defi cit in the translation of South Geor- connected to the Rocas Verdes backarc basin, part of the South America plate. Apatite thermo- gia accounted for by seafl oor spreading based chronometry results show that South Georgia had remained connected to South America until on a South American origin. Restoration using ca. 45–40 Ma; both record a distinct rapid cooling event at that time. Subsequent separation plate kinematic evidence can only account for from South America was accompanied by kilometer-scale reburial until inversion ca. 10 Ma, approximately half of the ~1600 km displace- coeval with the cessation of spreading at the West Scotia Ridge and collision between the South ment (Eagles et al., 2005). A position for South Georgia block and the Northeast Georgia Rise. Our results show that the South Georgia micro- Georgia on the Pacifi c margin of Gondwana continental block could not have been an emergent feature from ca. 40 Ma until 10 Ma. would require less transport to the east during opening of the Scotia Sea; it would mean that INTRODUCTION deformation structures in the Andean Cordil- the South Georgia block could not have served Considerable effort has been directed at un- lera, that drove inversion of the marginal ba- as an early proximal barrier to deep Pacifi c- derstanding the geological evolution of the Sco- sins, and the obduction of the Rocas Verdes Atlantic fl ow. To resolve these issues we exam- tia Sea region as seafl oor spreading in the West ophiolitic basement onto the continental mar- ined the provenance of Cretaceous turbidites Scotia Sea caused the opening of the deep Drake gin can be followed along strike from Tierra exposed on South Georgia using detrital zircon Passage oceanic gateway that paved the way for del Fuego into South Georgia (Dalziel et al., U-Pb geochronology and studied the island’s the thermal isolation of Antarctica by the deep 2013a). This phase of deformation is believed bedrock exhumation history using apatite ther- Antarctic Circumpolar Current (ACC) (Dalziel to have caused uplift of the North Scotia Ridge mochronometry. et al., 2013a, 2013b). Because models of the and may have also initiated eastward transla- evolution of the ACC are tied to the tectonic tion of the South Georgia microcontinent by GEOLOGY reconstructions that restore microcontinental left-lateral ductile shearing. The geology of South Georgia (Fig. 2) is blocks and volcanic arcs to pre–seafl oor spread- However, plate kinematic data have, con- central to the debate about the original loca- ing locations, it is essential that pre-drift loca- troversially, been used to suggest that the pre– tion of this microcontinental block and its role tions are well defi ned. Furthermore, because the three main fronts to the modern ACC are steered by regional bathymetry (Fig. 1), models of the 75°W 65°W 55°W 45°W 35°W 25°W ancient ACC need to incorporate constraints as to where and when crustal blocks were barriers South Maurice 50°S- to ocean currents. Today the Subantarctic Front American Falkland Plateau and the Polar Front follow gaps in the North Ewing NE Plate Bank Georgia Scotia Ridge while the Southern Antarctic Cir- Magallanes Rise N. Scotia Ridge cumpolar Current Front takes an eastward path Fuegian AndesBasin Study 54°S- before heading north, turning around the eastern Area end of South Georgia; however, how much of a W. Scotia Shack Scotia Plate E barrier these ridges were in the past is unknown South leton Fracture zoneSea Scotia Sandwich 58°S- due in part to uncertainty about their pre-break- Sub-Antarctic Front Drake Sea Plate up location and subsequent drift history. Passage The conventional view (Dalziel et al., 1975, Polar Front 2013a; Livermore et al., 2007), based on inter- S. Scotia Ridge Southern Antarctic pretations that match the geology of the South 62°S- Circumpolar Antarctic Plate Georgia microcontinent with South America, Current Front Weddell Sea is that originally South Georgia occupied a position to the immediate southeast of Tierra Figure 1. Scotia Arc region; study area (Fig. 2), principal topographic features, and main fronts of Antarctic Circumpolar Current are indicated (generated by GeoMapApp; del Fuego from the Jurassic until the Ceno- www.geomapapp.org). Red lines show positions of Sub-Antarctic Front and South- zoic, when seafl oor spreading created the west ern Antarctic Circumpolar Current Front (Orsi et al., 1995) and Polar Front (Moore Scotia Sea. Late Cretaceous compressional et al., 1997). GEOLOGY, April 2014; v. 42; no. 4; p. 299–302; Data Repository item 2014112 | doi:10.1130/G35091.1 | Published online 10 February 2014 GEOLOGY© 2014 Geological | April Society 2014 | ofwww.gsapubs.org America. Gold Open Access: This paper is published under the terms of the CC-BY license. 299 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/42/4/299/3545229/299.pdf by guest on 25 September 2021 38° W 37°30' 37° 36°30' 36° W 54° S SG5.14 36 ± 8 Ma N STROMNESS BAY AY SG467 SG369 12 ± 3 Ma 15 ± 3 Ma BARF PENINSULA CUMBERLAND B SG534 F 16 ± 2 Ma Late SG241 *SG389 10± 2 Ma 20 ± 3 Ma SG14 SG246 Annenkov SG1 *SG394 44 ± 5 Ma 17 ± 3 Ma Cumber- Sande- Island 80 ± 4 Ma Early 3.2 ± 0.2 Ma land Bay bugten Cooper Formation 27 ± 10 Ma Formation Formation Bay SG530 Formation 11 ± 1 Ma ANNENKOV Late SG9 Granitoid ISLAND Drygalski Larsen intrusions 79 ± 8 Ma PICKERSGILL Fjord Harbour Complex Complex ISLANDS SG11 Early 88 ± 4 Ma SG7 Salomon Novosilski Cooper 6.5 ± 0.5 Ma 10 ± 1 Ma Glacier Glacier Island Formation Formation Formation COOPER km ISLAND Sample No.: SG1 0102030 Pre-Jurassic Jurassic Cretaceous AFT Age: 80 ± 4 Ma SG22 AHe Age: 33 ± 19 Ma SG24 12 ± 2 Ma * SGxx used for detrital zircon U-Pb SG19 12 ± 2 Ma 21 ± 3 Ma Figure 2. Geological map of South Georgia Island; locations and apatite fi ssion track (AFT) central ages and ejection-corrected (U-Th)/He ages (AHe) of sampled rocks are indicated; map based on Curtis and Riley (2011). Age uncertainties are 1σ. during the opening of the Drake Passage. The south. The Rocas Verdes basin and continental edge of the Falkland Plateau, represented by a majority of the rock exposure is formed by two margin arc rocks terminate along the strike of Permian sample from the Falkland Islands (see laterally equivalent turbidite sequences depos- the mid-Cretaceous structures at the continental the GSA Data Repository1 for analytical details). ited by deep-sea fans in an Early Cretaceous margin immediately to the east of Isla Navarino, The results (Fig. 3) show a remarkable match to backarc basin. The 8-km-thick Cumberland leaving oceanic lithosphere to the south of Isla sources from the South Patagonian batholith, Bay Formation, which crops out over half of de los Estados and Burdwood Bank. The turbi- Jurassic volcanics, and the south Andean meta- the island, is a classic turbidite succession com- dite sequences that crop out on South Georgia morphic basement. The data do not fi t with an posed of andesitic volcaniclastic graywackes are therefore viewed as the missing part of the East Gondwana provenance (Eagles, 2010a) be- derived from a volcanic island arc (Tanner Fuegian Andes. By contrast, the alternative mod- cause Proterozoic to Cambrian age zircons are and MacDonald, 1982). The Sandebugten For- el for South Georgia, based on a passive margin largely absent. Our detrital zircon data thus sup- mation is also composed of turbiditic facies setting on the southern edge of the Falkland Pla- port a connection to the Rocas Verdes backarc rocks that are distinguished by their siliciclas- teau, suggests that other volcanic centers, such basin during the Early Cretaceous, as originally tic composition and the presence of trachytic as the Polarstern Bank near the southeast margin suggested by Dalziel et al. (1975). and dacitic fragments and felsitic and granitic of the Weddell Sea, could account for the silicic Apatite and zircon fi ssion track and apatite clasts sourced from the continental margin of a volcanic detritus (Eagles, 2010a). (U-Th)/He thermochronometry (AHe) results backarc basin (MacDonald et al., 1987). from bedrock samples (for analytical details, see The conventional view considers that the ge- RESULTS AND INTERPRETATION the Data Repository; see Fig.
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