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

Abstract

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.

Full text

Geology
doi: 10.1130/G35091.1
2014;42;299-302Geology
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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INTRODUCTION
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-
mochronometry.
GEOLOGY
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
2
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
ABSTRACT
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
|
doi:10.1130/G35091.1
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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
62°S-
58°S-
54°S-
50°S-
N. Scotia Ridge
S. Scotia Ridge
Falkland Plateau NE
Georgia
Rise
Drake
Passage
Antarctic Plate
W.
Scotia
Sea
E
Scotia
Sea
Weddell Sea
Maurice
Ewing
Bank
Fuegian Andes
Magallanes
Basin
Polar
Front
Southern Antarctic
Circumpolar
Current Front
Sub-Antarctic
Front
Study
Area
Scotia
Plate
South
American
Plate
Shackleton Fracture zone
South
Sandwich
Plate
Figure 1. Scotia Arc region; study area (Fig. 2), principal topographic features, and
main fronts of Antarctic Circumpolar Current are indicated (generated by GeoMapApp;
www.geomapapp.org). 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).
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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).
RESULTS AND INTERPRETATION
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
CUMBERLAND BAY
STROMNESS
BAY
COOPER
ISLAND
ANNENKOV
ISLAND
0102030
km
SG11
SG1
80 ± 4 Ma
27 ± 10 Ma
SG9
79 ± 8 Ma
SG467
12 ± 3 Ma SG369
15 ± 3 Ma
*SG389
20 ± 3 Ma
SG241
10± 2 Ma
SG7
10 ± 1 Ma
SG19
21 ± 3 Ma
SG24
SG246
17 ± 3 Ma
37°
SG22
12 ± 2 Ma
88 ± 4 Ma
6.5 ± 0.5 Ma
12 ± 2 Ma
SG534
16 ± 2 Ma
SG530
11 ± 1 Ma
SG5.14
36 ± 8 Ma
PICKERSGILL
ISLANDS
BARFF
PENINSULA
44 ± 5 Ma
3.2 ± 0.2 Ma
SG14
Sample No.: SG1
AFT Age: 80 ± 4 Ma
AHe Age: 33 ± 19 Ma
* SGxx used for detrital zircon U-Pb
N
*SG394
Cooper
Island
Formation
Novosilski
Glacier
Formation
Drygalski
Fjord
Complex
Larsen
Harbour
Complex
Granitoid
intrusions
Cumber-
land Bay
Formation
Sande-
bugten
Formation
Annenkov
Island
Formation
Cooper
Bay
Formation
Pre-Jurassic Jurassic Cretaceous
Late
Early
Late
Early
Salomon
Glacier
Formation
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
www.geosociety.org/pubs/ft2014.htm, or on request
from editing@geosociety.org or Documents Secre-
tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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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
histories.
DISCUSSION
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
Sandebugten
Fm (n=210)
0 150 300 450 600 750 900 1050
171
109
275
Fuegian Andes
(Barbeau et al., 2009,
post Albian data removed)
(n=611)
South
Patagonian
Batholith
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)
200
180
160
140
120
100
80
60
40
20
0
Summary of thermal histories
East coast SG West coast SG
Pickingills Islands
burial to
prehnite-pumpellyite
facies metamorphism
main
inversion
10-7 Ma
granite
emplace-
ment
timing of onset of burial
unconstrained by data
?
?ZFT
age
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.
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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
fl 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.
CONCLUSIONS
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.
ACKNOWLEDGMENTS
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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Manuscript received 5 September 2013
Revised manuscript received 3 January 2014
Manuscript accepted 11 January 2014
Printed in USA
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