Download Crustal Structure at the Continental Margin South of South Africa

Geophys. J . R. astr. Soc. (1976) 44, 601-623
Crustal Structure at the Continental Margin South of
South Africa
R. A. Scrutton
(Received 1975 September lo)*
Summary
Four gravity profiles over the continental margin south of South Africa
are interpreted in the light of existing seismic, gravity and magnetic results.
There is a marked difference in shallow and deep crustal structure between
the rifted segment of margin facing the Atlantic Ocean and the margin
offset facing the Indian Ocean. Both margin segments have a mass excess
beneath the continental rise and a mass deficit beneath the outer continental shelf and continental slope, but this distribution is much more pronounced at the margin offset. It can be explained by a rapid transition
from continental to oceanic crust which, in the Indian Ocean, probably
results from the mode of formation of the margin (strike slip faulting).
In the Atlantic Ocean the cause of the rapid transition is less obvious. The
shallow structure at the continent-ocean boundary changes around the
margin. At the rifted segment, the sedimentary sequence of the continental
rise north of 36" S gives way southwards to a basement high thought to be
of volcanic origin. At the margin offset the Agulhas fracture zone appears
to contain slivers of both oceanic and continental crust. The latter is in the
form of a marginal fracture ridge. Beneath the continental shelf a high
density lower crust may compensate for the low density sedimentary rocks
in the Outeniqua Basin.
I. Introduction
One of the programmes of the International Decade of Ocean Exploration (IDOE)
is a reconnaissance geophysical and geological study by the Woods Hole Oceanographic Institution of the Eastern Atlantic continental margin (EACM). Survey work
began in January 1972, with Cruise 67 of RV Atlantis II working between Cape St
Francis, South Africa (Fig. 1) and the Congo River. Continuous recordings of
bathymetric, seismic reflection, magnetic and gravity data were obtained together
with results from a number of expendable sonobuoy stations (Hoskins, Rogers &
Woo 1975). Emery et al. (1975) have presented an overall appraisal and interpretation
of these data.
On Leg 3 of Cruise 67 four crossings (Fig. 2) were made of the continental margin
around the Agulhas Bank south of South Africa. An interpretation of the gravity
* Received in original form 1975 June 20.
601
602
R. A. Scrutton
330s
34
35
36
37
38
39OS
FIG.1. Detailed bathymetric chart of the continental margin south of South Africa,
redrawn from a chart compiled by E. S. W. Simpson at the University of Cape
Town. Depths are in metres. The inset shows the position of the study area with
respect to Africa.
records obtained on the crossings is presented here as one of the more detailed studies
of the EACM project. The traverses cross the ' corner ' of southern Africa, where two
continental margins of markedly different origin meet (Scrutton et al. 1974). Facing
the Atlantic Ocean is a rifted margin, but facing the Indian Ocean is a 1200 km long
margin offset. With this study it is hoped to reveal some of the major differences in
crustal structure between these two margins and provide evidence for their modes of
evolution during and after continental break-up.
The morphology of the sea floor around Agulhas Bank is shown in Fig. 1. It has
been described by several authors (e.g. Simpson 1970; Scrutton et al. 1974), so only a
brief comment on the prominent features is given here. In the Atlantic Ocean, west of
20"E, there is a narrow continental shelf, moderately steep (up to 5"), canyonated
continental slope and a well-developed continental rise. These border the Cape
Basin which lies at a depth of 4500 to 5000rn. In the southern part of the Cape
Basin is the Cape Rise basement arch (Fig. 2) striking east-north-east towards Agulhas
Bank. The bathyrnetry in the Indian Ocean is quite different and much more complex.
The shelf break is often poorly defined, but if we use the 200-m isobath to define it,
the shelf is seen to narrow rapidly eastwards from a width of 250 km south of Cape
Agulhas to 50 km south of Cape St Francis. Where the shelf is widest, it terminates in
a steep continental slope, but where it narrows, there is a wide, gently inclined upper
603
Crustal structure south of South Africa
34
34
35
35
36
36
37
37
38
38
393
17'E'
18
19
20
21
22
23
24OE
39%
FIG.2. Structural elements of the continental margin defined by seismic reflection
and seismic refraction data. Sediment thicknesses were obtained from the
' Atlantis I1 ' seismic reflection profiles using an interval velocity of 2-0 km s-'.
Seismic refraction stations 4,5 and 6 are by Hales et al. (1970), I50 and 151 by
Ludwig et al. (1968), G & H by Green and Hales (1966), S by Spence (1970) and
Hales & Nation (1972), and the remainder (between Cape Town and Cape St
Francis) by Leyden et al. (1971). The 200-m isobath is shown.
slope and a narrow, steep slope (du Plessis & Simpson 1974). The continental rise
is poorly developed, being largely made up of the Mallory Seamount chain paralleling
the slope. Beyond the rise is the Agulhas Basin at over 5000 m depth and the Agulhas
Plateau rising to less than 2500 m depth.
The geological history of the region has been fully described by Dingle (1973b),
Scnitton (1973a), Dingle & Scrutton (1974) and Scrutton et al. (1974). All the features
to be studied here came into existence during Mesozoic and Cenozoic times. Immediately prior to the Mesozoic, the continental parts of the region existed within the Cape
fold belt (Fig. 2) within the supercontinent of Gondwanaland. Falkland Plateau
bordered Agulhas Bank at that time (Scrutton 1973a; DSDP Scientific Staff 1974)
and was also part of the late Palaeozoic-early Mesozoic Cape fold belt (Greenway
1972). During late Triassic-early Jurassic times, a phase of approximately east-west
tensional stress, possibly associated with the breakaway of east Gondwanaland from
west Gondwanaland, caused subsidence and the initiation of the Outeniqua Basin on
Agulhas Bank (Fig. 2). At this time, therefore, the faulted eastern flank of the Agulhas
Arch, bordering the Outeniqua Basin, began to evolve. The arch itself consists of
Palaeozoic strata folded on an east-south-east-west-north-weststrike underlain by
R. A. Scrutton
604
c field
1
1
36
37
38
17'E
18
19
20
21
22
23
24'E
FIG.3. Features of the magnetic field south of South Africa. The ' m ' sequence of
lineaticns in the Atlantic Ocean is from Larson & Ladd (1973). ' M ' is from
Talwani & Eldholm (1973) and is thought to mark the continent-ocean boundary.
The remainder of the features are from du Plessis & Simpson (1974). Note the
gently iindulating magnetic field with local intense anomalies over the Agulhas
Arch and inner shelf and the quiet field over the Outeniqua Basin and marginal
fracture ridge. The Cape slope anomaly marks the edge of oceanic crust in the
Indian Ocean. The 200-m and 4500-m isobaths are shown.
Precambrian granitic and metamorphic rocks. With the separation of South America
and Falkland Plateau from Africa in the late Jurassic-early Cretaceous (Larson &
Ladd 1973), both the Atlantic and Indian Ocean continental margins of Agulhas
Bank were formed. The newly-formed Agulhas fracture zone truncated both the
Agulhas Arch and the Outeniqua Basin. It is probable that the Agulhas marginal
fracture ridge came into existence at this time. As soon as the South American block
had broken away the new rifted margin segment west of Agulhas Arch would have
attempted to subside. However, since Falkland Plateau remained in contact with the
south-east edge of Agulhas Bank until mid-Cretaceous time, restraints would have
been imposed on this subsidence. Also, subsidence of the evolving continental edge in
the Indian Ocean would have been delayed. Only since the middle Cretaceous have the
margins around Agulhas Bank been free to evolve in a tectonically inactive setting.
A local igneous event took place on the Alphard Banks (Fig. 3) in the Palaeogene
(Dingle & Gentle 1972), but otherwise crustal subsidence and sediment accumulation
have predominated throughout the Upper Cretaceous and Cenozoic.
Crustal structure south of South Africa
605
FIG.4. Contour chart of free-air gravity anomalies offshore, b s e d on Afhntis II
traverses 10,12, 14 and 15, Graham &Hales (1965) and Talwani & Kahle (in press),
and Bouguer anomalies onshore, redrawn from Smit er a[. (1962). Where the
Arlanris ZZ traverses are dashed no gravity reccrd was obtained. The contour
interval is 20 mgal. The 200-m isobath is shown.
2. Previous geophysical studies
These studies have been reviewed for the whole of offshore southern Africa by
Scrutton et al. (1974), Siesser, Scrutton & Simpson (1974) and Emery et al. (1975).
It is now necessary to consider the seismic and previous gravity results from the
Agulhas Bank region where they have a bearing on the following gravity interpretation.
Two maps (Figs 2 and 3) summarize the seismic and magnetic work over the continental margin. The magnetic work is not considered in detail here.
Seismic refraction
The base of the continental crust beneath Agulhas Bank has been shown by
Hales & Nation (1972) to be at a depth of about 32 km. An unusually high mean Pwave velocity for the topmost mantle (8.95 km s-') was observed, however, causing
Hales and Nation to speculate on the possibility that a more normal P, phase of
about 8 . 0 km s-' had been masked in their records. If such a phase were present, the
calculated depth of 32 km to the 8.95 km s-l layer would be increased to about
35-5km and the top of the undetected 8.0 km s-' layer, which then becomes the
redefined base of the crust, would occur between 30 and 33 km depth. This depth
R. A. Scrulton
60 6
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200
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100
0
Crustal structure south of South Africa
607
estimate is consistent with the value of 30 km for the thickness of the crust at the
coastline obtained by Smit, Hales & Gough (1962) and Talwani (1962) from gravity
data. On the basis of these data, T = 30 km is used in the gravity interpretation
below.
Within the continental crust five major layers can be recognized seismically.
These are the postPalaeozoic sedimentary layer (V, < 4.0 km s-I), the Palaeozoic*
layer (V, N 4 . 3 to 5.3 km s-'), the Precambrian* layer (V, N 5 . 8 km s-I), the
main upper crustal layer (V, CI 6 . 2 km s-') and the main lower crustal layer ( Vp N
7-2kmsd'). The mean velocity of the sedimentary layer has been calculated by
averaging all the observed velocities in the constituent layers (Green & Hales 1966;
Ludwig et al. 1968; Spence 1970; Leyden, Ewing and Simpson 1971) weighted in
proportion to the thicknesses of these layers. 2.72 km s-l was obtained as the mean.
The density corresponding to this velocity on the extended Nafe-Drake curve (Ludwig,
Nafe & Drake 1971) is 2 . 2 g cmd3. This is the value used in the following gravity
calculations.
The Palaeozoic layer varies in thickness between 1 and 6 km, but Hales & Nation
(1972) have calculated that the underlying Precambrian layer also varies in thickness
such that its base occurs at approximately 8 . 3 km depth throughout the Agulhas
* The terms Palaeozoic and Precambrian are used for convenience; the ages of the rocks in these
layers only approximate to these periods.
20 1
01
0
100
Kilometres
FIG.5
200
608
R. A. Scrutton
.g%
-20
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- 20
”
Isostatic onomoly
4,
.......
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FIG.5. Profiles 15,14,12 and 10 of Atlantis ZI showing, from bottom to top in each
case: an isostatic model of the Earth‘s crust including seismic reflection data on the
thickness of the sediment layer (dashed lines), seismic refraction data (lu Ludwig
ct a/. (1968), le Leyden et a/. (1971), g Green & Hales (1966), s Spence (1970), h
Hales & Nation (1972), velocities in Icm s - l ) and densities in g ~ m (in- square
~
brackets);the observed free- air anomaly (dotted where interpolated from contour
chart) and the anomaly calculated for the isostatic model; and the isostatic anomaly.
Bank region. This horizon is incorporated in the crustal models in this study (Figs 5
and 6) and the Palaeozoic and Precambrian layers are grouped together into a
single layer of mean density 2.67 g ~ m - This
~ . density is chosen because it is in
common use for the upper part of the crust and because it corresponds to a P-wave
velocity of 5.6 km s- which is intermediate between the velocities in the Palaeozoic
and Precambrian Iayers. The main upper and lower crustal layers are also grouped
together for the initial stages of the gravity interpretation. A density of 2.87 g cm-3
is assigned to these since this is the value commonly chosen for the bulk of the lower
continental crust. It corresponds to a P velocity of 6.4 km s-’. Later in the gravity
interpretation, however, the 7.2 km s-l layer is introduced as a separate entity.
Hales & Nations’ (1972) results suggest that the layer occurs primarily beneath the
Outeniqua Basin as a form of compensation for the low-density sedimentary rocks. A
lower crustal layer of velocity about 7.4 km s-’ is frequently found below sedimentary basins and at inactive continental margins (Drake & Nafe 1968), providing part,
if not all, of the isostatic compensation for the sediments at the top of the crust.
There are no seismic refraction data on the deep structure of the oceanic crust
west of the Agulhas Arch but there are three stations in the north-east Agulhas
Basin (Fig. 2). The weighted mean velocity for the oceanic crust is 5.98 km s-’,
which falls midway between the 5.6 km s-’ and 6.4 km s-’ velocities for the Palaeo-
’,
609
Crustal structure south of South Africa
Profile 15
Free-air anomaly
:
1-20
___ Observed,----------Colculoted
3
10
[2.87]
%
c
E
._
Y
20
30
2 00
300
400
100
0
Kilometres
Profile 14
500
400
160
300
200
Kilometres
F I ~6.
100
0
610
R. A. Scrutton
20!
Profile 10
200
I00
Kilornetres
0
f
- _0
.-
__
-.
I00
200
~
..
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.
400
..
-~.-..
500
K ilometres
FIG.6. Profiles 15, 14, 12 and 10 of Atlantis II showing in each case: below, a
crustal structuremodel based on the seismic and gravity data (densities in g ~ m - ~ ) ;
above, the observed free-air anomaly and the anomaly calculated for the model.
CSA is the position of the Cape slope anomaly.
Crustal structure south of South Africa
61 1
zoic-Precambrian and main crustal layers beneath the continent. A two-layer model
for the oceanic crust, laterally continuous with the 2.67 g cm-j and 2.87 g cm"
layers beneath the continent, is therefore considered acceptable for the initial gravity
interpretation.
Seismic reflection
In a number of areas, the geological basement crops out extensively on the sea
floor; these areas are shown in Fig. 2. Those areas holding the greatest implications
for the gravity interpretation are the Agulhas Arch antiform of Palaeozoic and Precambrian rocks (Gentle 1970), the Agulhas marginal fracture ridge (Scrutton & du
Plessis 1973) and the Mallory Seamount chain.
Isopachs of sediment thickness over the Agulhas Arch show the extent of the
basement outcrop and the shape of the west and east flanks of the arch. On the west,
straight contours suggest a rather simple slope in basement; on the east, uneven
contours suggest downfaulting and the presence of minor anticlinal spurs extending
into the Outeniqua Basin. There are thick deposits in the Outeniqua Basin, but a
relatively thin sediment layer is developed beneath the continental slope on the west
and little or no post-Palaeozoic material is present on the steep fault-controlled
southern slope of Agulhas Arch (Emery et al. 1975).
Beneath the deposits of the Outeniqua Basin a number of small basement ridges
strike in a west-north-west to east-south-east direction as extensions of anticlinal
arches in the Cape Fold Belt (Dingle 1973a). It is doubtful, however, whether they will
have a noticeable effect on the gravity field, not only because of their small size but
because the density contrast between Palaeozoic basement and the most deeplyburied sediments is probably low. At the lower continental slope the Outeniqua Basin
is effectively closed by the Agulhas marginal fracture ridge (Scrutton & du Plessis
1973). The relationship between the ridge and the Agulhas Arch is at present unknown.
The Mallory Seamount chain lies on oceanic crust and parallels the marginal
fracture ridge. Du Plessis & Simpson (1974) have suggested that towards the southwest the seamounts pass into a small but continuous ridge that in places crops out
on the sea floor. Together, the seamounts and ridge constitute the southern edge of
the Agulhas fracture zone. Adjacent to the Agulhas Arch the fracture zone is in the
form of a fault-bounded trough containing at least 2 km of low-density sediments
(Scrutton et al. 1974). The marginal fracture ridge, seamount ridge, and trough all
act as sediment traps confining detritus to the shelf and slope so that the crust in the
Agulhas Basin carries only a thin veneer of sediment (Ewing et al. 1969). On the north
side of the fracture zone, in the southern Cape Basin, oceanic basement is deeper, with
about 2 km of predominantly clastic material on top of it (Emery et al. 1975). Towards
the continental slope bordering the Cape Basin, the sediment thickness varies between
0 km and at least 3 km due to the presence of ridges and troughs in basement.
Gravity
Previous gravity surveys have revealed few details of the crustal structure of this
continental margin. Graham & Hales (1965) presented a free-air anomaly contour
map for the Agulhas Bank region, but their small amount of interpretation was
hampered by a lack of seismic control. Even so, they were able to deduce that a steep
gradient occurs in the Moho beneath the continental slope in the Indian Ocean.
Hales & Nation (1972) made a further, brief interpretation of Graham and Hales'
data to show that their new seismic results were not ' in serious conflict with the gravity
data available '. No conclusions were drawn from this, however.
Lamont-Doherty Geological Observatory have published their gravity data from
the region in two forms: as a free-air anomaly contour chart (Talwani & Kahle, in
press) and as isostatic anomaly profiles (Talwani & Eldholm 1973). The latter show
612
R. A. Scrutton
that all round the Agulhas Bank a mass deficit exists beneath the continental slope
accompanied by a mass excess beneath the continental rise. It is interesting to note
that this mass distribution is the opposite of that discovered by early gravity measurements in the north-west Atlantic (Worzel 1965). Apparently, a mass deficit beneath
the rise is not an ubiquitous feature of rifted margins.
Emery ef al. (1975) have made a preliminary interpretation of the data used in
this paper.
3. Gravity data
A vibrating string accelerometer (VSA) mounted on a Sperry Mark 19 Mod 3C
gyrocompass was used aboard Atlantis I1 to measure variations in the earth‘s gravity
field. Data reduction to free-air and Bouguer anomalies (relative to the IGF 1930)
took place on board ship. Details of the gravimeter installation are given by Bowin,
Aldrich & Folinsbee (1972) and the data reduction system is described by Groman
et al. (1972).
It is thought that the accuracy of the gravity data is high, although there are no
track cross-overs to check the internal accuracy of the survey. A satellite navigation
system supplemented by ‘ Omega ’ and dead reckoning sensors provided navigational
accuracy of If: 1 km or better, but the inherently high drift rate of the VSA may have
introduced errors in the final gravity values. It is assumed that the free-air anomalies
are accurate to + 2 mgal. During interpretation no attempt has been made to fit
calculated gravity anonlalies to the observed to better than + 5 mgal. No regional
gradients caused by variations in mantle structure or composition have been removed
from the data during interpretation (See Regional Interpretation below).
A contour chart of free-air anomalies has been constructed for the continental
margin around Agulhas Bank (Fig. 4). The chart is redrawn from Graham & Hales’
(1965) and Talwani & Kahle’s (in press) charts, using the new Atlantis ZI data as
control. There do not appear to be any systematic errors between the three sets of
data; errors are randomly positive or negative and average out to approximately
zero. In regions of steep gravity gradient, however, individual errors of up to 35 mgal
are observed, probably as a result of navigational inaccuracies in the older data. In
view of this degree of comparison a contour interval of 20 mgal is considered appropriate for the contour chart.
Also shown in Fig. 4 are the Bouguer anomaly contours of the gravity survey of
South Africa (Smit et al. 1962). Where these intersect the coast they were used as a
further control on contouring the offshore data, knowing that Bouguer and free-air
anomalies should be equal at the coast-line.
4. Gravity interpretation
This has been carried out in three stages: a preliminary interpretation of the
gravity values on a regional scale, a qualitative description and interpretation of the
contour chart, and model fitting to the observed Atlantis 11, profiles 10, 12, 14 and 15.
Regional interpretation
In order to investigate the degree of isostatic balance of the region the mean of the
free-air anomalies measured by Atlantis ZI was calculated. Although there is a general
trend from positive free-air values over the shelf to negative values over the abyssal
areas, reflecting the continental edge effect, the mean obtained was - 1 mgal. This
is an indication that the region as a whole is in isostatic balance. Furthermore, this
might show that there are no density anomalies within the mantle beneath this region.
Evaluations of the regional free-air anomaly from two other sets of data, namely,
Crustal structure south of South Africa
613
measurements of satellite orbital characteristics and older marine data, support this
conclusion (Talwani 1971).
Free-air anornaZy contour chart (Fig. 4)
As is to be expected in a continental margin region the free-air anomaly chart is
dominated by the continental edge effect. Nevertheless, important anomalies can be
seen superimposed on this, indicating local variations in crustal structure.
In the Atlantic Ocean the edge effect is of an amplitude typical of rifted margins,
approximately 50 mgal peak-to-peak. The large positive anomalies observed north of
Cape Town (Scrutton 1973b),thought to be due to density variations in basement, are
not observed in this area. At 36" S over the continental slope the 0 mgal contour is offset approximately 100km in a left-lateral sense. The position of this offset correlates
well with the landward continuation of a fracture zone (Fig. 3) invoked by Larson &
Ladd (1973) to account for a small displacement of oceanic magnetic lineations in the
Cape Basin. Since there normally are gravity anomalies associated with fracture zones
and margin offsets (Cochran 1973), there could be a relationship between the gravity
and magnetic displacements observed here. A small offset might, therefore, be present
where the proposed fracture zone meets the continental edge at 36" S. On bathymetric
grounds, the fracture zone itself would appear to be the northern boundary of the
Cape Rise upwarp.
The negative component of the continental edge effect becomes very deep south
of the tip of Agulhas Bank where it is Aanked by steep gravity gradients of up to
8 mgal km-'. It is the net result of a large edge effect and the effect of the sedimentfilled depression within the Agulhas fracture zone. The gravity data support the
contention that the depression is fault bounded. On the southeast side of the depression
the steep gravity gradient coincides with the northern limb of the Cape slope magnetic
anomaly (du Plessis & Simpson 1974) (Fig. 3), marking the faulted edge of oceanic
crust at the Agulhas fracture zone. Just within the oceanic crust the Mallory Seamount
chain produces a gravity high, peaking at about 150 mgal above Shackleton Seamount
(Graham & Hales 1965). It is found that if the shape of this seamount is approximated
~ , seamount's topography can
by a vertical cylinder model of density 2-83 g ~ m - the
entirely account for the observed gravity anomaly. It seems unlikely, therefore, that
there is any crustal thickening or a solidified magma chamber beneath the seamount
chain, as has been observed elsewhere (Case et al. 1973).
Towards the north-east the above-mentionedgravity anomalies are diminished as a
result of the change in continental margin structure seen in Fig. 2 at about 22.5"E.
Despite this change, the gravity high associated with the seamounts can be traced
until it is again prominent beyond 24" E, where a small peak in the associated basement ridge may occur. The accompanying gravity low, north-west of the high, is also
apparent east of 24" E where, rather surprisingly, it occurs in part over the marginal
fracture ridge, which is a basement high. It reaches a peak value of -74 mgal over the
ridge's south-eastern flank. Since the ridge has no magnetic signature (Scrutton &
du Plessis 1973), the possibility arises of it being composed of predominantly sedimentary rock of moderate density, giving a reduced gravity effect. The gravity low
would then of necessity be caused by a deeper structure related to the formation of
the Agulhas fracture zone.
On the continental shelf, over the crest of the Agulhas Arch, the general level of
free-air gravity is +20mgal. This might appear low for an area of outcropping
basement, but since the exposed rocks are predominantly granitic and sedimentary
(Gentle 1970) it is a realistic value. We can see reflected in the gravity contours the
relative complexities of the western and eastern sides of the Agulhas Arch, the eastern
being the more complex. It is worth noting that no gravity anomaly is associated with
the Alphard Banks igneous province situated on the northern side of Agulhas Arch
a
614
R. A. Scrutton
(Dingle & Gentle 1972) (Fig. 3). The free-air anomalies in this area are near to zero.
Evidently, any magma chamber that fed the vents and plugs is small or no longer
present in the crust, or the magma was derived directly from the upper mantle.
To the east, the Cape fold belt anticlinal structures occurring beneath the narrowing
shelf produce a weak west-north-west-east-south-easttrend in the contours near the
coast. A small density contrast between the basement extensions and the most deeplyburied sediments of the Outeniqua Basin would account for the weakness of this
trend. South-eastwards, near 353"S, 23YE, where the Outeniqua Basin is about
2 km deep and basement has dropped to 4 km below sea level, the gravity effect of the
anticlinal extensions is lost altogether and low gravity values occur. An extremely
quiet magnetic field is consistent with the greater depth to basement in this area (Fig.
3). High gravity values, up to 34mga1, characterize the shelf region southwest of
Cape St Francis, but the reason for them is not apparent. Since there is no thinning of
low density sediments here, density variations in basement might be the cause.
Profile interpretations
Two-dimensional crustal structure models have been constructed for Atlantis ZZ
profiles 10, 12, 14, and 15. Profiles 12 and 14 cross the margin near to the right-angled
tip of Agulhas Bank where the assumption that structures are two-dimensional is not
perfectly valid. It is estimated, however, that because of this the calculated anomalies
for traverses 12 and 14 will only be about 5 mgal too high. Fortunately, errors of this
magnitude will not bother us, because a fit between calculated and observed profiles
of no better than 5 mgal is acceptable.
To remove the overshadowing edge effect from the observed free-air anomalies, and
thus detect departures of the crust from perfect isostatic equilibrium, an Airy isostatic
model of the crust was designed for each traverse. The free-air values due to the
models were subtracted from the observed values to give isostatic anomaly profiles.
A standard section for the crust, from which the models were derived, was established
at the coastline where analyses of gravity and seismic survey data have shown that
the crust is most probably 30 km thick and comprised of a layer 8 . 3 km thick of
density 2.67 g cm-3 and a layer 21 - 7 km thick of density 2-87 g cm-'. The mass of
a column of unit area of this standard crust together with 5 km of mantle of density
3.4 g cm-3 is approximately equivalent to the mass of other crust or crust plus
mantle standard sections 3 5 k m in thickness (Heiskanen & Vening Meinesz 1958;
Worzel & Shurbet 1955; Woollard 1959; Talwani, Sutton & Worzel 1959), that is
lo7 g m for a 1 cm2 column.
Fig. 5 shows the crustal models and their associated gravity profiles. For traverses
10 and 12 the calculated and observed profile are matched at the coastline but for
traverses 14 and 15 they are matched over the Agulhas Arch where basement comes
within 80 m of sea level. Also shown in Fig. 5 is the seismic control discussed earlier
in the paper. At the top of the crust sediment bodies are outlined and within the crust
refracting interfaces are indicated. In constructing the final crustal structure models
(Fig. 6) use was made of this control, firstly by calculating and removing from the isostatic gravity anomalies the effect of the sediment layer, then modifying the structure of
the remainder of the crust to compensate for the sediments and satisfy the isostatic
anomalies. Throughout this process other constraints on the structure were also
considered. Limiting depths to compensating and anomalous features were calculated
from the isostatic anomalies using Bott & Smith's rules (1958). It was found that,
theoretically, all the major anomalies could be satisfied by adjusting the topography
of the Moho or varying the density of the lowermost crust. In practice, however, this
would have produced a near vertical Moho in places, which is clearly unrealistic. In
general, therefore, the bulk of any anomaly was satisfied by adjusting the Moho or
density of the lower crust to a realistic extent and the remainder of the anomaly was
Crustal structure south of South Africa
615
accounted for by adjusting the mid-crustal structure. No adjustments were made to
the structure of the sediment layer since the seismic data define it well enough for the
purposes of this interpretation.
General features of the profiles and sections. The four isostatic anomaly profiles in
Fig. 5 vary in shape over the continental shelf but have much in common over the
slope and upper continental rise. A pattern consisting of negative isostatic anomalies
over the slope and positive anomalies over the rise is observed, although this is
developed to varying degrees on different profiles. Those profiles across the continental margin in the Atlantic Ocean have anomalies of smaller amplitude and gentler
gradients than those in the Indian Ocean. An inspection of Talwani & Eldholm’s
(1973) profiles reveals, however, that large anomalies and steep gradients occur again
north of traverse 15. Between the negative and positive anomalies is a slope in the
isostatic gravity profiles that Talwani & Eldholm believe marks the boundary separating continental from oceanic crust. We can see from Fig. 6 that this is in fact the case,
and that it is particularly clear in the Indian Ocean, on profiles 10 and 12, where the
gradient coincides with the Agulhas fracture zone (Cape slope anomaly). Talwani &
Eldholm (op. cit.) also think that the isostatic low on the continental side of the
boundary is due to the subsided but uncompensated edge of continental crust, with
perhaps, a thick section of sediments beneath the slope. This could also be true, but
it is less obvious on our crustal sections. In order to satisfy the isostatic low along the
Indian Ocean margin, where the slope is totally or partially free of sediments, it was
indeed necessary to place the Moho at a deeper level (move it seawards) beneath the
slope. In the Atlantic Ocean, however, where the sediment layer is thicker, any uncompensated deepening of the Moho is masked by the compensation for the sediments.
No explanation is given by Talwani & Eldholm for the isostatic gravity high on the
oceanic side of the boundary. From the interpretation presented here it would appear
that in general a shallower-than-normal Moho beneath the continental rise could
account for it. Shallow structure undetected by the seismic data or density variations in
the crust or mantle could also be causes, but refraction data in the Indian Ocean support the presence of a thin crust. Thus, whereas the continental edge has subsided to
excess, the adjacent oceanic crust may have been restricted in its subsidence. This
situation leads to a steep gradient to the Moho at the continental edge and has implications for the evolution of both segments of this inactive margin. These will be discussed
later.
When the isostatic profiles are corrected for the effect of the sediment layer, high
anomalies are obtained over the continental shelf. These correspond in extent to the
Outeniqua Basin and are indicative of compensation for the low-density sediments in
the basin. Hales & Nation’s (1972) seismic refraction results suggest that the form of
the compensation is an increase in density of the lower crust with no crustal thinning.
This is shown schematically in Fig. 6 as a near-vertical-sided layer of density 3-15
g ~ r n - ~The
. development of this high density phase in the lower crust may have
accompanied, and possibly initiated, the subsidence of the Outeniqua Basin. Since
Dingle & Scrutton (1974) have pointed out that the basin began as an intracontinental depression in early Mesozoic time, we could envisage the 3 - 15 g cm-3 compensating layer being locally present at the base of the crust during the formation of
the continental margin and the Agulhas fracture zone at the end of the Jurassic
period. This possibility is important to the evolution of the offset margin.
Individual features of the profles and sections. Traverse 15 reveals the simplest crustal
structure. To obtain a reasonable fit between the observed and calculated gravity
profiles it was found unnecessary to resort to introducing mid-crustal structure.
There is, however, scope in the fit to allow for the presence of a thick section of
sediments beneath the slope as suggested by Talwani & Eldholm (1973). The increase
616
R. A. Scrutton
in amplitude of their isostatic anomalies to the north, mentioned above, suggests
that traverse 15 lies near the southern end of such a structure. The fracture zone
proposed by Larson & Ladd (1973) rnight terminate the structure at 36" S. It was also
found unnecessary to incorporate a layer of high density in the lower continental
crust beneath the Outeniqua Basin, probably because the north-east end of the traverse
only just enters the north-western tip of the basin. Oceanward of the shelf break a
match with the observed gravity was obtained by thinning the crust by up to 4 km,
although small density changes could have been involved. From the resulting structure
the continent-ocean boundary would appear to be beneath the base of the continental
slope.
Traverse 14, crossing the margin between the fracture zone of Larson & Ladd
(1973) and the Agulhas fracture zone, shows a very different structure, with no scope
for a thick sequence of sediments beneath the slope and rise. A region of crust of
intermediate thickness occurs beneath the continental rise and appears to be bounded
to the north and south by the two fracture zones. At this point the Cape Rise meets
CONTINENTAL
CRUST
.........
.........,
SEDIMENTARY B A S I N
ON C O N T I N E N T A L CRUST
THICK O C E A N I C
ml
T H I N N E D , UPLIFTED & M E T A M O R P H O S E D
C O N T I N E N T A L CRUST
_--
CONTINENTAL
000
VOLCANIC
MAJOR
CRUSTAL
CRUST
FRACTURES
EDGE
IN
ATLANTIC
OCEAN
RIDGE
FIG.7. Sketch of proposed crustal types at the continental margin south of South
Africa.
Crustal structure south of South Africa
617
the continental edge. If, as has been suggested, the rise is the trace of the Bouvet
Island mantle plume (Johnson, Lowrie & Hey 1973), the crust of intermediate thickness can be interpreted as the landward limit of a swathe of thicker-than-normal
oceanic crust produced by excessive volcanic activity. A very good example of this
type of feature is the Greenland-Iceland-Faeroes Ridge (Bott, Browitt & Stacey
1971) in the North Atlantic.
On Section 12, a narrow zone of intermediate thickness crust is included beneath
the sediment-filled depression between the Cape slope magnetic anomaly and the
edge of the continental crust. It constitutes the Agulhas fracture zone at this point.
There is clearly one major fracture abruptly truncating the thin oceanic crust at the
Cape slope anomaly, while there may be another major fracture against true continental crust. A variable mid-crustal structure is incorporated in this profile in order to
satisfy the steeper gravity gradients, and this emphasizes the location of the major
fractures. The origin of this zone of crust of intermediate thickness is discussed in the
next section.
Traverse 10 crosses the Outeniqua Basin just west of its closure against the Recife
Arch (Fig. 7), a feature similar to the Agulhas Arch, and provides a section across
the Agulhas marginal fracture ridge. Beneath the ridge the crust is 23 km thick,
giving the impression that it is an integral part of the continent. It is possible a major
fracture occurs between the ridge and true continental crust, but it is not clear from
the gravity data. In front of the ridge we see a structure similar to that on traverse 12:
a major fracture abruptly truncates an upturned, thin oceanic crust. This fracture lies
beneath the Cape slope anomaly and within the Agulhas fracture zone.
5. Discussion of margin structure
Structure at rifted margin segment
The gross nature of the continent-ocean transition west of the Agulhas Arch is
revealed by traverse 15. A relatively thin sediment layer on the continental shelf
passes seawards into a thicker layer on the continental slope and rise. Overcompensation for this sediment thickening is apparent at the base of the crust where there
is a rapid seaward rise in the Moho to a depth less than the Airy isostatic depth
beneath the continental rise. Comparison of this transition with others at rifted
margins shows that it is rather different. Off eastern North America (Emery et al.
1970) and western southern Africa (Rabinowitz 1972; Scrutton 1973b; Emery et al.
1975),for example, there are thicker sediment wedges across the margins and relatively
high isostatic anomalies occur over the outer shelf and slope. The anomalies can be
totally or partially accounted for by the presence of a basement ridge beneath the
shelf break or an increase in basement density, neither of which are found off the
Agulhas Bank. Any anomalies remaining after this shallow structure and its compensation has been taken into consideration tend to show a mass deficit beneath the
continental rise, the opposite to that found here.
The observed configuration of the Moho west of Agulhas Bank, together with the
presence of a mass excess beneath the rise, is therefore somewhat unusual and has
implications for some of the mechanisms put forward by various authors for the
evolution of rifted continental margins. These mechanisms are thermal evolution, as
proposed by Sleep (1971), which leads to a relative elevation of the Moho as both
continental and adjacent oceanic crust cool and contract; sediment loading and
consequent crustal flexure, as advanced by Walcott (1972), leading to a relative
depression of the Moho beneath the continental shelf, slope and rise; transfer of
material by crustal creep from beneath the shelf and s!ope to beneath the rise, as
propounded by Bott (1971), leading to a relative elevation of the continental Moho
and depression of the oceanic Moho; migration of the Moho in response to sea-floor
618
R. A. SCrnttoa
spreading rate changes at the mid-ocean ridge, causing an overall elevation or depression; and interpretation of the Moho as the basalt-eclogite phase change whose
position changes in response to sediment loading at the top of the crust or temperature variations at the base of the crust, which might be applicable to continental
regions but not oceanic (Wyllie 1971). It would appear that none of these mechanisms
operating alone can explain restricted subsidence of the Moho beneath the continental
rise together with its excessive depression beneath the shelf and slope as observed
here. Perhaps two, or more of them, operating together, for instance thermal contraction and migration of the basalt-eclogite phase change, can explain it. If so, the
structure discovered here would support the hypothesis that a number of mechanisms
operate simultaneously as an inactive continental margin evolves (Rona 1974), some
outweighing others in their effect.
Deep structure at ofSset margin segment
The isostatic gravity anomalies and their associated structures seen on the rifted
segment of margin are even more pronounced on this offset segment. Particularly
apparent is the abrupt change across the Agulhas fracture zone from crust of continental thickness to crust of oceanic thickness. Moho gradients are steep and they could
have been made steeper by omitting the mid-crustal structure. However, it is primarily
the lack of thickening of the oceanic crust as the continental edge is approached that
produces the abrupt transition. In fact, the faulted edge of oceanic crust appears to be
upturned at the Agulhas fracture zone, causing further shallowing of the Moho.
An explanation of this accentuated structure might lie in the fact that a margin
offset perpendicularly links two tension-formed rift zones. In this situation it is not
subjected to all the margin-forming processes that affect rifted margin segments.
Firstly, the offset segment does not undergo crustal stretching but, instead, experiences
shearing stresses. Secondly, in the central part of the offset, away from the rift zones,
it is not subjected to the high temperatures that characterize rifts for a prolonged
period of time (c. 50 My) prior to crustal splitting. The crust at the central part of the
offset therefore does not experience significant thermal expansion and contraction.
Thirdly, an offset probably does not receive the overall heavy sedimentation that
produces the thick sediment wedges seen at rifted margins. This is because sediment
traps, such as the marginal fracture ridge, commonly develop at the continental slope
of margin offsets (Francheteau & Le Pichon 1972), thus preventing sedimentation on
the continental rise. It is likely, therefore, that crustal thinning and subsidence mechanisms dependent upon any of these three factors-rustal
stretching, high temperatures, heavy sedimentation-will be reduced in effect at margin offsets. This ultimately
leads to a steep Moho gradient and the development of a large mass excess beneath
the continental rise.
These arguments are not applicable to the rifted margin segment discussed above.
We must still appeal to a complex interaction of fully operative margin-forming
processes in order to explain the mass excess there.
Shallow structure at ofSset margin segment
The geological evolution of this segment of continental margin has been outlined
early in the paper. Although it is now clear that the Agulhas fracture zone controls
the trend of the margin, the origin of the structures closely associated with it is still
puzzling. Prior to the separation of South America from Africa, the Agulhas Arch
probably continued as a basement high on to Falkland Plateau. An examination of
the seismic refraction data of Ewing et al. (1971) shows that the arch could have been
continuous with the Falkland Islands high. If this is so, the 40 km wide strip of crust
of intermediate thickness seen beneath the trough on Section 12 must have come into
existence since the Palaeozoic and probably since movement between the continents
Crustal structure south of South Africa
619
along the Agulhas fracture zone began. Magnetic profiles 24-27 of du Plessis &
Simpson (1974) clearly show over the narrow strip a magnetic anomaly similar in size
to those associated with the nearby oceanic crust. From this observation, it can be
concluded that at least part of the basement beneath the sediment infilling the trough
is magnetic and probably basaltic in character. Basement here, therefore, differs from
that of the Agulhas Arch and the Agulhas marginal fracture ridge, both of which are
nonmagnetic. There is bathymetric continuity between the trough and the Cape
Rise of thickened oceanic crust, so there may also be a genetic connection. Assuming
this, the trough is envisaged as being floored by a sliver of thickened oceanic crust,
probably much faulted and dynamically metamorphosed within the Agulhas fracture
zone.
The trough appears to maintain its character to 22"E, then passes via a bathymetric step into the marginal fracture ridge. With the data now available it is possible
to put forward a hypothesis for the origin of this ridge. It appears to have a close
relationship with the adjacent continental crust, since it exists only where the Outeniqua Basin and the associated high-density lower crust occur beneath the shelf. It also
appears to have a relationship with the formation of the continental edge and the
Agulhas fracture zone because of their common parallelism. The lack of magnetic
signature and the crustal thickness of 23 km beneath the ridge both suggest affinities
with continental crust. Geophysical evidence suggests that the Ivory Coast-Ghana
Ridge is an analogous feature, hence the dredging from it of a suite of rocks including
coarse-grained and ferruginous micaceous sandstones and schists (Deltiel et ab. 1974)
is further evidence for a continental origin. Clearly, the weight of evidence is in
favour of a continental origin for the marginal fracture ridge.
If we attempt to explain the emplacement of the marginal fracture ridge in terms
of Le Pichon & Hayes' (1971) theory for the remobilization of continental crust in a
continent-confined shear zone, it is necessary to account for its restriction to the
margin of the Outeniqua Basin. Scrutton & du Plessis (1973) have already suggested
that the degree of development of such a ridge depends on the gross composition of
the crust through which the shear passes. If this is true, then the presence of the
embryonic Outeniqua Basin together with, perhaps, the high density lower crust
since early Mesozoic times may have facilitated remobilization to produce a significant
ridge at the time of continental break-up. Unfortunately, modem examples of major
continent-confined shear zones, the San Andreas fault and the Dead Sea fault system,
give no indication of the formation of ridge-like features. They do show, however,
lens faulting that can isolate a sliver of continental crust.
From the evidence presented above it would appear that the best explanation of the
marginal fracture ridge is that it is a sliver of continental crust, fault-bounded and
possibly dynamically metamorphosed. It would have been produced in the continentconfined shear zone that became the Agulhas fracture zone. The local geology of the
crust facilitated the metamorphism, thus restricting the ridge to a limited sector of the
offset margin segment.
7. Conclusions
Four major conclusions can be drawn from the preceding interpretation and
discussion of the Atlantis If gravity data. They concern both shallow and deep crustal
structure, a summary of which is given in Fig. 7, where an attempt is made to delineate
crust types around the southern tip of Africa. The conclusions are:
(1) South of South Africa the free-air gravity field over the continental shelf
and upper continental slope is dominated by high values over the Agulhas Arch
antiform and low values over the Outeniqua Basin of post-Palaeozoic sediments.
This pattern is modified by smaller structures and density variations in the pre-
620
R. A. Scrutton
Mesozoic basement. In deeper water areas, fracture zones and their associated
structures control the gravity anomalies.
(2) Isostatic gravity anomalies reveal a mass deficjt beneath the outer continental
shelf and continental slope and a mass excess beneath the continental rise. This
distribution of mass is somewhat unusual at inactive continental margins, but it
can be explained by interaction of the various processes of continental margin evolution. In the Indian Ocean, the observed mass distribution is more pronounced
than that in the Atlantic Ocean. This is attributed to the reduced effect of some of the
processes of margin evolution at this margin segment, which is an offset margin
segment as opposed to a rifted one.
(3) In the Agulhas fracture zone, along the Indian Ocean margin, there is a basement trough and a basement ridge. The former is thought to be underlain by a
sliver of thickened, dynamically metamorphosed oceanic crust, the latter by a sliver
of upfaulted, dynamically metamorphosed continental crust. These structures may be
typical of margin offsets.
(4) Two features of excessive volcanic activity are apparent in the oceanic regions
south of South Africa. One is the Cape Rise basement high, underlain by thickened
oceanic crust and forming the southern limit of the continental rise and slope sediment
prism in the Atlantic Ocean. The other is the Mallory Seamount chain, which
apparently is not associated with any anomalous deep structure.
Acknowledgments
I thank K. 0. Emery and E. S. W. Simpson for providing me with the opportunity
to sail on Atlantis 11. K. 0. Emery and C. 0. Bowin kindly allowed me to work
on the Woods Hole gravity data. I have had fruitful discussions with R. V. Dingle,
and A. P. Stacey has given valuable criticism of the paper. Diana Guthrie and
Kay Stavert have helped with the preparation of the manuscript.
The Eastern Atlantic Continental Margin Programme was funded by National
Science Foundation Grant GX-28193 to Woods Hole Oceanographic Institution.
Appreciation is expressed to all those who contributed to the collection of the gravity
data, especially E. Uchupi, Senior Scientist.
Grant Institute of Geology,
Uniuersity of Edinburgh,
Edinburgh EN9 3JW.
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