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IV: Marie Byrd Land

The Marie Byrd Land region lies between 180° and 240° E longitude. The
intermediate-spreading rate (~65-70 mm/yr) Pacific-Antarctic ridge traverses this region,
and to the south of it lies the Southeast Pacific basin. Although region IV is devoid of land
or grounded ice, the Geosat data coverage is incomplete due to sea ice which persisted from
1985S through 1986, particularly over parts of the northeastern Ross Sea. However, during
the ERM (1986-1990), Geosat data coverage extended further southward into the Ross Sea
than it did during the GM. Fortunately, the data coverage over the Pacific-Antarctic ridge
and ridge flanks is dense.

The high-resolution gravity field clearly depicts the spreading ridge, which is
associated with a positive gravity anomaly. Furthermore, the fine-scale resolution reveals
that most of the ridge segments are characterized by axial highs (gravity highs), although the
segment between 185° and 190° E is characterized by an axial valley (gravity low). A large
southwesterly-pointing propagating rift is located on the ridge axis at 195° E. Other older
propagating rifts that have been split apart by seafloor spreading are located on the ridge
flanks. For example, the arms of two northeasterly-pointing propagating rifts are located
south of FZ XII (which crosses the spreading ridge at 190° E), and there is evidence for
others (Marks and McAdoo, 1991). The large fracture zones that cross the Pacific-Antarctic
ridge are associated with gravity anomaly lows and, because these fracture zones are roughly
orthogonal to the descending Geosat passes, they are particularly evident in the image of
descending deflections. These fracture zones twist and bend symmetrically about the ridge
axis in response to past changes in the pole of rotation. A portion of the Udintsev fracture

" zone is located in the northeast corner of the region, and in the southeast corner there are
local gravity anomaly highs that are associated with the Amundsen ridges.

The early development of the Pacific-Antarctic ridge can now be deciphered in the
high resolution gravity field (Marks and Stock, manuscript in preparation). The New
Zealand microcontinent (including the Chatham rise and Campbell plateau) is believed to
have broken off West Antarctica along the Marie Byrd Land margin during the Cretaceous
(Mayes et al., 1990). The existence of several additional tectonic plates which moved
telative to the Pacific and Antarctic plates during the early Tertiary has been proposed. For
example, Stock and Molnar (1987) proposed a Bellingshausen plate which was active in the
early Tertiary but ‘froze’ to the Antarctic plate in the mid-Tertiary. Clear traces of this
Bellingshausen plate are not evident in our color images, perhaps owing to the persistent
sea ice and the consequently poor Geosat coverage of the southernmost portions of Region
Iv.

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53


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Foreword

The Southern Ocean has had fewer surface ship surveys than any of the other ocean
basins; as a result very little information on its morphology and tectonics is available. With
declassification of all Geodetic Mission radar altimeter data acquired by the Geosat satellite
south of 60° S, our knowledge and understanding of the seafloor and tectonic fabric of this
region has greatly increased. This report provides bathymetry plots, Geosat ground track
coverage, ascending and descending deflection of the vertical images, as well as the most
comprehensive gravity anomaly data available to date. This atlas serves as a companion to
Report MGG-6, Gravity Field over the Southern Ocean from Geosat, and is intended as an
initial source document for more detailed studies in the Southern Ocean.

The National Geophysical Data Center is pleased to make this report available as
a reference volume to the marine science community and public. It constitutes the seventh
in a series of scientific data reports published by the National Geophysical Data Center and
World Data Center-A for Marine Geology and Geophysics.

Michael A. Chinnery

Director

National Geophysical Data Center
Michael S. Loughridge

Director

World Data Center-A for
Marine Geology and Geophysics

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The Davis Sea region, which lies between 60° and 120° E longitude, encompasses
both the Davis and Cooperation Seas. These seas lie in the southern reaches of the South
Indian basin. As with the Gunnerus region to the west, the ERM ground track coverage
near the Antarctic margin is more extensive than the GM coverage because of sea ice cover
that was present in 1985 and 1986 while the GM data were collected. However, the areas
to the north have dense satellite coverage. Although smaller-scale anomalies are resolved,
most of the deflection and gravity anomalies in this region are unremarkable because there
are few prominent tectonic features. For example, the Valdivia abyssal plain (see
bathymetry plot) is associated with subdued gravity and deflection anomalies. The most
salient feature is the Kerguelen plateau, whose southern end is expressed as a bathymetric
and gravity high between 80° and 85° E. The Kerguelen-Heard ridge, and its conjugate, the
Broken ridge, are believed to have been generated at the Southeast Indian ridge prior to
40 Ma, and were subsequently split apart by seafloor spreading (Recq and Charvis, 1986).
The continental shelf (and submarine canyons) seaward of Antarctica are clearly seen in the
deflection and gravity images, as is the western portion of a gravity low or "moat" seaward
of the continental shelf. McAdoo and Marks (1991) have proposed that this gravity moat,
which extends at least as far east as 170° E (into region Ill), is due to the mechanical
response of the solid earth to ice sheets that were more extensive in the early Holocene.
Apparently, the viscous relaxation of the earth that was initiated by the melting of the East
Antarctic ice sheet is not yet complete.

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References

Cheney, R. E., Doyle, N. S., Douglas, B. C., Agreen, R. W., Miller, L., Timmerman, E. L.,
and McAdoo, D.C., 1991: The complete Geosat altimeter GDR handbook, NOAA
Man. NOS NGS-7, National Ocean Service, Rockville, MD.

Cheney, R., Douglas, B., Agreen, R., Miller, L., Milbert, D., and Porter, D., 1986: The
GEOSAT altimeter mission: A milestone in satellite oceanography, EOS Trans. AGU,
67, 1354-1355.

Marks, K. M. and McAdoo, D. C., 1992: Gravity field over the Southern Ocean from Geosat
(poster), Nat. Geophys. Data Center, World Data Center-A for Marine Geol. and
Geophys., Rep. MGG-6.

Marks, K. M. and McAdoo, D. C., 1991: Tectonic development of the Pacific-Antarctic ridge
(abstract), EOS Trans. AGU, 72, 266.

Marks, K. M., McAdoo, D. C., and Sandwell, D. T., 1991: Geosat GM data reveal new
details of ocean floor, EOS Trans. AGU, 72, 145-149.

Mayes, C. L., Lawver, L. A., and Sandwell, D. T., 1990: Tectonic history and new isochron
chart of the South Pacific, J. Geophys. Res., 95, 8543-8567.

McAdoo, D. C., and Marks, K. M., 1992: Gravity fields of the Southern Ocean from Geosat
data, J. Geophys. Res., 97, 3247-3260.

McAdoo, D. C., and Marks, K. M., 1991: Evidence for a more extensive east Antarctic ice
sheet in recent time from Geosat marine altimetry (abstract), EOS Trans. AGU, 72,
91,

Norton, I. O., and Sclater, J. G., 1979: A model for the evolution of the Indian Ocean and
the breakup of Gondwanaland, J. Geophys. Res., 84, 6803-6830.

Recq, M., and Charvis, P., 1986: A seismic refraction survey in the Kergulelen Isles,
southern Indian Ocean, Geophys. J. R. astr. Soc., 84, 529-559.

Stock, J., and Molnar, P., 1987: Revised history of early Tertiary plate motion in the south-
west Pacific, Nature, 325, 495-499.

55


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Contents
Preface 2... eee cece cece eee eee eee eee e eee eeeeees iii
Foreword «0... . cece cece eee eee eee teen ee ee eeenennes iv
Introduction 6.0.6... ccc ccc ccc eee eee eee ee eee nent eee n eens 1
Directory 6... 0. eee cece cee tee ener e teen ee eees 2
Computation of Gravity Fields ......... 0... cece cece eee eee 4
I Gunmerus Ridge 1.0.0... 0. cece cee cee eee eect eee 6
Tl: — Davis Sea 60... eee eee e eens 14
THI: George V Land ........ 0 cece cee eee cee eee eee 22
IV: Marie Byrd Land ....... 0... 0c cee cece eee eee ees 30
V: Thurston Island ...... 06... ccc cece eee eee eee e eens 38
VI: = Weddell Sea 2.0... cece cece cette eee ete e eens 46
Summary 6... eect eee eee teen teen nena 54
References 0... .. cece cece eect eee nent eeneeees 5S



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We have divided the circum-Antarctic study area into six regions, each spanning 60°
of longitude. These regions (numbered I-VI) are shown in Figure 1. For each of these
tegions we have compiled plots of ETOPOS seafloor bathymetry and Geosat ground track
coverage (both ERM and GM), as well as color images of deflections of the vertical (both
ascending and descending) and gravity anomalies. ETOPOS is a gridded bathymetry data
base that is available from the National Geophysical Data Center (Report 86-MGG-07).

In the color images, the gravity anomaly amplitudes range between -30 mGal
(magenta) and +30 mGal (red) (+30 microradians for the deflections of the vertical), and
are “illuminated” by a light source from the east. In many cases the gravity anomalies
exhibit fine-scale features which have no apparent counterpart in the corresponding
ETOPOS bathymetry plots. This lack of correspondence is largely attributable to the sparse
distribution of shipboard bathymetric observations used to construct the ETOPOS data base.

Because the ascending and descending deflections of the vertical may also be viewed
as horizontal gravity disturbances, the deflection images resemble those of the gravity
anomalies. However, the deflections of the vertical have a long-wavelength background field
removed, whereas the gravity anomalies have the background field restored.



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WORLD DATA CENTER-A FOR MARINE GEOLOGY AND GEOPHYSICS REPORT MGG-7

GRAVITY ATLAS
OF THE
SOUTHERN OCEAN

Karen M. Marks and David C. McAdoo

National Ocean Service

U.S. DEPARTMENT OF COMMERCE
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION


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Computation of Gravity Fields

We began with profiles of sea surface height (from ERM years 1, 2, and 3, and GM
GDRs), and ended with the gridded gravity anomalies that are shown in this atlas. The
processing steps and computation method used to derive these gravity anomalies are only
outlined here. For a more detailed discussion of the gravity derivations, see McAdoo and
Marks (1992),

The first step was to convert the GDRs into two-per-second sea surface heights by
fitting a sliding quadratic to the 10-per-second data. When the root-mean-square (RMS)
deviations of the 10 Hz data exceeded 0.1 m, the 2 Hz heights were edited. We made all
the appropriate corrections to the sea surface height data, including those for ocean and
solid earth tides, as well as ionospheric and tropospheric travel time delays. The heights
were also edited to remove outliers. Along-track deflections of the vertical were then
computed by differentiating the filtered sea surface heights with respect to distance along
the satellite pass. Furthermore, we filtered the data to pass only gravity anomalies in the
10-2000 km waveband. This was accomplished by subtracting a low-order background field,
and high-cut filtering with a Gaussian smoothing function.

The next step was to interpolate the along-track deflections (in two separate
operations: one on ascending and one on descending) onto a standard rectangular grid
having a grid interval of 0.05° latitude and 0.10° longitude (or approximately 5 km by 5 km).
Simple vector algebra was used to compute the east and north deflection components from
the gridded ascending and descending deflections.

. The last step was to forward Fourier transform the east and north deflection
components into the frequency domain, and multiply them by the inverse Vening Meinesz
filter. This filter converts the deflection components into gravity anomalies. Finally, the
gravity anomalies were inverse Fourier transformed into the spatial domain, and the low-
order field was restored. These gravity anomalies are available in digital and poster form
from NOAA’s National Geophysical Data Center (Marks and McAdoo, 1992).



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Summary

We have shown plots of bathymetry and of Geosat ground track coverage, as well as
images of deflections of the vertical (both ascending and descending), and gravity anomalies
for the region encircling Antarctica between 60° and 72° S. Release of the previously
classified, high-density Geosat GM data, combined with lower density ERM data, have
permitted us to construct high resolution gravity fields over the Southern Ocean. With these
results, we now have a better, more precise knowledge of the marine gravity field around
Antarctica than in any other large scale region of the world’s oceans. This is especially
exciting because the area around Antarctica is among the world’s most poorly charted by
ships and airplanes.

There are potentially many geophysical applications of these results. One particularly
important application is the mapping of the tectonic fabric imprinted in the seafloor and the
consequent improvement in our understanding of the relative plate motions which occurred
throughout the Cenozoic and late Mesozoic Eras following the breakup of the
supercontinent Gondwana.

Acknowledgments: We wish to thank Bruce C. Douglas of NOAA for his support as well as
our co-workers within the Geosciences Laboratory for their assistance. We thank the
Satellite and Ocean Dynamics Branch for producing the ERM and GM GDRs. This project
was funded by the Climate and Global Change project.

54


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The Geosat spacecraft was developed jointly by the U.S. Navy and the Johns Hopkins
Applied Physics Laboratory. It was launched on March 12, 1985, and performed two
missions in succession: the primary Geodetic Mission (GM), and the subsequent Exact
Repeat Mission (ERM). During the GM, classified altimeter data were collected along
closely-spaced ground tracks (2-3 km at 60° S) from April 1, 1985 to September 30, 1986.
Upon termination of the GM, the satellite was maneuvered into an orbit that repeated every
17 days (the ERM). During the ERM, which took place between November 12, 1986, and
January 5, 1990, unclassified altimeter data were collected along widely-spaced ground tracks
(~75 km at 60° S). Together, these Geosat missions mapped the surface topography of the
world’s oceans between 72° N and 72° § latitudes at an unprecedented accuracy and
resolution.

In August 1990, the U.S. Navy declassified the Geodetic Mission data south of
60° S. These data were produced by NOAA/NOS and are available in the form of
Geophysical Data Records (GDRs) from NOAA’s National Oceanographic Data Center
(Cheney et al., 1991). We have used these GM data along with those from the ERM
(Cheney et al., 1986) to derive marine gravity fields over the Southern Ocean that encircles
Antarctica. To date, no other region of the ocean has been so completely covered by such
accurate and closely-spaced altimeter observations that are unclassified. :

Tronically, this circum-Antarctic region is very poorly charted by ship or aircraft - as
poorly as any part of the world’s oceans. In roughly half of the 1° squares in this region, the
seafloor is completely uncharted, that is, there are no available shipborne/airborne gravity
observations or bathymetric soundings. In these uncharted areas, our gravity fields reveal
many previously unknown short-wavelength (10-500 km) features in the seafloor such as
seamounts, fracture zones, and mid-ocean ridge structures. The strong correlation between
short-wavelength gravity anomalies and seafloor topography enables us to ’see’ these
bathymetric features in the gravity fields. Moreover, because fracture zones, passive
continental margins, and other related features record the history of seafloor spreading, our
gravity field images detail the regional tectonic history for the last 100 or more million years.
Since the disintegration of Gondwana (the ancient supercontinent) some 140-160 million
years ago, the Antarctic plate(s) has grown via seafloor accretion as South America, Africa,
India, Australia, and New Zealand moved away. Many details of this accretion process can
be seen in the gravity field images.



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Preface

Before the end of this century, satellite altimeter observations should cover the entire
surface of the world’s oceans, aside from those regions that are masked by permanent or
persistent sea ice. Our first opportunity to evaluate these high-density data came in 1990,
when the United States Navy declassified all of the Geodetic Mission (GM) data acquired
by its Geosat satellite south of 60° S.1 Our results, as detailed in this atlas and companion
publications, surprised us. We knew that the Southern Ocean was among the most poorly
charted regions of the entire ocean, and we expected that the high-density Geosat data
would yield a considerably improved marine gravity field. What we did not know was that
these altimeter data would yield a marine gravity field of such excellent precision and
resolution that it could not be substantially improved by adding state-of-the-art shipboard
gravity observations.

We present high-resolution marine gravity fields in this atlas. However, because
short-wavelength gravity anomalies (10 km to 200 km) are highly correlated with bathymetry,
these fields yield much new information about the topography of the seafloor. So, later this
decade, when altimetric coverage of the world’s oceans is complete, we will be able to chart
the entire seafloor to a scale as fine as 10 km. Marine geophysics will have come along way
since that time forty years ago, before the seminal work of Heezen and Tharp, when the vast °
majority of the seafloor was uncharted.

Karen M. Marks
David C. McAdoo

Geosciences Laboratory
Ocean and Earth Sciences
National Ocean Service

1 As this goes to press, we have learned that the United States Navy has declassified all GM
data acquired south of 30°S; we gratefully acknowledge the Navy for their data and
cooperation.

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The Thurston Island region spans 240° to 300° E longitude, and encompasses the
Amundsen and Bellingshausen Seas. Both the ERM and GM data coverage is sparse south
of about 70° S, and also along the west coast of the Antarctic Peninsula, but the coverage
is complete elsewhere. A prominent feature in the deflections and the gravity field is a
north-south trending ridge of uncertain origin (at 269° E). This ridge, which is associated
with a gravity anomaly low, appears to split the De Gerlache seamounts. The ridge is not
evident in the contour plot of bathymetry, due possibly to poor data coverage, sediment
cover, or a subdued bathymetric expression. There is also an east-west trending chain of
seamounts at 67° S, 265° E, that terminates against the ridge. The tails of two large
southeasterly-trending fracture zones, the Tharp and the Heezen (of the Eltanin system),
appear as gravity lows, and have their ends (or tips) near the ridge. Farther to the west, the
tail of another large southeasterly-trending fracture zone, the Udintsev, can be seen.
Between the Tharp and Udinstev fracture zones are the southern limbs of two propagating
rifts, that trend generally north-south along 244° and 252° E longitude. These propagating
rifts have been split apart by seafloor spreading, and their conjugate, northern limbs lie to
the north of the Pacific-Antarctic ridge.

Another prominent feature in the gravity field is the South Shetland trench which lies
along the northwest margin of the Antarctic Peninsula, north of 62° S. The Drake plate, a
remnant of the Aluk plate, is presently (or has recently been) subducting beneath the South
Shetland Islands and Bransfield Strait at this trench. The Hero FZ which presumably

_ coincides with the southwestern boundary of the Drake plate, trends in a northwesterly
direction away from the South Shetland trench at ~62°S. Parallel to, and southwest of, the
Hero fracture zone are at least five more northwest-southeast trending fracture zones
(including the Tula FZ) which intersect the inactive portion of the Antarctic Peninsula’s
western margin. These fracture zones are evident in both the images of the gravity field and
the descending deflections; they were partially subducted beneath the Antarctic Peninsula
during the early and mid-Tertiary when plate convergence was active along this portion of
the Antarctic margin.

The tectonic features observed in the color images of this region reflect the
Cretaceous rifting between West Antarctica and New Zealand, the Campbell plateau, and
the Chatham rise microcontinents. Further analysis of this tectonic fabric can yield
additional insights into the regional plate motion history, including constraints on the nature
and existence of plates such as the Aluk and the Bellingshausen. These two plates have
been proposed to be separate tectonic blocks that existed throughout much of the Tertiary
but are now locked into the Antarctic plate (Mayes et al., 1990).

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U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Environmental Satellite, Data, and Information Service
Boulder, Colorado USA 80303



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National Geophysical Data Center
World Data Center-A for Marine Geology and Geophysics
Report MGG-7

Gravity Atlas
of the
Southern Ocean

Karen M. Marks and David C. McAdoo

Geosciences Laboratory

Office of Ocean and Earth Sciences
National Ocean Service, NOAA
Rockville, MD 20852

August 1992

U.S. DEPARTMENT OF COMMERCE
Barbara Hackman Franklin, Secretary

National Oceanic and Atmospheric Administration
John A. Knauss, Under Secretary

National Ocean Service
W. Stanley Wilson, Assistant Administrator


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III: George V Land

To the north of Wilkes Land and George V Land lies the South Indian basin, and
the Dumont D’Urville Sea. The George V Land region, located between 120° and 180° E
longitude, encompasses these features and also the southern part of the Southeast Indian
Ocean. Extensive ERM and GM data were successfully acquired close to the Antarctic
margin, except near Cape Hudson (see bathymetry and ground track plots). However, the
remainder of the region is densely covered with altimeter observations.

Tectonic features of major importance are revealed in the color images of this region.
In particular, portions of two active mid-ocean spreading ridges are evident. At
approximately 155° E and 63° S, the Southeast Indian ridge (SEIR) is intersected and offset
by the southeasterly-trending Balleny fracture zone. The axis of this easternmost segment
of the SEIR appears as a lineated, southwest-northeast trending high (a yellow color) in the
gravity field. Slightly to the east, at the Macquarie triple junction (approximately 160° E,
62° S), the SEIR terminates and the Pacific-Antarctic ridge begins. East of the Macquarie
triple junction, the Pacific-Antarctic ridge steps to the southeast in a series of small-offset
ridge segments, which are associated with gravity anomaly highs. These spreading segments
formed when a recent (<5 Ma) change in the pole of rotation created extension across a
large, old, NW-SE trending fracture zone, causing it to become "leaky" (Marks et al., 1991).
The split fracture zone shows up (in the descending vertical deflections) as a prominent
NW-SE trending high north of the small-offset ridge/transform system, and as a NW-SE
trending low to the south.

. Other major fracture zones are evident in the deflection and gravity anomaly images.
The Tasman and Balleny fracture zones which trend southeasterly between 150° and
155° E are prominent. Near the southern end of Balleny FZ lie the Balleny Islands, which
are comprised of a southeasterly-trending seamount chain associated with local gravity
anomaly highs. The Tasman and the Balleny FZs trace the separation of Australia from
East Antarctica during the Cenozoic. Prior to this separation, Australia was attached to
Antarctica along the Wilkes and George V Land margins.

Also note that a gravity ’moat’ lies seaward of the continental shelf offshore Wilkes
Land and George V Land in the Dumont D’Urville Sea. This moat is the eastern
continuation of the gravity low identified in Region II, and is attributed to the incomplete
viscous response of the solid earth to a large, early Holocene, East Antarctic ice sheet.



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I: Gunnerus Ridge

This region spans 0° to 60° E, and covers the Lazarev, Riiser-Larsen, and Cosmonaut
Seas, which make up the southernmost portions of the Atlantic and Indian Oceans. The
Geodetic Mission ground track coverage is dense in this region, but terminates
approximately 80 km seaward of Dronning Maud Land due to the sea ice cover which
persisted through the austral summers of 1985 and 1986. The ERM tracks fill in this gap,
thereby extending the satellite coverage to within approximately 25 km of Antarctica’s
coastline. Aside from this 25 km wide region bordering Antarctica, Geosat data provide
virtually complete coverage of the ocean in region I. The small-scale anomalies viewed in
the vertical deflections and gravity field are well constrained by the densely-spaced Geosat
data.

For example, the continental shelf seaward of Enderby Land, and the large submarine
canyons located on it, are clearly seen in the deflections and gravity field. The Gunnerus
ridge and the smaller Astrid ridge to the west (see bathymetry plot) are both associated with
north-south trending gravity anomaly highs. The Maud rise is also overlain by a gravity
anomaly high. Between 0° and 22° E, in the vicinity of the Maud rise, southwest- and
Southeast- trending fracture zones meet in an area of chaotically distributed gravity
anomalies. An extinct triple junction appears to be the source of these anomalies. The
Bouvet triple junction is located ~ 1500 km to the north, and it is possible that the extinct
triple junction is the forerunner of the Bouvet. The most prominent of the southwest
trending fracture zones (FZs) is the Astrid FZ which terminates near the northern tip of the

_ Astrid ridge and which can be seen well in both the gravity anomaly and ascending
deflection image. Other fracture zones are seen trending southwesterly towards the
Gunnerus ridge and also towards Enderby Land. The trends of these fracture zones reflect
the early (probably Cretaceous) direction of relative spreading between the Indian and
Antarctic plates.

The tectonic features discussed above are all a consequence of the Mesozoic breakup
between East and West Gondwana and the subsequent separation of Africa, Madagascar
and India from Antarctica (Norton and Sclater, 1979). Prior to this breakup, Dronning
Maud Land is believed to have adjoined Madagascar and the east coast of Africa, and
Enderby Land to have abutted India.



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VI: Weddell Sea

The Weddell Sea region is located between 300° and 360° E longitude, and
encompasses the Weddell Sea and the Weddell abyssal plain. The ERM and GM data
coverage, although dense in the east, central, and north Weddell Sea, is virtually absent over
much of the western Weddell Sea, i.e., from the Antarctic Peninsula eastward to ~310° E,
because of persistent sea ice. Note that some marine data were collected between 300° E
and ~310° E because polynyas occasionally opened in the western Weddell Sea. In this
region where data coverage is so sparse, the deflection and gravity anomalies are largely
manufactured by the interpolation algorithm. However, in the east and central Weddell Sea
where the satellite coverage is dense, one can see lineated small-scale gravity and deflection
anomalies which form a fish-skeleton pattern in the color images. North of 67° S, these
anomalies trend generally northwest-southeast, and south of 67° S they trend northeast-
southwest. It is likely that these anomalies are the expression of old fracture zones that
formed on spreading ridges that are now extinct (and probably subducted). The obvious
variations in the trend of these fracture zones indicates a complex history of spreading
between West Antarctica and South America.

Other features of tectonic importance can be seen in the images of gravity and
deflections of the vertical for this region. In the northwest corner, one can view the eastern
end of the South Shetland trench (a gravity low) which is intersected by the northwest-
trending Shackleton fracture zone. To the southeast of the South Shetland trench and
Islands is the Bransfield Strait. East of the Bransfield Strait is the Powell basin whose

. Southern edge is clearly outlined by a concave northward, arcuate gravity high which extends
from about 305° E to 325° E longitude. The northern edge of the Powell basin is formed
by the South Scotia fault which trends east-west along ~61° S (between 310° and 335° E).
At 335° E, and about 60° S, the southern end of the South Sandwich trench can be seen in
both the gravity anomaly and ascending deflection images. This trench forms the boundary
between the South American and Sandwich plates. Also, between 335° E and 346° E, along
the 61° S parallel, the east-west trending South Sandwich transform manifests itself very
clearly. This transform acts as a portion of the plate boundary between the South American
and Antarctic plates.

46


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