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Tecfonophyslcs, 155 (1988) 235-260
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
235
Evolution of the Southwest Indian Ridge from the Late
Cretaceous (anomaly 34) to the Middle Eocene (anomaly 20)
JEAN-YVES ROYER’, PHILIPPE PATRIAT ‘, HUGH W. BERGH 3
and CHRISTOPHER R. SCOTESE 4
’ Instllute for Geophysics, University of Texas at Austin, 8701 Mopuc Boulevard Austin, TX 7x759 (U.S.A.)
’ Lahorutoire de GPophysique Marine et CNRS UA 279, Institut de Physique du Globe de Paris, 4 place Jussletr.
75252 Paris Cedex 05 (France)
’ Bernard Price Institute of Geoph_wical Research, University of Witwatersrand, I Jan Smuts Avenue, Johunneshurg 2001 (South Afrrcu)
4 Shell Development Co., Bellaire Reseurch Center, P.0. Box 4X1. Houston. TX 77001 (U.S.A.)
(Received March 14, 1987: revised version accepted August 9. 1987)
Abstract
Royer. J.-Y., Patriat, P., Bergh, H.W. and Scotese, C.R.. 1988. Evolution of the Southwest Indian Ridge from the Late
Cretaceous (anomaly 34) to the Middle Eocene (anomaly 20). In: C.R. Scotese and W.W. Sager (Editors). Mesozoic
and Cenozoic Plate Reconstructions. Tectonophysics. 155: 235-260.
The determination of the motion of Antarctica relative to Africa is particularly important when considering the
breakup of Gondwana. Two models have been proposed that describe the pattern of seafloor spreading between Africa
and Antarctica during the Late Cretaceous (starting at chron 34, 84 Ma) through to the Middle Eocene (chron 20, 46
Ma). In the first model, the motion of Antarctica relative to Africa can be simply described by a rotation about a single
pole of rotation. In the second model, which we favor. the relative motion of Antarctica and Africa is more complex,
and a major change in spreading direction between chron 32 (74 Ma) and chron 24 (56 Ma) times is required.
In this paper we present ten plate tectonic reconstructions of the Southwest Indian Ridge that were produced using
a new compilation of magnetic, bathymetric and satellite altimetry data, in combination with interactive computer
graphics. These reconstructions illustrate that spreading directions started to change at chron 32 (74 Ma). Between
chrons 31 and 28 (69-64 Ma), spreading was very slow (c 1 cm/yr) and the direction of spreading changed from
NE-SW to a more N-S direction. Between chrons 26 and 24 (61-56 Ma) the direction of spreading shifted back to a
NE-SW orientation. These changes in spreading direction suggest that the present-day fracture zones in the area of the
Prince Edward Fracture Zone are younger features (Eocene) than their lengths might imply. Our result& also provide
important constraints concerning the Mesozoic reconstructions of the Indian Ocean and the motion of South America
relative to Antarctica prior to the Eocene.
Introduction
The history of relative motion between Africa
and Antarctica is the key to understanding the
dispersal of the fragments of Gondwana after
their initial breakup in Late Jurassic. The occur-
rence of a Paleocene fossil ridge in the Mascarene
Basin (Schlich, 1982), separating Madagascar from
the Seychelles-Saya de Malha Bank, makes it
0040-1951/88/$03.50 6: 1988 Elsevier Science Publishers B.V
difficult to resolve the motion of Africa relative to
India prior to chron 29 (66 Ma). Similarly. the
complex development of the Scotia Sea and the
interaction of the South Sandwich Trench with the
American-Antarctica Ridge (Barker et al., 1984)
do not permit direct determination of the relative
motion between South America and Antarctica
prior to chron 21 (50 Ma). Therefore, models of
the evolution of the Indian Ocean depend heavily
236
on an understanding of the relative motions of Jurassic. the relative motion of these IWO contl-
Africa with respect to Antarctica, South America nents is not well constrained. This paper descrihe,s
with respect to Africa (Ladd, 1974; Cande et al.. the seafloor spreading history along the Southwest
in press), and India with respect to Antarctica Indian Ridge during the Late Cretaceous and Early
(Patriat, 1983). Although magnetic anomalies indi- Tertiary (chron 34-chron 20).
cate that seafloor spreading between Africa and The early history of seafloor spreading between
Antarctica has been taking place since the Late Africa and Antarctica can be reconstructed using
Enderby Basin
____ Ridge Axis
-------. Bathymetric Deeps
Fig. 1. Tectonic elements of the Southwest Indian Ridge. Contours are coastlines, limits of the continental shdf and the 2000 and
4000 m isobaths. Dashed tines show &be structural grain according to &he bathymetric deeps. The present ridge axis (continuous line)
is traced from bathymetry, Seasat data, axial magnetic anomalies and earthquake epicenters.
237
the identifications of Mesozoic magnetic anoma-
lies (M22-MO) in the Mozambique Channel
(Segoufin, 1978; Simpson et al., 1979) and their
counterparts to the south, in the vicinity of Dron-
ning Maud Land (Bergh, 1977). Exactly how these
sets of magnetic anomalies must be matched,
however, is still controversial (Norton and Sclater.
1979: Segoufin and Patriat, 1981; Martin and
Hartnady. 1986). Disagreements arise from the
fact that there are no data from the Cretaceous
Quiet Zone (118-84 Ma) and that various recon-
structions have been proposed for the configura-
tion of the Southwest Indian Ridge at chron 34
(84 Ma).
In contrast. the Eocene to Recent development
of the Southwest Indian Ridge is better under-
stood, and between 46” and 70” E has been in-
vestigated in detail (Fig. 1) (Patriat, 1979, 1983;
Tapscott et al.. 1980; Sclater et al., 1981). From
chron 20 (46 Ma) to the present, the Southwest
Indian Ridge has been spreading very slowly (< 1
cm/yr) in a N-S direction. Although plate recon-
structions for this time interval are based on data
from only the eastern part of the Southwest In-
dian Ridge, the rotation parameters are well
constrained by symmetric sets of magnetic
anomalies, and the results are in good agreement
with rotations calculated by solving the plate cir-
cuit Africa-India-Antarctica (Patriat, 1983;
Patriat and Segoufin. this issue).
Because of the remoteness of Antarctica and
the paucity of oceanographic surveys at high
southerly latitudes, the magnetic anomaly data
that describe the period of seafloor spreading be-
tween the Late Cretaceous (chron 34, 84 Ma) and
the Middle Eocene (chron 20, 46 Ma) come prim-
arily from the area north of the Southwest Indian
Ridge. Late Cretaceous and Early Tertiary mag-
netic anomalies have been described from the
Agulhas Basin, Mozambique Basin and in the
southwest section of the Madagascar Basin (Bergh
and Norton, 1976; LaBrecque and Hayes, 1979;
Patriat. 1979). Magnetic anomalies in these areas
are often difficult to identify due to slow spread-
ing rates, the occurrence of numerous volcanic
edifices. and the extremely rugged topography re-
sulting from the complex system of deep, en eche-
lon fracture zones (Fig. 1).
Because of the poor constraints provided by the
magnetic data, some of the first reconstructions of
Africa and Antarctica were based on the assump-
tion that the continuous trends of well-defined
fracture zones, such as the Prince Edward Frac-
ture Zone, were “flowlines” that described the
relative motion between Africa and Antarctica
since the Late Cretaceous. This assumption led to
the conclusion that the motion between Africa
and Antarctica during the last 80 m.y. could be
modeled by a single rotation about a stationary
Euler pole (Norton and Sclater, 1979; Fisher and
Sclater, 1983). This simple assumption was chal-
lenged, however. by new identifications of
anomalies 33 (80 Ma) and 34 (84 Ma) in the
vicinity of the Prince Edward Fracture Zone (Fig.
1) by Patriat et al. (1985). Their identifications
required that a significant change in spreading
directions must have taken place between chron
32 (74 Ma) and chron 24 (56 Ma). This change in
spreading direction, also illustrated by Larson et
al. (1985) suggests that large-offset fracture zones,
such as the Prince Edward Fracture Zone, are not
as continuous as they appear, and consequently,
they cannot be used to derive rotation parameters
for long periods of time.
In this paper we present a new compilation of
magnetic, bathymetric and satellite altimetry data
(Seasat) that document the change in spreading
direction along the Southwest Indian Ridge dur-
ing the Late Cretaceous and Early Tertiary. These
data. in conjunction with the Evans and Suther-
land interactive computer graphics system, have
been used to produce plate tectonic reconstruc-
tions of Africa and Antarctica for chrons 34 (84
Ma), 33 (80 Ma), 32 (74 Ma), 31 (69 Ma), 29 (66
Ma), 28 (64 Ma), 26 (61 Ma). 24 (56 Ma), 21 (50
Ma) and 20 (46 Ma). The area of investigation is
centered on the Prince Edward Fracture Zone and
extends from 5” to 65” E, and from 20” to 65”s
(Fig. 1).
Data compilation
Buth,vnetq~ and mugnetic unomalies
Our compilation is based mainly on bathymet-
ric and magnetic data published during the last
23X
Fig. 2. Profiles of residual magnetic anomalies at right angles to tracks in the Southwest Indian Ocean. South African research vessel
tracks are represented by dashed lines and National Geophysical Data Center data by continuous lines.
239
Fig. 3. interpretation of
South North
h
8myr 8m7.W
cl ZOOKm Southern Flank
South North
8 mm&r 10 nlwyr
Northern Flank
I . ’ .
2WKm 0
magnetic anomalies from some profiles in the African-Antarctic Basin. All profiles
direction N20 o E. Extent of profiles A-E are located in Fig. 2.
are projected onto
decade. Our bathymetric interpretations are based
on the work of Driscoll et al. (in press), Fisher and
Sclater (1983) and Sclater et al. (1981) as well as
the GEBCO oceanographic survey (LaBrecque and
Rabinowitz, 1981; Hayes and Vogel, 1981; Fisher
et al., 1982). The magnetic data compilation is
based on information from the National Geo-
physical Data Center, and unpublished magnetic
240
TABLE 1
Ages of the magnetic reversal boundaries picked on our mag-
netic profiles (after the reversal timescale from Kent and
Gradstein, 1986). Ages correspond to the oldest boundary of a
normal polarity event. except those marked with a y (youngest
extremity)
Anomaly Age
(Mai
20 46.17
21 50.34
24 56.14
26 60.75
2Q 64.29
29 66.17
31y 68.52
32 73.55
33 80.17
34Y 84.00
data collected by South African research vessels
south of the Southwest Indian Ridge and in the
Agulhas Basin. These new profiles are shown in
Fig. 2 and some of the profiles from the
African-Antarctic Basin are interpreted {Fig. 3).
All the magnetic anomaly data have been reinter-
preted using the magnetic reversal time scale of
Kent and Gradstein (1986). Table 1 shows the
reversal boundaries that have been picked on the
magnetic profiles. These boundaries are the same
as those used by Patriat (1983), Patriat et al.
(1985) and Barker and Lawver {in press). The
magnetic anomalies east of 46”E or those related
to the Central and the Southeast Indian Ridge, are
from Patriat (1983). The magnetic anomaly data
in these areas have been published by Schlich
(1975, 1982).
Satellite altimetry data
Satellite altimetry data have proved exception-
ally useful in remote and uncharted oceanic areas.
The excellent correlation between the small-wave-
length features of the geoid and seafloor topogra-
phy has been widely used to map to~~~c
features such as fracture zones (e.g., Sandwell and
Schubert, 1982) or uncharted seamounts (e.g.,
Lazarewicz and Schwank, 1982). Recently, Dris-
co11 et al. (in press) have tested the usefulness 01
this approach by comparing deflection of the
vertical with bathymetr~ in a particularly rugged
and well-charted area near the Southwest Indian
Ridge between 20’ and 50 o E. They showed that
the lineations seen in the geoid gradient closely
follow the fracture-zone trends although their ac-
tual locations remain difficult to determine from
the geoid data alone. This is especially the case
where the fracture zones are closely spaced, as in
the area between the Du Toit and Prince Edward
fracture zones. Using similar analysis described by
Gahagan et al. (this issue), we have correlated the
highs and lows of the deflection of the vertical in
order to determine the azimuth of the fracture
zones where only sparse bathymetric data were
available.
The Southwest Indian Ridge between 5 ’ and- 65 ’ E
Fracture zones
The topography of the Southwest Indian Ocean
floor is characterized by a series of deep and
well-delineated fracture zones. As outlined by the
GEBCO chart (Fisher et al., 1982) the main frac-
ture zones are expressed by narrow and deep
troughs, with depths sometimes greater than 6OQO
m, bordered by elevated rims, in some places
shallower than Zoo0 m. A comprehensive survey
of the westernmost Du Toit, Bain and Prince
Edward fracture zones has been made by Driscoll
et al. (in press), and similar work on the Dis-
covery, Indomed and Gall&i fracture zones has
been published by Fisher and Mater (1983).
Sclater et al. (1981) described the easternmost
Atlantis If and Melville fracture zones (Fig. 1).
All these fracture zones extend over great dis-
tances, have nearly continuous trends and are
subparallel. The structural grain, emphasized by
deep troughs, strikes 15 O-20 a NE near Prince
Edward Fracture Zone. This regular pattern and
the alignment of these fracture zones with older
structures such as the Mozambique Pla&au, has
led to the interpretation that these fracture zones
represent flowlines that describe the motions of
Africa relative to Antarctica since the Late Creta-
741
ceous (Norton and Sclater, 1979; Fisher and
Sclater, 1983).
There are relatively little bathymetric data south
of 55”S, as illustrated by the artificially smooth
and uniform contours on the most up-to-date
bathymetric map of this area (Hayes and Vogel,
1981). This lack of detail contrasts with the com-
plex meandering of the bathymetric contours north
of the Southwest Indian Ridge (Fisher et al., 1982).
South of the Southwest Indian Ridge it is not
possible to locate the counterpart of the Mozam-
bique Fracture Zone or to follow the southern
extension of the Du Toit Fracture Zone. Satellite
altimetry data (Seasat), however, provide ad-
ditional data as far south as 65” S. The
satellite--gravity map of Haxby (1985), in addition
to highlighting the linearity of the fracture zones
north of the South Indian Ridge, revealed a major
NNE-SSW trending structure in the Enderby
Basin. The southern extension of this feature ap-
pears to join the Astrid Ridge and corresponds to
the Astrid Fracture Zone described by Bergh
(1987). Although the Astrid Fracture Zone ap-
pears to be aligned with the Prince Edward Frac-
ture Zone, a clearly defined westward shift of the
Astrid Fracture Zone at about 56 “S indicates that
its northerly continuation is the Bain Fracture
Zone (Fig. 4). The observed westward shift of
fracture-zone trends at 56”s is the best evidence
for the proposed change in spreading direction
during the Late Cretaceous.
At present, the largest offset of the Southwest
Indian Ridge axis lies between the Prince Edward
and Du Toit fracture zones. The active parts of
these fracture zones intersect the ridge axis at
about 45 “S, 35’ E and 53”S 27’ E, respectively.
Over the relatively short distance between these
two fracture zones (< 200 km), the ridge axis is
offset by more than 7” ( = 800 km). Rough topog-
raphy makes it difficult to identify additional ridge
segments between the Prince Edward and Du Toit
fracture zones.
It is interesting to note that the location of the
large offset in the Southwest Indian Ridge axis
has shifted through time. The minor offset of
anomaly 34 in the Mozambique Basin (Bergh and
Norton, 1976) indicates that the offset of the Late
Cretaceous ridge axis, north of the Prince Edward
Fracture Zone. was relatively small ( < 250 km).
During the Late Cretaceous, the major offset in
the ridge axis was between the Mozambique Ridge
and the magnetic bight in the Agulhas Basin (Fig.
4). The magnetic anomaly data require that either
a ridge jump or a change in spreading direction
took place sometime between the Late Cretaceous
(84 Ma) and the Eocene (chron 20,46 Ma).
Several aseismic ridges symmetrically disposed
on either side of the Southwest Indian Ridge
occur in the western Indian Ocean. The western-
most pair of aseismic ridges are the Agulhas
Plateau and the Maud Rise (Fig. 4). These fea-
tures lie north and south, respectively, of chron 34
and therefore are probably Middle Cretaceous in
age. Further to the east are two other features. the
Mozambique Ridge and the Astrid Ridge (Fig. 4).
Although it has not been clearly established
whether these ridges are ~ontinent~~l or oceanic in
origin, their position with respect to anomaly 34
indicates that they are older than Late Cretaceous.
We believe that during the Middle Cretaceous the
Agulhas Plateau was joined to the Maud Rise, and
the Mozambique Ridge was aligned with the Astrid
Ridge. This realignment requires that motion was
oblique to present-day fracture-zone trends some-
time after the Late Cretaceous.
Further to the east are the Madagascar Ridge
and the Del Caiio and Conrad rises which lie
south of the ridge axis. The northern section of the
Madagascar Ridge is composed on an anomalous
type of crust that may be partly continental in
origin, while the southern part of the ridge is
considered to be thickened oceanic crust (Cioslin
et al., 1981: Recq and Go&n, 1981: Sinha et al..
1981). Ocean floor of early Late Cretaceous age
(anomaly 34) occurs to the west of the southern
Madagascar Ridge (Bergh and Norton. 1976). 7‘0
the east of the southern Madagascar Ridge the
ocean floor is of Paleocene age (anomaly 29)
(Patriat, 1979). The oldest magnetic anomaly im-
mediately to the south of the Madagascar Ridge is
Eocene in age (anomaly 22) (Fisher and Sclatcr,
1983).
The Del Cafio Rise consists of volcanic edifices
and seamounts that trend E-W and extend be-
242
1
30%
)“E I
* 50”E
..: C’ /
,.:
( ,.y
,:’ /;
Fig. 4. Compilation of magnetic anomalies and Seasat lineations. Lineations from Seasat data are interpreted from the ascending
subsatellite tracks only: dotted lines join aligned maxima of the deviation of the vertical, and dashed lines the minima.
243
tween the Prince Edward and Marion islands in
the west and the Crozet Archipelago and Crozet
Bank in the east. The deep seismic structure of the
Del Cane Rise resembles that of typical oceanic
crust and because of its similarity to the southern
section of the Madagascar Ridge. it has been
proposed that both features were formed at the
same time along the axis of the Southwest Indian
Ridge (Goslin et al., 1981). The orientation of the
Del Catio Rise relative to the ridge axis suggests
either that spreading has been very asymmetric or
that the Del Cano Rise has been growing in a
westerly direction; however, there are no pub-
lished magnetic anomalies between the Del Cane
Rise and the ridge axis that might lend support to
these hypotheses. The entire region north of the
Del Cane Rise is also anomalously elevated
(Anderson et al., 1973) and corresponds to a posi-
tive anomaly in the geoid (Roufosse et al.. 1987).
Unlike the Del Caiio Rise, the age of the ocean
floor surrounding the Crozet Bank is well known.
The Crozet Bank is bounded by anomaly 32 to the
southwest and by anomaly 31 to the northeast
(Schlich. 1975) (Fig. 4). Both of these magnetic
anomalies were generated along the axis of the
Southeast Indian Ridge. Just to the northwest of
the Crozet Bank anomaly 29 occurs (Fig. 41, pro-
duced by spreading along the Southwest Indian
Ridge (Patriat, 1979).
South of the Del Cane Rise and Crozet Bank
lies the broad, WNW-ESE trending Conrad Rise
that consists, in part, of the Ob, Lena and Marion
Dufresne seamount chain (Fig. 1). During the
Late Cretaceous the Lena and Marion Dufresne
seamounts formed the southern limit of the Crozet
Basin (Schiich, 1975, 1982) which was related to
spreading along the Southeast Indian Ridge. To
the north and east of the Ob Seamount, Patriat et
al. (1985) have identified magnetic anomalies 33
and 34. produced by spreading along the South-
west Indian Ridge. Gravity studies indicate that
the Ob, Lena and Marion Dufresne seamounts are
locally isostatically compensated (G&in. 1979;
Diament and Goslin, 1986).
Kinematic considerations led Goslin and Patriat
(1984) to suggest that the Marion Dufresne, Lena
and Oh seamounts, as well as the southern section
of the Madagascar Ridge and Del Caiio Rise were
formed at ridge axes rather than by intraplate
volcanism. They proposed that the southern
Madagascar Ridge and Del Ca50 Rise were pro-
duced along the Southwest Indian Ridge during a
period of slow spreading in the Early Eocene
(chrons 24-23, 5.5 Ma). The Ob, Lena and Marion
Dufresne seamaunts are older and may have been
generated during a major reorganization of the
plate boundaries during the Cretaceous Quiet Zone
(Diament and Goslin, 1986). The time of the
uplift of the Crozet Bank remains uncertain. Ge-
oid studies (Cazenave et al.. 1980) suggest that the
bank may be the result of recent intraplate
volcanism, whereas the pattern of magnetic
anomalies and evidence of local isostatic com-
pensation suggest a much older age (Goslin, 1981).
Mugnefic unomulies
In the African-Antarctic Basin, seafloor
spreading since the Late Cretaceous is now well
documented by numerous magnetic profiles (Fig.
2) (LaBrecque and Hayes. 1979; Bergh, 1986:
LaBrecque and Cande. 1986). The magnetic
anomaly sequences are complete on both sides of
the ridge from anomaly 34 to present and show a
slow decrease in spreading rates from 4.7 cm/yr
at chron 34 (84 Ma) to 0.8 cm/yr at present. As
illustrated in Fig. 3 a period of very slow spread-
ing occurred between chrons 31 and 28. During
the same time interval. seafloor spreading ceased
in both the Cape Basin, south of the
Falkland-Agulhas Fracture Zone. and in the
Mascarene Basin between Madagascar and the
Seychelles-Saya de Maiha Bank (Schlich. 1982).
LaBrecque and Hayes (1979) and Bergh and Bar-
rett (1980) have mapped the magnetic bight on the
African plate formed by anomalies 33 and 34.
southwest of the Agulhas Plateau. Bergh and Bar-
rett (1980) also identified the corresponding bight
on the Antarctic plate, north of Maud Rise. The
location of the magnetic bight indicates that the
triple junction between the African. Antarctic and
South American plates at chron 33 and 34 times
separated the Agulhas Plateau and Maud Rise.
Further to the east. on the Antarctic plate. Norton
and Sclater (1979) identified anomalies 34-31.
The easternmost identification of anomaly 34 lies
west of the Astrid Ridge, just 2” north of the
Mesozoic anomalies identified by Bergh (1977).
The first good identifications of Late Creta-
ceous and Early Cenozoic magnetic anomalies
along the Southwest Indian Ridge were made in
the Mozambique Basin, east of the Prince Edward
Fracture Zone (Bergh and Norton. 1976). They
suggested that there were two abrupt changes in
spreading rates; one during the Middle-Late
Eocene (between anomalies 21 and 15) and a
second during the Late Oligocene-Early Miocene
(between anomalies 8 and 5). Recent reinterpreta-
tions of these data by Segoufin (1981) and Fisher
and Sclater (1983) have favored a more continu-
ous spreading rate between anomalies 24 and 13.
followed by a significant decrease after anomaly
13 (1.4-0.8 cm/yr). The revision of the spreading
rates has permitted the identification of magnetic
anomalies 21-13 along profiles south of the ridge
axis, these rates being consistent with spreading
rates in the African-Antarctic Basin during the
same time interval (Fig. 3) (LaBrecque and Hayes,
1979; Bergh, 1986).
The other well-defined anomalies in the
Mozambique Basin are of Late Cretaceous to
Paleocene age (anomalies 34-28). SCgoufin (1981)
identified another set (anomalies 34-32) east of
the southern part of the Mozambique Ridge. This
anomaly sequence has been considered as being
offset from anomalies 34-31 further to the east by
the northern extension of the Prince Edward Frac-
ture Zone (Fig. 4). Spreading rates for these
anomalies are equivalent to those reported for the
African-Antarctic Basin during the same time
interval (4.0 cm/yr from anomaly 34 to 33 and 1.5
cm/yr from anomaly 33 to 28). Between anoma-
lies 28 and 24 in the Mozambique Basin, no clear
interpretation can be made. The distance between
these anomalies implies a high spreading rate (3.6
cm/yr) which is not observed either in the Afri-
can-Antarctic Basin or in the Madagascar Basin.
South of the ridge axis and east of the Prince
Edward Fracture Zone, anomalies 33 and 34 are
clearly identified (Bergh and Barrett, 1980;
Segoufin and Patriat, 1981). Patriat et al. (1985)
confirmed these identifications with new profiles
collected on the R.V. Marion Dufiesne between
the southern extension of the Prince Edward Frac-
ture Zone and the Conrad Rise. In addition. the\
mapped two other 34-32 sequences that stair-step
to the north and east. These identifications play a
key role in constraining reconstructions for chron
33 and 34 times. One of their profiles runs from
anomaly 34 to the ridge axis, and indicates a
period of very slow spreading between chron 32
and 24. This contrasts with the accelerated spread-
ing rates to the north of the ridge axis during the
same time interval. This asymmetry in spreading
rates suggests that a ridge jump to the south
occurred east of the Prince Edward Fracture Zone
between chron 28 and 24.
East of the Indomed Fracture Zone (46 ’ E).
Patriat (1979. 1983) identified several sets of 13-29
anomalies on either side of the Southwest Indian
Ridge and calculated a spreading rate of 1.4 cm/yr
for that time interval. It is interesting to note that
the age of the oldest magnetic anomalies decreases
to the east: anomaly 29 is not observed east of the
Gallieni Fracture Zone (53OE). anomaly 24 is not
observed east of the Atlantis II Fracture Zone
(57OE), and anomaly 20 is not observed east of
the Melville Fracture Zone (61” E). This pattern is
the result of the rapid eastward migration of the
Indian Ocean triple junction, combined with slow
spreading along the Southwest Indian Ridge.
Sclater et al. (1981) recognized that a similar pat-
tern is observed for anomalies 19 and younger
between the Melville Fracture Zone and the
Rodriguez triple junction (25 OS, 70 o E).
Isochrons from the northern flank of the
Southwest Indian Ridge intersect the isochrons
created along the Central Indian Ridge in the
Madagascar Basin. These intersections form acute,
chevron-shaped bights. Isochrons from the south-
ern flank of the Southwest Indian Ridge intersect
the isochrons generated along the Southeast In-
dian Ridge in the Crozet Basin where they form
obtuse bights. The apices of these bights represent
the past location of the triple junction between the
African, Antarctic and Indian plates (Patriat and
Courtillot, 1984) and provide important con-
straints for plate tectonic reconstructions.
In the African-Antarctic Basin the variable
trends of anomalies 32-24 (Fig. 2) are consistent
with the suggestion that spreading directions
changed dramatically during this interval. How-
ever, east of the Prince Edward Fracture Zone,
magnetic anomaly data do not provide any direct
evidence for a change of spreading direction. The
only indication that a major plate reorganization
took place is the asymmetry of spreading rates,
north and south of the ridge axis, between chron
28 and chron 24 times.
Reconstructions
Method
The poles and angles of the finite rotations
(Table 2) were calculated with the method de-
scribed by Patriat (1983). The parameters of rota-
tion were determined by matching magnetic picks
from both sides of the Southwest Indian Ridge
and minimizing the area of misfit (sum of triple
scalar products; p. 526, McKenzie and Sclater,
1971). Data from the fracture zones were not used
in these calculations because of the difficulty in
precisely locating the traces of conjugate transform
faults. We believe that the reconstructions pre-
sented in Figs. 7a-j are well constrained due to
the geometrical constraints provided by large off-
sets in the ridge axis, the relative abundance of
magnetic data, the variable orientation of mag-
netic lineations, and the length of plate boundary
under study (2000-3500 km).
An attempt has been made to avoid drastic,
and probably spurious, changes in spreading rates
TABLE 2
245
and directions. In this regard, the evolution of
spreading rates and directions through time (Fig.
6, Table 4) has been used as an independent check
on the plate reconstruction model. This is espe-
cially true for those intervals where magnetic picks
are sparse or where symmetric magnetic picks are
absent (e.g., anomaly 31). Also, as mentioned
earlier, the reconstruction of the triple junctions at
either end of the Southwest Indian Ridge provides
important additional constraints.
Relative motions of Antarctica and Africa from the
Late Cretaceous (anomaly 34) to the Middle Eocene
(anomaly 20)
The set of finite rotations presented in Table 2
and illustrated in Fig. 5a describes the relative
motion of Antarctica and Africa for ten stages
during the Late Cretaceous and Early Tertiary
(chron 34 (84 Ma)-chron 20 (46 Ma)). The chron
34 and 33 finite poles are within I” of the poles
determined by Patriat et al. (1985). The finite
poles for chron 29-chron 20 are also quite well
constrained by the magnetic data. On the other
hand, the chron 32 and chron 31 poles are less
well constrained and two alternative models are
possible. In the first model, the direction of
spreading remains nearly constant between chrons
33 and 32. The finite pole at chron 32 time is
4.3”S, 41.3” W (angle = 13.51”), and the finite
pole at chron 31 time is 5.6” N, 48.0 o W (angle =
Finite rotations describing the motion of Antarctica relative to Africa
Anomaly Age (Ma) Lat. (+, ON) Long.(+. OE) Angle (O ) N $2
20 46.2 11.4 - 43.1 7.81 8 0.020
21 50.3 10.3 - 42.9 8.77 7 0.014
24 56.1 6.7 - 40.6 9.97 I 0.035
26 60.8 3.8 - 39.7 10.63 4 0.038
2x 64.3 0.6 39.2 11.32 5 0.017
29 66.2 -0.4 -- 39.4 11.59 5 0.045
31 68.5 1.1 -- 41.6 11.84 3 0.011
32 73.6 - 1.8 -- 41.4 13.47 5 0.020
33 80.2 - 4.7 -- 39.7 16.04 11 0.004
34 84.0 - 2.0 -- 39.2 17.85 12 0.003
Angles are positive counterclockwise. N is the number of points from the rotated contour matched with N + 1 to 2N points from the
fixed contour (i.e., N triple scalar products). The misfit area is evaluated by the mean value ( 8) of the N triple scalar products. D is
expressed as R7. n2/1802 km3 (R = radius of the Earth).
246
- 30DN
Fig. 5. Wandering paths of finite (a) and stage (b) poles of rotation (see Tables 2 and 3 for coordinates). Stage poles are shown for
northern flank or African side of the Southwest Indian Ridge. the
11.74’) (Fig. 5a). This model requires that a dras-
tic change in spreading direction occurred be-
tween chrons 31 and 29. In the Agulhas Basin, for
instance, a change in the spreading direction of
47” and an increase in the spreading rate from 2
to 5 cm/yr would have been necessary. We favor
an alternative model which suggests that spread-
ing rates and directions changed more continu-
ously between chrons 33 and 29, as observed in
Figs. 2 and 3. The stage poles (Table 3) that we
derive follow a hook-shaped curve that doubles
back on itself between chron 31 and 29 (Fig. 5b).
A similar cusp is observed for the same time
interval on the stage pole w~dering path that
describes the opening of the South Atlantic Ckean
(Cande et al., in press).
Table 4 and Fig. 6 show the rates and direc-
tions of spreading calculated along two flowlines,
one located in the Agulhas Basin and the other
east of the Prince Edward Fracture Zone. The
resulting curve confirms the change in relative
plate motion proposed by Patriat et al, (3985). The
change in spreading directions is nearly continu-
ous and can be divided into three episodes (Fig.
6). During the first episode (&on 34-29, 84-66
Ma) spreading directions progressively shift from
SSW-NNE to SSE-NNW, and spreading rates
fall by a factor of 3. During this episode an
TABLE 3
Stage poles and angles of rotation computed from the finite rotations in Table 2
Anomaly Time span (Ma) African plate
Lat. (-t, ON)
20-O 46.2 11.4
21-20 4.1 1.9
24-21 5.8 - 15.6
26-24 4.7 -31.1
28-26 3.5 - 38.2
29-28 1.9 - 36.7
31-29 2.3 25.8
32-31 5.1 -21.5
33-32 6.6 -17.8
34-33 3.8 20.7
Angles are positive counterclockwise.
Angle Antarctic plate
Long.{+, OE) (“1 (‘/Ma) Lat. (+, “N) Long. (-t. ‘E)
-43.7 7.81 0.17 11.4 -43.7
- 36.0 0.98 0.24 1.0 - 37.3
- 23.5 1.39 0.24 - 18.2 - 27.6
- 23.8 0.86 0.18 - 33.4 - 31.0
- 27.3 0.93 0.27 - 39.6 - 36.5
-43.5 0.34 0.18 - 35.1 -51.7
- 103.0 0.60 0.26 36.0 - 99.7
- 37.5 1.75 0.34 - 21.8 - 42.4
- 28.8 2.71 0.41 - 20.3 - 32.9
- 38.6 1.98 0.52 19.4 -31.4
TABLE 4
Directions (D) and full rates (0) of spreading in the Agulhas and the Mozambique basins. A is the distance in degrees between the
stage pole of motion and the point of measurement
Anomaly Time span (Ma) Agulhas Basin Prince Edward Fracture Zone
A (“) u (cm,&) D Afr. D. Ant. A (“) ~1 (cm%) D. Afr. D Ant.
(-, ow; +, OE) (-. OW; +. OE)
20-O 46.2 81 1.9 -149N 31N 91 1.9 -165N 1SN
21-20 4.1 69 2.5 - 151N 35N 79 2.6 - 167N 1SN
24-21 5.8 48 2.0 - 152N 35N 58 2.3 - 173N 9N
26-24 4.7 37 1.2 -169N 2ON 50 1.5 170N --7N
28-26 3.5 3s 1.7 178N 8N 49 2.2 16ON - 16N
29-28 1.9 41 1.4 169N ON 61 1.7 154N -21N
31.-29 2.3 131 2.2 175N 6N 144 1.7 17ON -6N
32-31 5.1 52 3.0 - 171N 19N 65 3.5 172N -4N
33-32 6.6 49 3.4 - 161N 31N 60 4.0 180N 4N
34-33 3.8 85 5.8 - 143N 52N 92 5.8 -15SN 31N
important step in spreading direction occurs at the extinction of the Mid-Atlantic Ridge in the
chron 33. Interestingly, the next change in spread- Cape Basin, south of the Agulhas-Fa~k~and Frac-
ing direction does not appear to be synchronous ture Zone (Du Plessis, 1977: Barker. 1979:
along the ridge. Changes in spreading direction LaBrecque and Hayes, 1979). From chron 29 to 24
first take place in the Agulhas Basin (chron 31) (66-56 Ma), the direction of spreading shifts back
and then propagate eastward towards the Prince to SSW-NNE. The second phase ends with a
Edward Fracture Zone (chron 29). These changes reorganization of the plate boundary beginning at
in spreading direction are contemporaneous with about chron 26 (61 Ma). The third and final
Spreeding ratea
2 L
cmlyi
1
L-_--J
1 0
Dkaotlona of apreadlng
r-i
from north
F 160’
South
34 33 32 31 2028 26 24 21 20
,I(9 l,lII,lll ,I,,,I1,, rl,,,,,,,‘,~,!‘,,,r
I Anotnaly +
80 TO 80 50 40 Tim* Ma
Fig 6. Evolution of the rates and directions of spreading of the Southwest Indian Ridge between the Late Cretaceous and the Middle
Eocene. These parameters (Table 4) are computed for the Agulhas Basin (dashed line) and the Mozambique Basin (continuous line).
Directions of spreading correspond to the northern flank of ridge.
248
episode of spreading begins at chron 24 (56 Ma),
and by &on 20 (46 Ma) the direction and rate of
spreading are similar to those of present-day plate
motions. East of Prince Edward Fracture Zone,
our computed direction of spreading is 15” NE.
This is the same as the spreading direction esti-
mated by Bergh and Norton (1976), and similar to
the direction (17” NE) reported by Sclater et al.
(1981). It is interesting to note that these direc-
tions of spreading are nearly the same as the
spreading directions between chrons 34 and 33.
a
4d
Rec~n~t~ction~ of the southwest Indiutz Ridge cot
~gurutio~
In this section we present plate tectonic recor
structions of the Southwest Indian Ridge at chro
34, 33, 32, 31, 29, 28, 26, 24, 21 and 20 (Fig:
7a-j). These reconstructions are based on ou
compilation of available magnetic, bathymetri
and satellite altimetry data (Figs. 1 and 2, and 4:
For each reconstruction we have drawn isochron
that illustrate the past configuration of the South
20”E 40*
Anomaly 34 - 84 Ma
Fig. 7. a-j. Configurations of the Southwest Indian Ridge at ten stages of its evolution between the Late Cr&xceous and the Middle
Eocene. Dots are magnetic picks from the African plate, triangkz from the Antarctic plate and squares from the Indian plate.
Contours are identical to those in Fig. 1. Finite rot+ions at &on 34, 33, 32, 31 and 29 between India and Antarctica, and from
&on 28 to 20 between India and Africa are from Patriat (1983).
249
40” SO”
Anomaly 33 - 60 Ma
Fig. 7 (continued),
west Indian Ridge. The location of transform faults
has been deduced from bathymetric and satellite
altimetry data, and the trends of the transform
faults are small circles about the stage poles that
describe the relative plate motions during the in-
tervening time intervals. The ridge segments are
drawn on the basis of observed and rotated mag-
netic picks, or in a manner consistent with the
previous ridge configuration.
Chrons 34 and 33 (Figs. 7a and 7b) have both
been drawn using the 34-33 stage pole to de-
termine fracture-zone trends. One of the main
features of the chron 34 reconstruction is the huge
offset of the ridge axis between the Mozambique
Basin and the Agulhas Basin. This offset, which
we call the Agulhas-Mozambique Fracture Zone,
was created as a result of differential spreading in
the Agulhas and Mozambique basins. Seafloor
spreading began in the Mozambique and Enderby
Basin during the Late Jurassic as a result of the
separation of Africa and Antarctica. Rifting, how-
ever, did not begin in the Agulhas Basin until the
separation of South America (Falkland Plateau)
and Africa during the Early Cretaceous. In the
African-Antarctic Basin, there is no evidence of
Mesozoic anomalies related to Africa-Antarctica
motions; furthermore, chron 34 lies at the same
distance from the African and Antarctic coastlines
(= 10”).
The direction of spreading before chron 34 time
was probably parallel to the trend of the Mozam-
bique Ridge, which we consider to be the northern
20”E 40” SO”
Anomaly 32 - 74 Ma
counterpart of the Astrid Fracture Zone. At chron
34 time, the Conrad Rise was adjacent to the
Madagascar Ridge, although it is likely that these
two features had separated shortly before chron
34 time, Goslin and Patriat (1984) argue that the
ridge jumped north, from the Enderby Basin to
the Conrad Rise, sometime near the end of the
Cretaceous Quiet Zone (= 90 Ma). This ridge
jump may have been ~~~0~s with the ini-
tiatiun of spreading between Madagascar and the
Seychelles-India block.
The change in spreading dire&on between
&on 33 and 32 required a break-up of the Agul-
has-Mozambique Fracture Zone south- of the
Mozambique Plateau into a stair-case succession
of small ridge and transform fault segments (Fig.
7~). Our representation of the number and ioca-
tion of the ridge segments is arbitrary: however,
the trend of the transform faults matches the
trend of the fracture-zone shift between the As&id
and Bin fracture zones (Fig. 4). It is important to
note that the configuration of the ridge axis at
chron 32 severely contra su~~t -config-
urations of the Southwest In&n Ridge,
The configuration of the Southwest Endian
Ridge at chron 31, 29 a& 28 times is very similar
(Figs. 7d-f). Due to the rapid change in sp&t&ng
directions and as a result of the very slow rates,
spreading was highly oblique during this period.
Ridge segments increased in length as the direc-
20°s 20%
1 I I
d - + + +
Anomaly 31 - 69 Ma
60” Fig. 7 (continued).
tion of motion shifted to NW-SE. The Crozet
Bank may have been created at the ridge axis
between chron 32 and 31; however, because there
is no counterpart on the Indian plate, it is more
likely to be the result of younger intraplate
volcanism.
At chron 26 (Fig. 7g), the configuration of the
ridge in the African-Antarctic Basin began to
change. However, as in the case of the earlier
reconstructions, there are only a few identified
anomalies between 32” and 46”E. Chron 24 time
marks the end of the phase of oblique spreading.
By that time, spreading directions and the orienta-
tion of the Southwest Indian Ridge became simi-
lar to the present configuration (Fig. 7h). Between
chron 26 and 24 several ridge jumps occurred in
the Mozambique Basin. East of 38” E a ridge
segment jumped to the south and, between the
Madagascar Ridge and Del Catio Rise, another
two ridge segments jumped to the north. The large
offset in the ridge along the Gallieni Fracture
Zone was created by a ridge jump at this time.
The dashed lines in Figs. 7h-j indicate the extinct
ridge axes. As a result of the change in plate
motion, the ridge segments west of Prince Edward
Fracture Zone shortened. while the offsets be-
tween the ridge segments grew longer (Figs. 7h-j).
By chron 20, the growing fracture-zone offsets had
coalesced to form the basis of the modern
Bain-Prince Edward Fracture Zone system.
20”E 40” 60”
60
c
0
Anomaly 29 - 68 Ma
Fig. 7 fcontinued).
One of the special aspects of the Southwest
Indian Ridge is the fact that since the Late Creta-
ceous, it has been bounded by triple junetions
each involving three mid-oceanic ridges. At chrons
34, 33 and 32 we ,assume that the Indian-Afri-
can--Antarctic triple junction had a R-F-F *
configuration and that its location remained fixed
with respect to the African plate. This ~~fi~a-
tion resulted in the lengthening of the ant
Indian Ridge. The rapid eastward n&r&on of the
Indian-African-Antarctic triple junction began
after chron 32 when the spreading rates slowed
* R-ridge: F-fault.
down between Africa and Antarctica, while in-
creasing rapidly between Africa and India and
between India and Antarctica. Since that tirtx the
Indian triple junction has had either a R-R-F or
a R-R-R configuration (Patiat and Courtihot,
1984). The rate of lengthening of the
Africa-A.ntar&ica plate boundary was very high
between chron 31 and 28 (= 140 km/Ma) (TabIe
5) with spreading rates along the Cerrtral and
Sox&heast Indian ridges 8 times gre&er than
spreading rates along the Southwest In&n Ridge.
When the directions and rates of spreading
began to change akmg the §outhwest Indiaa Ridge
(chron 26) and along the Central and §outh~t
Indian ridges (chron 24), the eastward rn&ation
20’E 4o” SO”
6C
Anomaly 28 - 64 Ma
Fig. 7 (continued).
of the rndia~-Afghan-Antarctic triple junction
began to slow down. The large fracture zones
south of the Madagascar Basin, such as the Atlan-
tis II and Melville fracture zones, originated dur-
ing this time interval (chron 24 and 20 respec-
tively). The Indian-African-Antarctic triple junc-
tion has been ~grating eastward at a nearly con-
stant rate ( = 30 km/Ma) since the Middle Eocene.
The V-shaped domain. apparent in both the
bathymetric and magnetic data, is due to the fact
that the eastward migration rate of the triple
junction has always been greater than the spread-
ing rate along the Southwest Indian Ridge.
At the western end of the Southwest Indian
Ridge, the Africa-South America-Antarctic triple
junction has migrated westward during the same
TABLE 5
Rate of lengthening ( R ) of the eastern extremity of the South-
west Indian Ridge from the Early Paleocene (anomaly 32) to
the present day. The length of the plate boundary (L) is
estimated by adding the ridge and transform fault segments
from the lndomed Fracture Zone __--
Anomaly L (km) Time span (Ma) R (km/Ma)
32
31
29
28
26
24
21
20
Present
0 5.1
308 60
2.3
641 145
1.9
908 140
3.5
1250 94
4.7
1671 90
2095 5.8 73
2209 4.1 2x
46.2
3644 31
Anomaly 26 - 61 Me
time interval. Between chron 34 and 33, and possi- vicinity af the Prince Edward Fracture Zone. Ini-
bly until chron 31, the triple junction was a R-F-F tially, they were considered to be stable fattxres
type (LaBrecque and Hayes, 1979; Bergh and whose location and orientation had not substan-
Barrett, 1980). After the westward jump of the tially changed since chron 34 time (Morton and
ridge that occurred at about chron 31 time, the &later, 1979; Fisher and Sclater, 1983; Martin
triple junction migrated south of Meteor Rise. At and Hartnady, 1986). However, Patriat et al. (1985)
that time, the Mid-Atlantic Ridge and the South- have shown that these fracture zones are not sta-
west Indian Ridge were probabiy linked by a long ble and &at the edge-fracture ge&etry has
fracture zone running south of Ueteor Rise. changed with time.
I tiona and discltssion
One aspect of this work concerns the behavior
and evolution of the important fracture zones that
characterize the Southwest Indian Ridge in the
Several lines of evidence have led us to con-
clude that the fracture zones along the central
section of the Southwest Indian Ridge arr: not
stable features. A change in plate motitio9, and a
consequent change in fractur*zone geometry; is
required if one compares the mconstructmn of the
20.E 4d 60’
, I I S/f $ I
Anomaly 24 - 56 Ma
80. I\.. ni I I I
Fig. 7 (continued).
Southwest Indian Ridge at chron 33 or 34 with its We believe that the reconstructions at chron 34
present configuration. At chron 33 and 34, the and 33 are particularly well constrained because
long fracture zone that separated magnetic the large offset along the Agulhas-Mozambique
anomalies in the Mozambique and Agulhas basins Fracture Zone, combined with the relative ahun-
(Agulhas-Mozambique Fracture Zone) ran along dance of magnetic picks, results in a unique fit of
the eastern edge of the Mozambique Ridge (Figs. Africa and Antarctica. Patriat et al. (1985) also
7a and b). This configuration was inherited from noted that the character of the magnetic signature
the Early C‘retaceous, during which time the east- (amplitudes and symmetry) is similar for con-
em edge of the Mo~mbique Ridge was aligned jugate magnetic anomalies from the Mozambique
with the Astrid Fracture Zone. At present, how- and Enderby basins. Furthermore, chrons 28 and
ever, the long-offset fracture zone {Prince Edward 29 are consistent and well constrained, and pro-
Fracture Zone) is no longer aligned with the vide the required intermediate steps between
Mozambique Ridge. This suggests that there has chrons 31 and 26. The good agreement of our
been either a shift in the location of the fracture rotation parameters with those obtained by solv-
zone or reorganization of the fracture-zone geome- ing the plate circuit Africa-India-Antarctica
try. (Patriat, 1983; Patriat and Segoufin, this issue)
20"E 40' 60"
i
80’
VW Anomaly 21 - 50 Ma
Fig. 7 (continued).
provides an independent verifkatiorr of our results
between chrons 29 and 20 (Figs. 7e-j).
Although a variety of magnetic data evidence is
available, there is little bathymetric evidence for
the proposed change in plate motion. When the
chrons 20 or 21 are superimposed on the new
bathymetric summary of Driscoll et al. (in press},
it beames apparent that the well-mapped fracture
zones, trending N?dE-4SW, lie between these iso-
chrons and therefore are younger features. Be-
tween the eastern edge of the Mmtique Ridge
and chrons 20 and 21, bathymetric data are scarce
and no particular structural trends emerge. To the
north, the only bathymetric evidence for the
change in motion is a topographic -high bordered
by a deep fracture that shifts from a NNE-SSW
direction (48*S, 31”E) to a N-S direction (45’S,
32” E). In the vicinity of Antarctica, det&ed
bathymetry is absent but satellite altimetry data
provide clear evidence for the clrangr: in plqte
motion (Fig. 4). Considering the draa
spreading direction between chron 29
siow spreading rates, aI& the time
(= 30 Ma), it is not s~~ that
of fracture zones did not &v&q
fracture zones did not become reea
the modern system of pla& motion b&e e&ah-
lished at chron 21.
An indirect argument in favor of t-he- prqxxM
change in plate motion is that other plate re-
20’E 40” 60”
Anomaly 20 - 46 Ma
Fig. 7 (continued).
organizations occurred in the southern oceans be-
tween chron 31 and 24. In the South Atlantic
Ocean, spreading rates dropped abruptly at about
chron 31 and the direction of spreading changed
slightly (Cande et al., in press). Sometime between
chrons 31 and 28, the Mid-Atlantic Ridge in the
Cape Basin stopped spreading; spreading resumed
west of Meteor Rise at chron 26-25 (Barker, 1979;
LaBrecque and Hayes, 1979). In the Indian Oc-
ean, seafloor spreading ceased in the Mascarene
Basin shortly after chron 28 (Schlich, 1982). The
fast northward drift of India also started at chron
31. Evidence for a rapid and nearly simultaneous
increase in spreading rates at chron 31 is found in
the Grozet, Madagascar and Central Indian basins,
as well as in the Mascarene and Wharton Basin
(McKenzie and Sclater, 1971; Sclater and Fisher,
1974; Schlich, 1975, 1982). At chron 22 spreading
rates in these basins began to slow down. This
phase of spreading ended in the Middle Eocene
due to the collision of India and Asia and the
resulting plate boundary reorganization.
In conclusion, it must be emphasized that this
rapid change in motion may have modified and
reshaped the topography as well as the fracture-
zone pattern. The Agulhas-Mozambique and
Astrid fracture zones have been spared because
they stand outside and now bound the restruc-
tured area. Therefore, except for these two old
fracture zones, the present fracture zones (i.e.,
Prince Edward Fracture Zone) only reflect the
direction of spreading during the last 45 m.y., and
are much younger than their lengths might imply.
By matching magnetic anomalies symmetrically
disposed along the Southwest Indian Ridge we
have derived a set of rotation parameters that
confirm the important change in relative motion
between Africa and Antarctica during the Late
Cretaceous and Early Tertiary, first pointed out
by Patriat et al. (1985) and Larson et al. (1985).
This change in relative plate motion began at
chron 32 (74 Ma), and during the latest Creta-
ceous (chron 31-28, 71-66 Ma), the direction of
spreading shifted nearly 45” to the northwest.
During the earliest Tertiary (chron 26-24, 61-56
Ma) the direction of motion shifted back to the
original spreading direction. By the Early Eocene
(chron 21-20, 50-46 Ma), the Southwest Indian
Ridge had adopted its present-day configuration.
By accurately reconstructing the history of rela-
tive motion between Antarctica and Africa. we
have provided new constraints concerning the
breakup of Gondwana. Our reconst~ctions are a
starting point for further work. in particular on
the Mesozoic evolution of the Indian Ocean, the
evolution of the plate boundary between South
America and Antarctica, and on the development
of the Weddell Sea and Scotia Sea.
The complex evolution of the Southwest Indian
Ridge, especially in the vicinity of the Prince
Edward Fracture Zone, emphasizes the fact that
fracture-zone trends cannot always be taken at
face value and should be used with great care for
determining relative plate motions. The last direc-
tion of motion can easily overprint and erase
previous transform fault trends. Furthermore, in
areas of slow spreading rates and rough topogra-
phy, complex changes in past plate motions may
be easily overlooked.
Acknowkdgements
The authors wish to thank Mavis Driseoll and
John LaBrecque for their helpful comments and
suggestions. This work was supported. r.n part, by
a Shell Distinguished Professorship to John G.
Sclater under NSF grant OCE-86 17193 and by
the sponsors of the Pale~eanographic Mapping
Project (POMP), University of Texas at Austin.
The senior author also received support from the
French Foreign Office (‘ bourse Lavoisier’) while
at the Institute for Geophysics, University of
Texas. This is UTIG contribution 726.
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