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Cenozoic tectonic history of the South Georgia microcontinent and potential as a barrier to Pacific-Atlantic through flow


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Cenozoic opening of the central Scotia Sea involved the tectonic translation of crustal blocks to form the North Scotia Ridge, which today is a major topographic constriction to the fl ow of the deep Antarctic Circumpolar Current that keeps Antarctica thermally isolated from warmer ocean waters. How this ridge developed and whether it was a topographic barrier in the past are unknown. To address this we investigated the Cenozoic history of the South Georgia microcontinental block, the exposed part of the ridge. Detrital zircon U-Pb geochronology data confirm that the Cretaceous succession of turbidites exposed on South Georgia was stratigraphically connected to the Rocas Verdes backarc basin, part of the South America plate. Apatite thermochronometry results show that South Georgia had remained connected to South America until ca. 45–40 Ma; both record a distinct rapid cooling event at that time. Subsequent separation from South America was accompanied by kilometer-scale reburial until inversion ca. 10 Ma, coeval with the cessation of spreading at the West Scotia Ridge and collision between the South Georgia block and the Northeast Georgia Rise. Our results show that the South Georgia microcontinental block could not have been an emergent feature from ca. 40 Ma until 10 Ma.
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doi: 10.1130/G35091.1
Andrew Carter, Mike Curtis and James Schwanethal
as a barrier to Pacific-Atlantic through flow
Cenozoic tectonic history of the South Georgia microcontinent and potential
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Considerable effort has been directed at un-
derstanding the geological evolution of the Sco-
tia Sea region as seafl oor spreading in the West
Scotia Sea caused the opening of the deep Drake
Passage oceanic gateway that paved the way for
the thermal isolation of Antarctica by the deep
Antarctic Circumpolar Current (ACC) (Dalziel
et al., 2013a, 2013b). Because models of the
evolution of the ACC are tied to the tectonic
reconstructions that restore microcontinental
blocks and volcanic arcs to pre–seafl oor spread-
ing locations, it is essential that pre-drift loca-
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
ancient ACC need to incorporate constraints as
to where and when crustal blocks were barriers
to ocean currents. Today the Subantarctic Front
and the Polar Front follow gaps in the North
Scotia Ridge while the Southern Antarctic Cir-
cumpolar Current Front takes an eastward path
before heading north, turning around the eastern
end of South Georgia; however, how much of a
barrier these ridges were in the past is unknown
due in part to uncertainty about their pre-break-
up location and subsequent drift history.
The conventional view (Dalziel et al., 1975,
2013a; Livermore et al., 2007), based on inter-
pretations that match the geology of the South
Georgia microcontinent with South America,
is that originally South Georgia occupied a
position to the immediate southeast of Tierra
del Fuego from the Jurassic until the Ceno-
zoic, when seafl oor spreading created the west
Scotia Sea. Late Cretaceous compressional
deformation structures in the Andean Cordil-
lera, that drove inversion of the marginal ba-
sins, and the obduction of the Rocas Verdes
ophiolitic basement onto the continental mar-
gin can be followed along strike from Tierra
del Fuego into South Georgia (Dalziel et al.,
2013a). This phase of deformation is believed
to have caused uplift of the North Scotia Ridge
and may have also initiated eastward transla-
tion of the South Georgia microcontinent by
left-lateral ductile shearing.
However, plate kinematic data have, con-
troversially, been used to suggest that the pre–
seafl oor spreading location was at the eastern
end of the North Scotia Ridge and that South
Georgia once belonged to part of an extended
continental margin along the Falkland Plateau
that formed as Gondwana broke up in Jurassic
time (Eagles, 2010a). The motivation for this
model was driven by the need to explain an ap-
parent defi cit in the translation of South Geor-
gia accounted for by seafl oor spreading based
on a South American origin. Restoration using
plate kinematic evidence can only account for
approximately half of the ~1600 km displace-
ment (Eagles et al., 2005). A position for South
Georgia on the Pacifi c margin of Gondwana
would require less transport to the east during
opening of the Scotia Sea; it would mean that
the South Georgia block could not have served
as an early proximal barrier to deep Pacifi c-
Atlantic fl ow. To resolve these issues we exam-
ined the provenance of Cretaceous turbidites
exposed on South Georgia using detrital zircon
U-Pb geochronology and studied the island’s
bedrock exhumation history using apatite ther-
The geology of South Georgia (Fig. 2) is
central to the debate about the original loca-
tion of this microcontinental block and its role
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
British 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
Cenozoic opening of the central Scotia Sea involved the tectonic translation of crustal blocks
to form the North Scotia Ridge, which today is a major topographic constriction to the fl ow of
the deep Antarctic Circumpolar Current that keeps Antarctica thermally isolated from warmer
ocean waters. How this ridge developed and whether it was a topographic barrier in the past are
unknown. To address this we investigated the Cenozoic history of the South Georgia microcon-
tinental block, the exposed part of the ridge. Detrital zircon U-Pb geochronology data confi rm
that the Cretaceous succession of turbidites exposed on South Georgia was stratigraphically
connected to the Rocas Verdes backarc basin, part of the South America plate. Apatite thermo-
chronometry results show that South Georgia had remained connected to South America until
ca. 45–40 Ma; both record a distinct rapid cooling event at that time. Subsequent separation
from South America was accompanied by kilometer-scale reburial until inversion ca. 10 Ma,
coeval with the cessation of spreading at the West Scotia Ridge and collision between the South
Georgia block and the Northeast Georgia Rise. Our results show that the South Georgia micro-
continental block could not have been an emergent feature from ca. 40 Ma until 10 Ma.
GEOLOGY, April 2014; v. 42; no. 4; p. 299–302; Data Repository item 2014112
Published online 10 February 2014
© 2014 Geological Society of America. Gold Open Access: This paper is published under the terms of the CC-BY license.
75°W 65°W 55°W 45°W 35°W 25°W
N. Scotia Ridge
S. Scotia Ridge
Falkland Plateau NE
Antarctic Plate
Weddell Sea
Fuegian Andes
Southern Antarctic
Current Front
Shackleton Fracture zone
Figure 1. Scotia Arc region; study area (Fig. 2), principal topographic features, and
main fronts of Antarctic Circumpolar Current are indicated (generated by GeoMapApp; Red lines show positions of Sub-Antarctic Front and South-
ern Antarctic Circumpolar Current Front (Orsi et al., 1995) and Polar Front (Moore
et al., 1997).
on March 15, 2014geology.gsapubs.orgDownloaded from
April 2014
during the opening of the Drake Passage. The
majority of the rock exposure is formed by two
laterally equivalent turbidite sequences depos-
ited by deep-sea fans in an Early Cretaceous
backarc basin. The 8-km-thick Cumberland
Bay Formation, which crops out over half of
the island, is a classic turbidite succession com-
posed of andesitic volcaniclastic graywackes
derived from a volcanic island arc (Tanner
and MacDonald, 1982). The Sandebugten For-
mation is also composed of turbiditic facies
rocks that are distinguished by their siliciclas-
tic composition and the presence of trachytic
and dacitic fragments and felsitic and granitic
clasts sourced from the continental margin of a
backarc basin (MacDonald et al., 1987).
The conventional view considers that the ge-
ology of South Georgia represents a missing part
of the Fuegian Andes once located to the south
of Isla de los Estados and Burwood Bank (Dal-
ziel et al., 1975). Both areas share common rock
types, depositional ages, and structures that fi t
with a once-extended Rocas Verdes basin that
dates back to the breakup of Gondwana when the
Patagonian Andes underwent extension. By the
Early Cretaceous, this extension had led to the
formation of the quasi-oceanic rift basin fi lled
by large volumes of silicic volcaniclastic sedi-
ments, including turbidites that thicken to the
south. The Rocas Verdes basin and continental
margin arc rocks terminate along the strike of
the mid-Cretaceous structures at the continental
margin immediately to the east of Isla Navarino,
leaving oceanic lithosphere to the south of Isla
de los Estados and Burdwood Bank. The turbi-
dite sequences that crop out on South Georgia
are therefore viewed as the missing part of the
Fuegian Andes. By contrast, the alternative mod-
el for South Georgia, based on a passive margin
setting on the southern edge of the Falkland Pla-
teau, suggests that other volcanic centers, such
as the Polarstern Bank near the southeast margin
of the Weddell Sea, could account for the silicic
volcanic detritus (Eagles, 2010a).
To discriminate between competing plate re-
construction models and remove uncertainty sur-
rounding the pre-breakup location of the South
Georgia microcontinental block, we compared
detrital zircon U-Pb age signatures of the Cre-
taceous turbidite sequences exposed on South
Georgia with potential source areas; namely,
the Cordillera. These are the Cordillera Darwin
complex and the eastern Magallanes foreland
basin of South America (Barbeau et al., 2009;
Hervé et al., 2010; Klepeis et al., 2010), and an
East Gondwana passive margin setting on the
edge of the Falkland Plateau, represented by a
Permian sample from the Falkland Islands (see
the GSA Data Repository1 for analytical details).
The results (Fig. 3) show a remarkable match to
sources from the South Patagonian batholith,
Jurassic volcanics, and the south Andean meta-
morphic basement. The data do not fi t with an
East Gondwana provenance (Eagles, 2010a) be-
cause Proterozoic to Cambrian age zircons are
largely absent. Our detrital zircon data thus sup-
port a connection to the Rocas Verdes backarc
basin during the Early Cretaceous, as originally
suggested by Dalziel et al. (1975).
Apatite and zircon fi ssion track and apatite
(U-Th)/He thermochronometry (AHe) results
from bedrock samples (for analytical details, see
the Data Repository; see Fig. 2 for locations and
summary ages) lend additional support to this
interpretation. On the northeastern side of South
Georgia, a compilation of the results from the
Cretaceous Sandebugten Formation from the
Barff Peninsula identifi es two distinct phases
36° W36°30' 37°30'38° W
54° S
80 ± 4 Ma
27 ± 10 Ma
79 ± 8 Ma
12 ± 3 Ma SG369
15 ± 3 Ma
20 ± 3 Ma
10± 2 Ma
10 ± 1 Ma
21 ± 3 Ma
17 ± 3 Ma
12 ± 2 Ma
88 ± 4 Ma
6.5 ± 0.5 Ma
12 ± 2 Ma
16 ± 2 Ma
11 ± 1 Ma
36 ± 8 Ma
44 ± 5 Ma
3.2 ± 0.2 Ma
Sample No.: SG1
AFT Age: 80 ± 4 Ma
AHe Age: 33 ± 19 Ma
* SGxx used for detrital zircon U-Pb
land Bay
Pre-Jurassic Jurassic Cretaceous
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.
1GSA Data Repository item 2014112, analytical
methods, Figures DR1 and DR2, Table DR1 (AFT
data), Table DR2 (AHe data), and Table DR3 (U-Pb
ages for detrital zircon grains), is available online at, or on request
from or Documents Secre-
tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
on March 15, 2014geology.gsapubs.orgDownloaded from
April 2014
| 301
of cooling (Fig. DR1 in the Data Repository)
constrained by zircon fi ssion track data (ZFT)
and differential fi ssion track annealing kinetics
arising from variations in apatite composition.
Petrography shows that these rocks were buried
to low-grade metamorphic temperatures within
the prehnite-pumpellyite facies (Stone, 1980)
and the ZFT data record the end of this meta-
morphism as ca. 45 Ma, marked by rapid exhu-
mation to shallow (<2 km) crustal levels; apatite
grains with more resistant compositions form a
distinct population of ca. 40 Ma. Although re-
burial followed soon after, it was not suffi cient
to reset fi ssion tracks across the entire range of
apatite compositions (Fig. DR1), so maximum
burial temperatures could not have been much
above ~100 °C. Peak burial-related heating in
the late Miocene was followed by inversion
ca. 10–7 Ma, constrained by the youngest popu-
lation of FT grain ages (least resistant to FT re-
setting) and apatite AHe ages.
The southwestern side of South Georgia also
records post-Eocene reburial, but the depth of
burial is less. A west coast sample from the
mostly apatite-barren Cumberland Bay For-
mation has an FT central age of 44 Ma, diag-
nostic of the Eocene cooling event, but the ac-
companying AHe age of 3.2 ± 0.2 Ma requires
some small-scale reburial and recent exhuma-
tion in this region. Sampled 90–105 Ma igne-
ous rocks from the Annekov and Pickersgill
Islands yielded apatite FT (AFT) ages with
long (>14 µm) mean track lengths, close to
the rock formation ages, that testify to long-
term residence at near surface temperatures,
although the 6.5 ± 0.2 Ma AHe age from the
Pickersgill Islands also shows that there must
have been some burial. Thermal history models
(Fig. DR1) confi rm this and show that burial
from ca. 40 Ma reached peak temperatures of
~70 °C prior to the initiation of fi nal inversion
ca. 10–7 Ma. Figure 4 summarizes the thermal
Our provenance and exhumation history re-
sults confi rm that the South Georgia block was
once connected to the Rocas Verdes backarc
basin, most likely east of Navarino Island and
south of the Burdwood Bank (Dalziel et al.,
1975). Evidence from South America shows
that this basin was inverted and obducted onto
the continental margin of South America, meta-
morphosed (upper amphibolite grade), and the
equivalents of the Early Cretaceous turbidites
of South Georgia folded before intrusion of
the Late Cretaceous Beagle Suite granitoids
(Mukasa and Dalziel, 2009); therefore, South
Georgia was likely a topographic feature by the
Late Cretaceous. This is supported by nearly
contemporaneous AFT ages from 90–105 Ma
granitoids on the Annekov and Pickersgill Is-
lands that show that these rocks were exhumed
to shallow crustal levels (<1 km) soon after
their emplacement.
Between the Late Cretaceous and early Eo-
cene, the South Georgia block must have been
reburied to low-grade metamorphic tempera-
tures as the exhumation data require cooling
from ~250 °C close to 45 Ma. This is coincident
with exhumation data from the Fuegian Andes
that record rapid cooling ca. 45–40 Ma (Gom-
bosi et al., 2009) and a dramatic sediment prov-
enance shift ca. 39 Ma in the Magallanes fore-
land basin, interpreted as evidence of rock and
surface uplift of the Cordillera Darwin complex
and adjacent hinterland thrust sheets (Barbeau
et al., 2009). A shared exhumation history thus
requires fi nal separation from South America to
postdate 45 Ma. The contractional regime that
drove this exhumation and development of the
Patagonian orocline by counterclockwise ro-
tation of the Fuegian Andes (Gombosi et al.,
2009) may have contributed to the fi nal breakup.
The thermochronometry data from South
Georgia require a second period of kilometer-
scale reburial following Eocene exhumation, a
period that extended through the Oligocene as
seafl oor spreading took place in the West Scotia
Sea. The fi nal phase of exhumation recorded by
both AFT and AHe data initiated ca. 10 Ma and
was likely related to the effects of collision with
the Northeast Georgia Rise ca. 12–9 Ma (Krist-
offersen and LaBrecque, 1991; Dalziel et al.,
2013a). Thrust earthquakes are recorded from
both sides of the South Georgia microcontinent,
but the main thrusting appears to be onto the
central Scotia Sea fl oor (Eagles, 2010b), consis-
tent with deeper, more recent exhumation in the
northeastern section of the island. To the south-
west, the Annekov and Pickersgill Islands appear
to have been much more stable, and record lower
levels of reburial and exhumation compared to
mainland South Georgia to the east. This mark-
edly different thermal history may be related to
movement on some of the major structures in the
region that trend northeast-southwest. A possible
structural candidate is the mid-Late Cretaceous
Cooper Bay shear zone (Curtis et al., 2010), the
largest exposed structural feature in South Geor-
gia. However, exhumation data collected from
both sides of the onshore parts of this structure
in the Cooper Bay–Drygalski Fjord region give
ages similar to those from South Georgia. Alter-
natively, the inferred offshore contact between
the ophiolitic Larsen Harbour Complex and the
arc assemblage of the Annekov and Pickersgill
Islands (Simpson and Griffi ths, 1982) provides a
likely structural boundary that is consistent with
the exhumation data.
A common provenance and exhumation his-
tory requires South Georgia to be placed much
closer to Tierra del Fuego during the early
Fm (n=210)
0 150 300 450 600 750 900 1050
Fuegian Andes
(Barbeau et al., 2009,
post Albian data removed)
Southern Andean
Metamorphic Complexes
1200 1350 1500
Permian, West Falkland (n=110)
(East Gondwana provenance)
Age (Ma)
Time (Ma)
100 90 80 70 60 50 40 30 20 10 0
Temperature (°C)
Summary of thermal histories
East coast SG West coast SG
Pickingills Islands
burial to
facies metamorphism
10-7 Ma
timing of onset of burial
unconstrained by data
Figure 3. Kernel density
plots of detrital zircon
U-Pb ages from Sand-
ebugten Formation of
South Georgia Island
compared with data
from rocks in Fuegian
Andes that include Cor-
dillera Darwin complex
and eastern Magallanes
foreland basin (Barbeau
et al., 2009) and Perm-
ian sandstone from West
Falkland representative
of typical provenance of
eastern Gondwana.
Figure 4. Plot summarizing thermal histories
of sampled rocks from South Georgia Island
(SG) and Annekov and Pickersgill Islands.
Timings for inversion and burial events are
similar, although magnitudes of burial vary
with east to west location. ZFT—zircon fi s-
sion track.
on March 15, 2014geology.gsapubs.orgDownloaded from
April 2014
Cenozoic than shown by recent reconstructions,
which acknowledge the lack of previous con-
straints (Lawver et al., 2011). What took place
after separation is important to help understand
the development of the deep ACC. Our results
show that following breakup, during its drift
eastward South Georgia could not have been an
elevated block, as the thermal history models
require increased levels of heating due to kilo-
meter-scale reburial throughout the Oligocene
and early Miocene. This reheating must be due
to burial, because (1) elevated geotherms asso-
ciated with seafl oor spreading are constrained
by slow rates of conductive transfer which are
unlikely to have much impact on the adjacent
continental crust; and (2) more important, our
data from the Annekov and Pickersgill Islands
(closest to oceanic crust) show no evidence of
Oligocene reheating. At geothermal gradients in
the range of 20–30 °C/km, the thermal history
models require between 2 and 4 km of burial.
Therefore, during the opening of the Scotia
Sea and Drake Passage, South Georgia would
have been submerged; however, whether it was
a major barrier to Pacifi c to Atlantic through
ow would depend on the bathymetry at that
time. Submerged features such as the Camp-
bell Plateau south of New Zealand (bathymetry
500 m) can defl ect the ACC, but such plateaus
have low sedimentation rates, and they would
need to subside further in order to accommodate
the several kilometers of sediment indicated by
the apatite thermochronometry data.
We have confi rmed that the South Georgia
microcontinent was originally connected to the
Rocas Verdes backarc basin of South America
until the Eocene. As part of the South America
plate it would have been a topographic feature
during the two main regional deformation events
in the Late Cretaceous and Eocene. Kilometer-
scale reburial after the Eocene is required by
the thermochronometry data, so South Georgia
must have been submerged until the fi nal stage
of surface uplift that initiated ca. 10 Ma, linked
to the collision between the South Georgia mi-
crocontinent and the Northeast Georgia Rise.
Some of the fi eld work (by Curtis) in this study was
conducted as part of the British Antarctic Survey Po-
lar Science for Planet Earth programme, funded by the
Natural Environmental Research Council. We thank
Alan Vaughan for help with sampling and location data.
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Tanner, P.W.G., and MacDonald, D.I.M., 1982, Mod-
els for the deposition and simple shear deforma-
tion of a turbidite sequence in the South Georgia
portion of the southern Andes back-arc basin:
Geological Society of London Journal, v. 139,
p. 739–754, doi:10.1144/gsjgs.139.6.0739.
Manuscript received 5 September 2013
Revised manuscript received 3 January 2014
Manuscript accepted 11 January 2014
Printed in USA
on March 15, 2014geology.gsapubs.orgDownloaded from
... position immediately east of Cape Horn and due south of Isla de los Estados and Burdwood Bank, which we take to be our null hypothesis; however, other plate reconstruction models resulting from study of geophysical data from the South Atlantic Ocean, the Scotia Sea and the Weddell Sea would place the SGM off Maurice Ewing Bank much farther to the east (Eagles, 2010b;Eagles & Eisermann, 2020). Constraining the initial position of the SGM is important for understanding the paleogeography of this southern margin of the South American continent as it evolved during the late Mesozoic and Cenozoic, playing a critical role in the onset and development of the Antarctic Circumpolar Current (Carter et al., 2014;Dalziel, Lawver, Norton, et al., 2013;Dalziel, Lawver, Pearce, et al., 2013) and hence influencing global climate. ...
... There is thus a close correlation with the rocks of the Fuegian Andes (Dalziel et al., 2021). Moreover, the provenance of the turbidites on South Georgia can be closely matched with sources in Tierra del Fuego that are absent in the island's present location and differ markedly from those to be expected in the more easterly original location implied by the plate reconstruction models of Eagles (2010a) and Eagles and Eisermann (2020) (Carter et al., 2014;Dalziel et al., 1975; see further discussion in Dalziel et al., 2021). Dalziel et al., 2021) ...
... Gross tilting of the entire SGM subsequent to acquisition of the magnetization is of course a theoretical possibility that cannot be excluded by the paleomagnetic data. However, despite thermochronologic evidence of late Eocene or younger burial and subsequent tectonic uplift associated with collision with the Northeast Georgia Rise (Figure 1; Carter et al., 2014;Dalziel et al., 2021) there is no evidence of this. The low plunge of the Andean folds in the turbiditic strata is comparable to that in the equivalent formations in Tierra del Fuego and the bathymetry of the continental shelf is homogeneous around the island (Curtis, 2011;Fretwell et al., 2009). ...
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South Georgia exists as a microcontinent along the North Scotia Ridge ∼1,700 km east of Cape Horn. The tectonostratigraphic units of South Georgia have long been correlated with those of the Fuegian Andes of southernmost South America. Accordingly, South Georgia has been regarded as a continuation of the Late Jurassic–Early Cretaceous Rocas Verdes marginal basin system, formerly situated south of Burdwood Bank and east of Cape Horn. To test this, paleomagnetic analysis of samples from the Larsen Harbour Complex, Drygalski Fjord Complex, and Annenkov Island Formation of South Georgia showed that 21 sites yield a mean direction of D = 328.5°, I = −62.1° (a95 = 3.5°) and a paleomagnetic pole at 068.2°E, 67.2°N, A95 = 4.7°. The consistency of directions and strong polarity bias, plus indications of a negative differential tilt test, point to a secondary magnetization acquired in the Late Cretaceous. Comparison of predicted versus observed directions for South Georgia relative to stable South America indicate 27.2 ± 11.2° of counter‐clockwise rotation (and 10.5° ± 4.5° of northward tilting) since the acquisition of magnetization. These results are consistent with paleomagnetic studies from the Fuegian Andes and support a paleoposition of the South Georgia microcontinent south of Burdwood Bank as strongly indicated by the geologic evidence. Partitioning this rotation between oroclinal bending during the Rocas Verdes basin collapse in the Late Cretaceous and left‐lateral translation along the North Scotia Ridge is not possible on paleomagnetic grounds, but the co‐linearity of Andean structures between the restored microcontinent and Tierra del Fuego indicates the former.
... This is also shown in a number of previous reconstructions (e.g. Dalziel et al., 1975;Diraison et al., 2000;Barker, 2001;Vérard et al., 2012;Nerlich et al., 2013;Carter et al., 2014;Maldonado et al., 2014). We note that the exact geometry of the blocks, fragments, banks and microplates that were part of, and formed, the continental bridge, as well as the distribution, extent, geometry and type of tectonic boundaries separating these units during subsequent fragmentation of the bridge, are associated with a significant amount of uncertainty, both in previous reconstructions, as well as in our own reconstructions presented in Fig. 3. ...
... Following the opening of the Rocas Verdes basin, the eastern boundary of CHm has thus been interpreted to change from a subduction zone plate boundary (Fig. 3a,b) to a collisional plate boundary (Fig. 3c) to a sinistral transform plate boundary ( Fig. 3d-h) that eventually becomes the Magallanes-Fagnano fault (Fig. 1). Geological investigations imply that sedimentary rocks from South Georgia Island formed part of the Rocas Verdes basin (Carter et al., 2014;Dalziel et al., 2021). Therefore, we reconstruct South Georgia microcontinent in the early Late Cretaceous at a position at the southern end of the closure of the Rocas Verdes basin (Fig. 3c,d). ...
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Subduction zones and their associated slabs are the main drivers of plate tectonics and mantle flow, but how these zones initiate remains enigmatic. In the Scotia Sea region, subduction started in the Late Cretaceous/Early Cenozoic in a pristine ocean basin setting devoid of other subduction/collision zones. How this subduction zone initiated remains intensely debated, as exemplified by the variability of published plate tectonic reconstructions. Despite such variability, several works argue for a subduction initiation mechanism, in which a South America-Antarctica relative plate motion change, in combination with a particular plate boundary geometry in the western Weddell Sea, caused convergence across a transform plate boundary segment that subsequently evolved into a subduction zone. Here we discuss this kinematic model of subduction initiation, and, following geometric and kinematic arguments, highlight several unsolved issues that call for alternative explanations. Furthermore, we present new tectonic reconstructions of the Scotia region involving a simpler middle-Late Cretaceous plate boundary configuration, which avoid the geometric and kinematic problems of earlier reconstructions and that call for a new mechanism of subduction initiation. We refer to this mechanism as Subduction Invasion Polarity Switch (SIPS), which involves a long-lived and wide subduction zone (South American-Antarctic subduction zone) with lower mantle slab penetration, which imposes major horizontal trench-normal compressive deviatoric stresses on the overriding plate. This plate consists of a narrow continental lithospheric (land) bridge at the trench (Cretaceous-Early Cenozoic Antarctica-South America land bridge) with oceanic lithosphere behind it (Weddell Sea-Atlantic Ocean). The stresses cause shortening and thrusting at the continent-ocean boundary in the backarc region of the overriding plate, forcing oceanic lithosphere under continental lithosphere, starting the subduction initiation process, and eventually leading to a new, self-sustaining, subduction zone (Scotia subduction zone) with an opposite polarity compared to the long-lived subduction zone. The model thus involves invasion of a new subduction zone into a pristine ocean basin (Atlantic Ocean), with the primary driver being a long-lived subduction zone in another ocean basin. To test the physical viability of the SIPS model, we have conducted numerical geodynamic simulations of buoyancy-driven subduction. Numerical results demonstrate that the SIPS model is viable, with compressive stresses in the overriding plate resulting from strong trenchward basal drag induced by subduction-driven whole-mantle poloidal return flow and compression at the subduction zone plate boundary. Subduction initiation starts in the overriding plate after ~100 Myr of long-lived subduction, eventually resulting in the formation of a new, opposite-dipping, subduction zone. This new subduction zone develops at the continent-ocean boundary for models without and with a pre-imposed weak zone. We further propose that the SIPS model might explain subduction initiation elsewhere, including the New Caledonia subduction zone in the Southwest Pacific, the Lesser Antilles-Puerto Rico subduction zone in the Caribbean region, and the subduction zones that consumed the Rocas Verdes and Arperos backarc basins in South America and Central America, respectively. We further postulate that active backarc shortening in the Japan Sea, with eastward under-thrusting of Japan Sea oceanic lithosphere below the Japan arc, represents an early stage of SIPS.
... New geochronological work indicates an important peak at 266 Ma (Chemale et al., in prep.), which is difficult to reconcile with a position just east of South Africa [42,62] (and cites therein). Permian ages are common at these latitudes in the Darwin Cordillera, with a more prominent peak at c. 270 Ma [65]; this is well documented for the Patagonian Andes, with a large well-defined 260-300 Ma population likely derived from the Gondwanide belt formed during the Carboniferous-Permian assembly of Patagonia [66], bracketed by Suárez et al. [67] between 255-268 Ma, and even closer, in the South Georgia Island, at its previous position adjacent to eastern Tierra del Fuego Island, with a dominant younger peak at 275 Ma [68,69]. ...
The latest studies on the tectonic evolution of the Malvinas (Falkland) Islands and their adjacent continental plateau further east are analyzed to assess a long controversy regarding the origin of these islands. Although there has been a controversy for several decades on this subject, new technologies and exploratory drilling have brought new data, however the debate of the geological evolution of this area remains open. The two dominant hypotheses are analyzed by assessing the eventual collision between the islands and the South American continent, the presence of a large transcontinental fault such as Gastre, the potential 180º rotation of the Malvinas Islands, and the occurrence of a mega-decollement with opposite vergence. These hypotheses are contrasted with the processes that have occurred in Patagonia, especially those based on the new isotopic data on the Maurice Ewing Bank at the eastern end of the Malvinas Plateau, and the current knowledge of the adjacent Malvinas Basin. The new data highlights the inconsistencies of certain models that proposed these islands migrated from the eastern African coasts near Natal, to their current position and rotated 180º on a vertical axis. The new observations are consolidating the hypothesis that postulates that the islands have been part of the South American continent since before the Paleozoic.
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Plain Language Summary In this study, we update the northern Antarctic Peninsula‐South Shetland Islands plate rotation process since ∼90 Ma. Five new enhanced magmatic events are identified in the northern Antarctic Peninsula‐South Shetland Islands, and we also reconstruct the migration of magmatism. Then, we compare plate rotation and magmatic migration to the Phoenix Plate‐Antarctic Peninsula convergence, and find a strong correlation between these events. The abrupt change in the convergence rate is attributed to the enhanced magmatic events. The clockwise rotation of the northern Antarctic Peninsula also corresponds to the Late Cretaceous initiation of the ancestral South Sandwich subduction zone and the late Paleocene separation of the northern Antarctic Peninsula from South America, indicating a causal relationship. The counterclockwise rotation of the northern Antarctic Peninsula after ∼47 Ma facilitated lithospheric extension and basin opening in the South Scotia Ridge region, contributing to the opening of the Scotia Sea. Therefore, this study provides a comprehensive interpretation of the geological process in Scotia Sea regions, from slab subduction and overlaying plate rotation to magmatic evolution and continental separation.
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The Cenozoic development of the Scotia Sea and opening of Drake Passage evolved in a complex tectonic setting with sea‐floor spreading accompanied by the dispersal of continental fragments and the creation of rifted oceanic basins. The post‐Eocene tectonic setting of the Scotia Sea is relatively well established, but Late Mesozoic paleo‐locations of many continental fragments prior to dispersal are largely unknown, with almost no geological control on the submerged banks. Detrital zircon analysis of dredged metasedimentary rocks of Bruce Bank from the South Scotia Ridge demonstrates a geological continuity with the South Orkney microcontinent (SOM) and also a clear geological affinity with the Trinity Peninsula Group metasedimentary rocks of the Antarctic Peninsula and components of the Cordillera Darwin Metamorphic Complex of Tierra del Fuego. Kinematic modelling indicate an Antarctic Plate origin for Bruce Bank and the SOM is the most plausible setting, prior to translation to the Scotia Plate during Scotia Sea opening.
The Miocene to Pliocene (Neogene) occurred between 23.04 and 2.58 million years ago and includes intervals of peak global warmth where Earth’s average surface temperature was up to 8℃ warmer than present. Major cooling steps also occurred, across which Antarctica’s ice sheets advanced to the continental shelf for the first time and sea ice expanded across the Southern Ocean. Knowledge of Antarctic environmental change and ice sheet variability through this dynamic period in Earth history has advanced over the past 15 years. Major field and ship-based efforts to obtain new geological information have been completed and significant advances in numerical modelling approaches have occurred. Integration of ice proximal data and coupled climate-ice sheet model outputs with high-resolution reconstructions of ice volume and temperature variability from deep sea δ¹⁸O records now offer detailed insight into thresholds and tipping points in Earth’s climate system. Here we review paleoenvironmental data through key episodes in the evolution of Neogene climate to include the Miocene Climatic Optimum (MCO), Middle Miocene Climate Transition (MMCT), Tortonian Thermal Maximum (TTM), Late Miocene Cooling (LMC), and Pliocene Warm Period (PWP). This review shows that Antarctica’s climate and ice sheets remained dynamic throughout the Neogene. Given the analogous nature of warm episodes in the Miocene and Pliocene to future projections, the environmental reconstructions presented in this chapter offer a stark warning about the potential future of the AIS if warming continues at its current rate. If average global surface warming above pre-industrial values exceeds 2℃, a threshold will be crossed and AIS instabilities would likely be irreversible on multi-century timescales.
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The Antarctic coastal fauna is characterized by high endemism related to the progressive cooling of Antarctic waters and their isolation by the Antarctic Circumpolar Current. The origin of the Antarctic coastal fauna could involve either colonization from adjoining deep-sea areas or migration through the Drake Passage from sub-Antarctic areas. Here, we tested these hypotheses by comparing the morphology and genetics of benthic foraminifera collected from Antarctica, sub-Antarctic coastal settings in South Georgia, the Falkland Islands and Patagonian fjords. We analyzed four genera (Cassidulina, Globocassidulina, Cassidulinoides, Ehrenbergina) of the family Cassidulinidae that are represented by at least nine species in our samples. Focusing on the genera Globocassidulina and Cassidulinoides, our results showed that the first split between sub-Antarctic and Antarctic lineages took place during the mid-Miocene climate reorganization, probably about 20 to 17 million years ago (Ma). It was followed by a divergence between Antarctic species ~ 10 Ma, probably related to the cooling of deep water and vertical structuring of the water-column, as well as broadening and deepening of the continental shelf. The gene flow across the Drake Passage, as well as between South America and South Georgia, seems to have occurred from the Late Miocene to the Early Pliocene. It appears that climate warming during 7–5 Ma and the migration of the Polar Front breached biogeographic barriers and facilitated inter-species hybridization. The latest radiation coincided with glacial intensification (~ 2 Ma), which accelerated geographic fragmentation of populations, demographic changes, and genetic diversification in Antarctic species. Our results show that the evolution of Antarctic and sub-Antarctic coastal benthic foraminifera was linked to the tectonic and climatic history of the area, but their evolutionary response was not uniform and reflected species-specific ecological adaptations that influenced the dispersal patterns and biogeography of each species in different ways.
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The Palawan microcontinental block is thought to have separated from the South China margin due to seafloor spreading and opening of the South China Sea. However, it is uncertain when and from which section the Palawan microcontinental block rifted from the South China margin, and little is known about sediment routing across the rifted margin before continental breakup. To address these aspects we studied the biostratigraphy and provenance of syn-rift sedimentary rocks collected from the Panas-Pandian Formation in central-southern Palawan. Micropaleontological evidence indicates a Middle Eocene–earliest Oligocene (47.7–32.9 Ma) age for the Panas-Pandian Formation. Based on this and the oldest age of the post-rift Nido Limestone (~32 Ma), the breakup unconformity on the Palawan microcontinent block is dated around 33–32 Ma. This timing of breakup unconformity is close to that of the Pearl River Mouth Basin (~30 Ma) and IODP Site U1435 (~34Ma), suggesting a conjugate relationship between the Palawan microcontinental block and the Pearl River Mouth Basin. Trace fossils and benthic foraminifera from the Panas-Pandian Formation indicate a middle bathyal to abyssal environment on the continental slope of the South China margin. Multidisciplinary provenance analysis reveals that the Panas-Pandian Formation was derived from both local Mesozoic basement uplifts and the interior Cathaysia Block. It indicates that a paleo-Pearl River has been established at least since the Middle Eocene (47.7–42.1 Ma) and could deliver sediments from the interior Cathaysia Block to the continental slope, across the wide rifted margin with a low topographic gradient.
The mountainous, glaciated island of South Georgia is the crest of one of the most isolated fragments of continental crust on Earth. It is located approximately 1700 km east of the southern termination of the Andean Cordillera of South America. The island is primarily composed of Lower Cretaceous turbidites, the infill of a marginal basin floored by stretched continental and ophiolitic crust. Remnants of a volcanic arc are preserved on offshore islets to the southwest. The Pacific hinterland of the southernmost Andes is missing in Tierra del Fuego, terminating at a submarine escarpment forming the continental margin immediately east of Cape Horn. The arc and back-arc basin infill rocks of South Georgia correspond exactly to part of the missing Cordilleran hinterland. The mechanism of transport of the South Georgia microcontinent eastward relative to South America remains obscure, but likely involved some form of ‘escape tectonics’ during mid- to Late Cretaceous counterclockwise rotation of the arc that led to closure and inversion of the marginal basin.
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Northeast Georgia Rise is located on inferred oceanic crust that is considered Albian in age and to have formed during the separation of Africa and South America. Basalt overlying a weathered regolith was recovered at Site 698 and a basaltic substratum at other sites is inferred from the downhole variation in pore-water chemistry. The provenance of a 2-m-thick gravel bed containing abundant clasts of continental lithologies displaced into lower Oligocene ooze at Site 699 is an enigma. We infer that at least part of the Northeast Georgia Rise was formed at a spreading center by excessive volcanism. At least two episodes of deformation have subsequently modified the topography of the rise. -from Authors
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Joint inversion of isochron and flow line data from the flanks of the extinct West Scotia Ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron C8 (26.5 Ma), and extinction around chron C3A (6.6–5.9 Ma). When they are placed in a regional plate circuit, the rotations predict plate motions consistent with known tectonic events at the margins of the Scotia Sea: Oligocene extension in Powell Basin; Miocene convergence in Tierra del Fuego and at the North Scotia Ridge; and Miocene transpression at the Shackleton Fracture Zone. The inversion results are consistent with a spreading history involving only two plates, at rates similar to those between the enclosing South America and Antarctica plates after chron C5C (16.7 Ma), but that were faster beforehand. The spreading rate drop accompanies inception of the East Scotia Ridge back-arc spreading center, which may therefore have assumed the role of the West Scotia Ridge in accommodating eastward motion of the trench at the eastern boundary of the Scotia Sea. This interpretation is most easily incorporated into a model in which the basins in the central parts of the Scotia Sea had already formed by chron C8, contrary to some widely accepted interpretations, and which has significant implications for paleoceanography and paleobiogeography.
The two main lithostratigraphic units of South Georgia, one composed of volcaniclastic greywackes and the other of quartzose greywackes, are now considered to be facies variants with a single turbidite sequence deposited during the Upper Jurassic and Lower Cretaceous. A continuous lateral variation in the composition of the detrital components of the greywackes has been established and it is likely that the two formations were derived from opposite sides of the depositional basin. Deep burial of the turbidite sequence resulted in extensive prehnitization, and during polyphase deformation the metamorphic grade was locally elevated to lower greenschist facies. A number of dolerite sheets were intruded early in the structural history of the area, and these have been metamorphosed to epidiorites. P.Br.
The results of a marine geophysical survey of the S Georgia continental shelf are presented. The area can be roughly divided by an E-W line into two distinct regimes. The N half is characterized by low values of the Bouguer gravity anomaly (<100mgal) of long wavelength and by little magnetic expression. Both geology and gravity data indicate that the Cumberland Bay-Sandebugten sedimentary rocks extend under the N half of the area, thickening to a maximum at or near the shelf edge. The S half of the continental block is characterized by high values of the Bouguer gravity anomaly (>100mgal) of shorter wavelength. In general, the rocks here are highly magnetic, the anomalies reaching their maximum amplitudes (approx 1000nT) in the SE corner of the shelf. The gravity data, the known geology of S Georgia, and a comparison of magnetic profiels from the area with Project Magnet flights across southern S America strongly support the views already advanced on the geologic similarities of the two areas. It is suggested that the marked magnetic high lying along the SE edge of the shelf may be evidence of an underlying intrusive belt comparable with the Patagonian batholith. The ophiolite suite, mapped as the Larsen Harbour Formation, is not strongly magnetized; it has no consistent and recognizable magnetic signature but seems to be associated with a gravity high.-Authors
The time of the opening of Drake Passage between South America and the Antarctic Peninsula is problematic. Mammals were able to migrate between South America and Antarctica until sometime in the early Eocene. Various continental fragments may have formed an effective barrier to substantial deepwater circulation through Drake Passage until at least 28 Ma. Alternatively, a medium-depth to deep water passage may have existed through Powell Basin to the south of the present Drake Passage as early as 33 Ma, but it is difficult to constrain the time of opening of Powell Basin. Simple opening of a shallow seaway between southern South America and the Antarctic Peninsula does not produce a vigorous Antarctic Circumpolar Current (ACC). Other gateways must be open to medium-depth to deepwater circulation such as one between the South Tasman Rise and East Antarctica. Even mid-ocean plateaus may play a role in the ultimate development of a circum-Antarctic current. The most probable southern ocean feature that may have affected global circulation was the opening of a deep seaway between the Kerguelen Plateau and Broken Ridge at about the Eocene-Oligocene boundary. While a complete deepwater (2000 m) circuit was certainly developed by the end of the early Oligocene, it may have been the closure of a major deep seaway north of Australia in the middle Miocene that finally produced the environment for the development of a vigorous ACC.
Zircon U-Pb and muscovite 40Ai/39Ar isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ±1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic "back stop" of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ±1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ±1.0 Ma, but their upper intercept is not well known. Inas-much as this age is similar to the 40Ar/39Ar age of secondary muscovite growing in extensional fractures of pulled-apart feld× spar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ±0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the high-grade metamorphic complex of Cordillera Darwin in Tierra del Fuego. Copyright © 1996 Elsevier Science Ltd & Earth Sciences & Resources Institute.
The central Scotia Sea, located between the South American and Antarctic plates, is an inte- gral part of the marine conduit that permits eastward deep-water flow from the Pacific Ocean to the Atlantic Ocean. The geologic history of the central Scotia Sea is therefore critical for a full understanding of the initiation and subsequent evolution of the complete, deep Antarctic Circumpolar Current, widely believed to have been a key factor in the history of Antarctic glaciation. Here, we present new evidence on the nature and age of the central Scotia Sea floor. Multibeam surveys and the first dredged samples indicate that a now-submerged remnant volcanic arc may have formed a barrier to deep eastward oceanic circulation until after the mid-Miocene climatic optimum. Inception and development of a full deep Antarctic Circum- polar Current may therefore have been important, not in the drop in global temperatures at the Eocene-Oligocene boundary as long surmised, but in the subsequent late Miocene global cooling and intensification of Antarctic glaciation.
Zircon U-Pb and muscovite isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic “back stop” of that wedge.
The Cumberland Bay formation on South Georgia is an Upper Jurassic - Lower Cretaceous andesitic turbidite sequence >8 km thick, derived from a contemporary volcanic arc. A palinspastic reconstruction is attempted.-R.A.H.
Rocks on South Georgia Island at the eastern end of the North Scotia Ridge are no older than late Mesozoic. The Cumberland Bay and Sandebugten graywacke and mudstone sequences there are comparable in general lithology and structural style to the Lower Cretaceous Yahgan Formation of the Beagle Channel area in southernmost South America. The Cumberland Bay rocks, which form most of South Georgia Island, were thrust northeastward over the Sandebugten sequence. The Cumberland Bay and Yahgan sequences contain Cretaceous fossils, whereas the Sandebugten rocks are unfossiliferous. The dominant dispersal of Cumberland Bay detritus was toward the northwest. The Sandebugten dispersal pattern was more complex but was dominated by a south-directed component. In Early Cretaceous time, however, the South Georgia microcontinent apparently was attached to South America along the present southern margin of the Burdwood Bank. The Cumberland Bay, Sandebugten, and farther westward along strike, the Yahgan, apparently were deposited in a marginal small ocean basin between a calc-alkalic 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. According to this interpretation, deformation of the sediments occurred when the arc moved back toward the continent in middle Cretaceous time, and the basin was closed and uplifted with the arc.