Cenozoic tectonic history of the 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 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 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 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 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 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 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. 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

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54° S

SG5.14 36 ± 8 Ma N 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 . 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. 2 for locations and ology of South Georgia represents a missing part To discriminate between competing plate re- summary ages) lend additional support to this of the Fuegian Andes once located to the south construction models and remove uncertainty sur- interpretation. On the northeastern side of South of Isla de los Estados and Burwood Bank (Dal- rounding the pre-breakup location of the South Georgia, a compilation of the results from the ziel et al., 1975). Both areas share common rock Georgia microcontinental block, we compared Cretaceous Sandebugten Formation from the types, depositional ages, and structures that fi t detrital zircon U-Pb age signatures of the Cre- identifi es two distinct phases with a once-extended Rocas Verdes basin that taceous turbidite sequences exposed on South dates back to the breakup of Gondwana when the Georgia with potential source areas; namely, 1GSA Data Repository item 2014112, analytical Patagonian Andes underwent extension. By the the Cordillera. These are the Cordillera Darwin methods, Figures DR1 and DR2, Table DR1 (AFT Early Cretaceous, this extension had led to the complex and the eastern Magallanes foreland data), Table DR2 (AHe data), and Table DR3 (U-Pb ages for detrital zircon grains), is available online at formation of the quasi-oceanic rift basin fi lled basin of South America (Barbeau et al., 2009; www.geosociety.org/pubs/ft2014.htm, or on request by large volumes of silicic volcaniclastic sedi- Hervé et al., 2010; Klepeis et al., 2010), and an from [email protected] or Documents Secre- ments, including turbidites that thicken to the East Gondwana passive margin setting on the tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/42/4/299/3545229/299.pdf by guest on 25 September 2021 Sandebugten Fuegian Andes contemporaneous AFT ages from 90–105 Ma Fm (n=210) (Barbeau et al., 2009, granitoids on the Annekov and Pickersgill Is- post Albian data removed) 109 (n=611) Figure 3. Kernel density lands that show that these rocks were exhumed plots of detrital zircon to shallow crustal levels (<1 km) soon after 171 Permian, West Falkland (n=110) U-Pb ages from Sand- their emplacement. (East Gondwana provenance) ebugten Formation of Between the Late Cretaceous and early Eo- South Georgia Island compared with data cene, the South Georgia block must have been from rocks in Fuegian reburied to low-grade metamorphic tempera- Andes that include Cor- tures as the exhumation data require cooling dillera Darwin complex from ~250 °C close to 45 Ma. This is coincident 275 and eastern Magallanes foreland basin (Barbeau with exhumation data from the Fuegian Andes et al., 2009) and Perm- that record rapid cooling ca. 45–40 Ma (Gom- ian sandstone from West bosi et al., 2009) and a dramatic sediment prov- Falkland representative enance shift ca. 39 Ma in the Magallanes fore- of typical provenance of land basin, interpreted as evidence of rock and eastern Gondwana. 0 150 300 450 600 750 900 1050 1200 1350 1500 surface uplift of the Cordillera Darwin complex and adjacent hinterland thrust sheets (Barbeau South Southern Andean Age (Ma) Patagonian Metamorphic Complexes et al., 2009). A shared exhumation history thus Batholith requires fi nal separation from South America to postdate 45 Ma. The contractional regime that of cooling (Fig. DR1 in the Data Repository) DISCUSSION drove this exhumation and development of the constrained by zircon fi ssion track data (ZFT) Our provenance and exhumation history re- Patagonian orocline by counterclockwise ro- and differential fi ssion track annealing kinetics sults confi rm that the South Georgia block was tation of the Fuegian Andes (Gombosi et al., arising from variations in apatite composition. once connected to the Rocas Verdes backarc 2009) may have contributed to the fi nal breakup. Petrography shows that these rocks were buried basin, most likely east of Navarino Island and The thermochronometry data from South to low-grade metamorphic temperatures within south of the Burdwood Bank (Dalziel et al., Georgia require a second period of kilometer- the prehnite-pumpellyite facies (Stone, 1980) 1975). Evidence from South America shows scale reburial following Eocene exhumation, a and the ZFT data record the end of this meta- that this basin was inverted and obducted onto period that extended through the Oligocene as morphism as ca. 45 Ma, marked by rapid exhu- the continental margin of South America, meta- seafl oor spreading took place in the West Scotia mation to shallow (<2 km) crustal levels; apatite morphosed (upper amphibolite grade), and the Sea. The fi nal phase of exhumation recorded by grains with more resistant compositions form a equivalents of the Early Cretaceous turbidites both AFT and AHe data initiated ca. 10 Ma and distinct population of ca. 40 Ma. Although re- of South Georgia folded before intrusion of was likely related to the effects of collision with burial followed soon after, it was not suffi cient the Late Cretaceous Beagle Suite granitoids the Northeast Georgia Rise ca. 12–9 Ma (Krist- to reset fi ssion tracks across the entire range of (Mukasa and Dalziel, 2009); therefore, South offersen and LaBrecque, 1991; Dalziel et al., apatite compositions (Fig. DR1), so maximum Georgia was likely a topographic feature by the 2013a). Thrust earthquakes are recorded from burial temperatures could not have been much Late Cretaceous. This is supported by nearly both sides of the South Georgia microcontinent, above ~100 °C. Peak burial-related heating in but the main thrusting appears to be onto the the late Miocene was followed by inversion central Scotia Sea fl oor (Eagles, 2010b), consis- ca. 10–7 Ma, constrained by the youngest popu- tent with deeper, more recent exhumation in the lation of FT grain ages (least resistant to FT re- Summary of thermal histories northeastern section of the island. To the south- East coast SG West coast SG setting) and apatite AHe ages. Pickingills Islands west, the Annekov and Pickersgill Islands appear 0 The southwestern side of South Georgia also to have been much more stable, and record lower records post-Eocene reburial, but the depth of 20 levels of reburial and exhumation compared to

burial is less. A west coast sample from the 40 mainland South Georgia to the east. This mark- mostly apatite-barren For- edly different thermal history may be related to mation has an FT central age of 44 Ma, diag- 60 movement on some of the major structures in the

nostic of the Eocene cooling event, but the ac- 80 region that trend northeast-southwest. A possible companying AHe age of 3.2 ± 0.2 Ma requires structural candidate is the mid-Late Cretaceous timing of onset o 100 unconstrained by data

some small-scale reburial and recent exhuma- Temperature (°C) Cooper Bay shear zone (Curtis et al., 2010), the tion in this region. Sampled 90–105 Ma igne- 120 largest exposed structural feature in South Geor- main ous rocks from the Annekov and Pickersgill 140 inversion gia. However, exhumation data collected from Islands yielded apatite FT (AFT) ages with f buria 10-7 Ma both sides of the onshore parts of this structure 160 l long (>14 µm) mean track lengths, close to ? ZFT in the Cooper Bay–Drygalski Fjord region give burial to age the rock formation ages, that testify to long- 180 granite ? ages similar to those from South Georgia. Alter- emplace- prehnite-pumpellyite term residence at near surface temperatures, ment facies metamorphism natively, the inferred offshore contact between 200 although the 6.5 ± 0.2 Ma AHe age from the 100 90 80 70 60 50 40 30 20 10 0 the ophiolitic Complex and the Pickersgill Islands also shows that there must Time (Ma) arc assemblage of the Annekov and Pickersgill have been some burial. Thermal history models Figure 4. Plot summarizing thermal histories Islands (Simpson and Griffi ths, 1982) provides a (Fig. DR1) confi rm this and show that burial of sampled rocks from South Georgia Island likely structural boundary that is consistent with from ca. 40 Ma reached peak temperatures of (SG) and Annekov and Pickersgill Islands. the exhumation data. Timings for inversion and burial events are ~70 °C prior to the initiation of fi nal inversion similar, although magnitudes of burial vary A common provenance and exhumation his- ca. 10–7 Ma. Figure 4 summarizes the thermal with east to west location. ZFT—zircon fi s- tory requires South Georgia to be placed much histories. sion track. closer to during the early

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ACKNOWLEDGMENTS the Pacifi c margin of Gondwana: Geological Some of the fi eld work (by Curtis) in this study was Society of London Journal, v. 167, p. 555–568, Manuscript received 5 September 2013 conducted as part of the Po- doi:10.1144/0016-76492009-124. Revised manuscript received 3 January 2014 lar Science for Planet Earth programme, funded by the Klepeis, K., Betka, P., Clarke, G., Fanning, M., Manuscript accepted 11 January 2014 Natural Environmental Research Council. We thank Hervé, F., Rojas, L., Mpodozis, C., and Thom- Alan Vaughan for help with sampling and location data. son, S., 2010, Continental underthrusting and Printed in USA

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