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Reconstruction of the Central Indian Ocean
211
Tectonoph_wics, 155 (1988) 211-234 Elsevier Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
reconstruction of the Central Indian Ocean PHILIPPE
PATRIAT and JACQUES
SEGOUFIN
Institut de Physique du Globe de Paris, Laboratoire de GtGophysiqueMurine, CNRS U.A. 729, 4 place Jumeu. 75252 Paris Cedex 05 (France) (Received
September
5,1987;
revised version
accepted
of the Central
Indian
November
20,1987)
Abstract
Patriat,
P. and Stgoufin,
(E.ditors).
Mesozoic
J., 1988. Reconstruction and Cenozoic
Sixteen plate tectonic the latest Cretaceous into
five distinct
Eocene
tectonic
Early Miocene-Recent rates of spreading motion
occurred
reconstructions
(anomaly
(A27-A22),
Plate Reconstructions.
epochs:
(3) Middle (A8-AS). along
between
are presented
29) to the present. (1) latest
Eocene-Late During
the Central,
illustrating
The history
spreading
Paleocene
(A22-AU),
Scotese
of the Central in the Indian
(A29-A27),
and W.W.
Sager
epochs,
and Southwest
Indian
A21 and Al8 time due to the collision
subtle changes ridges.
Indian
Ocean
Miocene
Paleocene-Early (Al8-A@,
and (5)
took place in the directions
The most
from
Ocean can be divided
(2) Middle
(4) Late Eocene-Early
each of these tectonic
Southeast
In: C.R.
the development
of seafloor
Cretaceous-Early Eocene
Ocean.
Tectonophysics, 155: 211-234.
dramatic
changes
and
in plate
of India and Eurasia.
Introduetion
The Indian Ocean is the result of seafloor spreading between the African, Antarctic, and Indian plates. Identifiable magnetic anomalies of Late Cretaceous and Cenozoic age have been mapped on both sides of the Southwest Indian Ridge (SWIR), Southeast Indian Ridge (SEIR) and Central Indian Ridge (CNIR). Using these magnetic anomalies, we have produced plate tectonic reconstructions describing the tectonic evolution of each ridge (Segoufin, 1981; Patriat, 1987; Royer et al., this issue). This paper puts together these sets of reconstructions in order to illustrate the tectonic development of the Central Indian Ocean around the Central Indian Triple Junction. Anomaly 29 is the oldest identifiable magnetic anomaly mapped in the western Central Indian Basin. It is only from this epoch onwards that 004%1951/88/$03.50
0 1988 Elsevier Science Publishers
B.V.
complete sequences of magnetic anomalies exist on both sides of the three ridges. As a consequence we are able to present sixteen plate tectonic reconstructions which describe, in detail, the plate tectonic evolution of the Central Indian Ocean from the Late Cretaceous (A29) to the Present (Fig. 1). Of particular interest are the reconstructions that describe the change in plate motions that took place between anomaiy 22 and 18 as a result of the progressive collision of India with Eurasia. This paper represents the first synthesis of magnetic anomaly data from the Southwest, Southeast and Central Indian ridges. We will explain the method that was used to produce the plate reconstructions (Figs. S-20), we will discuss the most important constraints, and we will describe, in a step-by-step fashion, the tectonic development of the Central Indian Ocean.
212
_
--
-7
Fig. 1. Magnetic anomalks and isccbrons for the Central Indian Ocean.
---
213
TABLE 2
Methods used to produce the reconstructions
Finite rotation poles. India-Africa
General considerations
Epoch
Lat. (+,
At
any
boundary
given is made
segments
that
the spreading in this paper
history
were produced
is based
described
by Patriat
on the minimization
7.42
A6
16.2
44.2
11.09
chrons
are listed were then
drawn
1-3.
about
46.0
15.40
11.3
51.6
21.31
Al8
14.9
49.9
23.23
A20
17.0
48.1
24.09
A21
12.6
48.3
28.34
A22
23.8
38.4
23.53
magnetic
A23
14.9
41.9
29.93
A24
15.9
38.6
31.11
A25
17.6
33.9
32.70
A26
17.6
31.9
34.63
A27
14.0
33.4
40.22
A28
15.3
31.0
40.50
A29
13.1
31.4
45.65
(1987) which scalar
Magnetic
by linking
14.8
presented
of the triple
in Tables
A8 Al3
spread-
product (McKenzie and Sclater, 1971). The finite poles of rotation that were calculated using this method
(“)
of ridge
anomaly picks from each side of the spreading axis. To determine the goodness of fit, we have used the method
-1 “W)
51.4
disposed
by refitting
“E;
8.2
floor as a series of magnetic
are symmetrically
(+,
A5
will be pre-
axis. The reconstructions
OS)
plate
faults. As seafloor
the spreading
served in the ocean anomalies
accretionary
up of a combination
and transform
ing progresses,
an
time,
Angle
Long. ON; -,
together
iso-
From Patriat (1987).
ob-
served or rotated magnetic anomalies of the same age. As a consequence, in this paper the isochrons
picks at the beginning of the normal polarity sequence, with the exception of anomalies 13, 25,
are the result of the reconstructions and are then identical on both sides of each ridge.
27 and polarity
In order to facilitate comparisons with other studies. we have defined
28, for which the end of the normal sequence was chosen (Fig. 2). Due to
of our result the anomaly TABLE 3 Finite rotation poles, Africa-Antarctica
TABLE 1
Epoch
Finite rotation poles, India-Antarctica Epoch
Lat. (+,
Long. ON; -,
“S)
(+,
“E; -,
Lat. (+.
Angle “W)
(“)
Angle
Long. ON; -,
OS)
(+.
“E;
-1 “W)
(“)
A5 (1)
9.1
-41.6
1.49
A6 (1)
13.4
-55.1
2.79 4.42
A5
9.2
40.7
7.13
14.6
- 52.2
A6
14.7
32.1
11.97
Al3 *
15.1
-51.0
5.86
A8
16.2
30.6
15.65
Al8 (1)
15.3
- 50.4
6.92 7.84
A8
l
Al3
17.0
30.5
19.62
A20 (1)
10.2
- 42.8
A18
19.2
21.4
22.88
A21 (1)
7.9
- 39.6
8.86
A20
16.4
28.8
25.41
A22 *
9.3
-41.7
9.31
A21
12.8
28.5
28.67
A23 *
10.4
- 43.4
9.12
A22
14.5
24.6
29.56
A24 (1)
11.3
- 44.9
10.11 10.38
A23
13.4
23.3
32.17
A25 *
6.9
-41.8
A24
12.8
21.9
34.48
A26 (2)
3.8
- 39.7
10.63
A25
12.4
19.3
37.52
A27 *
1.7
- 39.4
11.07
A26
12.4
17.5
39.45
A28 (2)
0.6
- 39.2
11.32
A27
10.1
18.0
43.40
A29 (2)
- 0.4
- 39.4
11.59
-41.6
11.84
- 41.4
13.47
A28
11.1
15.4
44.24
A31 (2)
1.1
A29
11.2
13.0
47.44
A32 (2)
- 1.8
A31
9.4
13.7
51.59
A32
9.8
10.6
55.31
From Patriat (1987).
(1) From Patriat (1987). (2) From Royer et al. (this issue). *
Interpolated.
214
-0Ma
40 Ma
’
pi
17
2
1=3
t8
PA
H
IS 20
21
4A
E3 E
-
0
D cc
$zJ
5
- DMa
25 “I
continuing controversies concerning the correlation between the magnetic reversal time scale and the absolute time scale. we have chosen to describe our results only in terms of the magnetic reversal time scale. It is important to point out, that in our calculations, as often as possible, only magnetic anomaly picks were used to determine the best-fit rotation parameters. The shape of the fracture zone between magnetic anomalies was not used because, in general, the shape of the fracture zone is a poor predictor of instantaneous plate motion. The geometry of a fracture zone is the result of the evolution of a series of active transform faults. The larger the offset between magnetic anomalies, the more likely it is that the fracture-zone trend is a composite feature made up of the superposition
PRESENT DAY CONFIGURATION
26 -1
CNIR
SE
f3
_ 2OMa
15ochron t _
8OMo
CNlR
-
6 6A
27 CrJ
= :
RECONSTWCT~ON EPOCH f
- 4oMa
- 6OMa
Fig. 2. Magnetic reversal chrom&g. ~y~ti~~~ drawn. Black points refer to the point where the mpsaetic anomalies were picked.
Fig. 3. A method for rcconst~~ctiott of the SoutbwM Indian Ridge (SWIR), using previously rcwnstructed iswbrom (heavy lines) on the Southeast Indian IGdgz (SEIR) md Cu@ti Indian Ridge (CNIR). The large dot is the trace of the Ittd& Ocean Triple Junction on the thne plates. The magnetic anomaly picks to be rfmscm bled are indicated by &cles (African plate) and triangles (Antarctic p&e).
215
216
L
ll
217
--I i ; LI I
’
;?
-1
218
of successive trends of the active transform boundary. Due to the composite nature of fracture-zone trends their use to estimate relative plate motions may produce inaccurate results (Patriat et al., 1985).
When there are numerous magnetic anomalies observed on either side of a ridge axis, it is possible to calculate the best-fitting rotation parameters using magnetic anomaly data only. A unique fit of magnetic anomalies is possible because each
60
a
-20
R 0
+
-30
-40
+
c
Fig. 5. Reconstructions
of the Central Indian Ocean with respect to Africa at anomaly
29 time. The Seychelles
has been rotated by 5.62” around a pole at 18.4O N, 16.8” E to allow for the dying Mascarene apply for this figure and for Figs. 6-20.
Heavy continuous
location of axis, if spreading had been symmetrical (see text for explanation);
Location
of magnetic
at the time of the reconstruction;
and geometry unchanged
heavy dashed line-tripie
the Indian plate rotated to Africa via Antart&; de La Foumaise).
line-axis
junction fine lines-0
anomalies:
Circle-
strutted isochrons;
since the last step; arrow-direction
and 2500 m isobaths; asterisk-location triangle-Africa;
dashed line-interpolated
and spreading rate
Triple Junction trace on
of La Reunion hotspot (Piton
square-India;
isochrons.
symbols
heavy dotted line-predicted
traces; heavy dotted and dashed line-Indian Antarctica;
Plateau (dashed line)
Basin Ridge. The following
continuous
iine-recon-
219
ridge segment has a specific trend. In these cases, a visual check is made to be certain that the major fracture zones are realigned when the plates are reconstructed. However, in cases where there are few magnetic anomalies, or where the ridge length is small, the relative spacing of fracture zones is taken into account. In these cases, the calculations require that the end points of one or two conjugate fracture zones line up when the plates are reconstructed. The calculations do not require, however, that the trends of the fracture zones .I 0
superimpose. In this way, the location of the fracture zones is taken into consideration, but no importance is placed on the trend. Special constraints
One of the most important constraints for reconstructing the Central Indian Ocean is reassembly of the fracture-zone segments that bound anomalies 20-29. One of these fracture-zone segments (La Boussole) lies on either side of the
50
b0
0
Fig. 6. Reconstruction
of the Central
been rotated
Indian
by 2.53O around
Ocean
with respect
to Africa
at anomaly
70
2X. The Seychelles
a pole at 18.4O N, 16.8” E to allow for the dying Mascarene
8 0
Plateau
(dashed
Basin Ridge.
line) has
220
CNIR; it is located southeast of Reunion and its conjugate member southeast of the Chagos-Laccadive Islands (Fig. 1). The other fracture-zone segments (L’Astrolabe) lie on either side of the SEIR. The southernmost part of the L’Astrolabe Fracture Zone extends northward from the eastem edge of Crozet; its northern counterpart lies halfway between the Chagos-Laccadive Islands and the Ninety-East Ridge (Fig. 1). The trace of the Indian Ocean Triple Junction in the Central Indian Basin is located between the northern extensions of the La Boussoie and L’Astrolabe frac-
30 +
0
40
50
+
b
ture zones since their conjugate part?; lie on both sides of the SWIR. The unique geometry of the La Boussole and L’Astrolabe fracture zones was used to reconstruct the CNIR and SEIR, and subsequently the trace of the Indian Ocean Triple Junction in the Central Indian Ocean Basin, between anomalies 20 and 29. This was done by realigning the southern sections of these fracture zones with their northern counterparts when magnetic anomaly data was sparse. Moreover, the remonstrated isochrons of the SEIR and CNIR were used to reassemble the 60 +
70 +
//
-10,
-20
-30
-40
.a0
+
+
+
Fig. 7. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 27.
80
221
magnetic
anomalies
along
the SWIR
Fig. 3. In this way, although each ridge were performed tions
were required
the Central
The rotation magnetic 1-3.
parameters along
Indian
The confidence
parameters
varies
on the special
the Southeast,
of the
Central
we place
in these
rotation
from ridge to ridge depending
characteristics
of each
spreading
In general,
ridge between
the rotations because
magnetic
defined
fracture-zone
The rotation of the CNIR
Ocean
Fracture
anomaly
data
Junction abunwell-
offsets.
parameters
for the reconstruction
are well constrained
due to the very
simple ridge geometry
before Al8 time and due to
the different
of the successive
ments
obliquity
after Al8
Ocean with respect
ridge seg-
time (Fig. I). The exceptions
50
Indian
Triple
of the
and numerous
50
of the Central
length
Zone provides
&
Fig. 8. Reconstruction
for the SEIR are
the great
the Indian
and the Amsterdam dant
are listed in Tables
30 I
system.
best constrained
Junction.
used to reconstruct
Ridges
in of
all rota-
the coherency
Ocean Triple
isochrons
and Southwest
independently,
to maintain
Indian
as shown
the reconstructions
to Africa
at anomaly
26.
are
at A29 where the data are too sparse and at A5 where the great dispersion of the reassembled picks is unexpected and unexplained. Despite its slow spreading rate, the reconstructions of the SWIR are fairly well constrained due to the great length of the ridge. However, because it has lengthened considerably since the Early Tertiary, it is difficult to make accurate reconstructions prior to anomaly 26 time. As a consequence the rotations describing the reconstruction of the ridge for anomalies 5-24 are taken from Patriat (1987) while the rotation parameters for anomalies 26-29 are based on the work of Royer
30 +
0
Explanation of the figures
In Figs. 5-20, plate tectonic reconstructions of the Central Indian Ocean are represented for anomalies 5, 6, 8, 13, and 18 and for anomalies 20-29. The ridge axis in each figure has been drawn by reassembling the magnetic anomaly picks
50
SO
40
et al. (this issue), which is based on additional data between the Bouvet Triple Junction and the Gallieni Fracture Zone.
70
f
I /I’ /
- 10
/’
+
-40,
0
JO *
1)
+
Fig. 9. Reconstruction of the Central Indian Ocean with respectto Africa at anomdy 25.
80
223
from each pair of conjugate plates. Triangles, squares and circles represent magnetic anomaly picks from the African, Indian and Antarctic plates, respectively. On successively younger reconstructions the reconstructed ridge axes become symmetric magnetic isochrons. Magnetic isochrons have been drawn for each anomaly epoch. with the exception of anomalies 8, 13, 22 and 23. and 25-27 along the SWIR (Fig. 1), where no reassembly of magnetic picks has been performed. In each figure, spreading directions and rates are indicated by vectors whose absolute lengths
represent the amount of oceanic crust generated in 5 million years. Two sets of vectors have been plotted. In addition to vectors at the ridge axes, the direction and rate of spreading during the last anomaly epoch are also indicated. These two sets of vectors help to highlight times during which there were major changes in spreading direction or rate (e.g. Fig. 15). The fine dotted line plotted at the active spreading center represents the previous configuration of the ridge axis. The difference in the shape and location of the ridge between the dotted line (previous configuration) and the active 60 ,
r
-20
40
-50,
‘
+
Fig. 10. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 24.
224
spreading center may indicate ridge jumps, periods of asymmetric spreading, or errors in our interpretation of the ridge shape. The continents, aseismic ridges, magnetic isochrons and plate boundaries have been reconstructed in a reference framework in which Africa has been held fixed. The Mascarene Plateau and the Saya de Malha Bank, as well as the Chagos-Laccadive Ridge are assumed to have been produced by movement of the African and Indian plates over the Reunion hotspot (Duncan, 1981; Morgan, 1983). As a consequence on succes-
sively older reconstructions, parts of the Mascarene Bank and Chagos-Laccadive Ridge have been removed according to the age progression suggested by the Reunion hotspot track. In a similar manner, the older parts of the Ninety-East Ridge have been removed to avoid overlap with the Kerguelen Plateau. The goodness of the fit can be estimated by the alignment, along the ridge axes, of the symbols that represent magnetic anomaly picks from the African, Indian and Antarctic plates (Figs. 5-20). In general, the magnetic anomaly picks are colin-
10,
-20
-30
-50
+
+
Fig. 11. Reconstructionof the Canti
Indian Ocean with respect to Africa at anon&y 23.
22s
ear and the transform offsets predicted from the magnetic data coincide with the location of known fracture zones. As an additional estimate of the goodness of fit, the trace of the Indian Ocean Triple Junction on the Indian plate has been drawn twice, using two independent data sets. The first trace (heavy dashed line) represents the path of the triple junction rotated by the relative motion between the Indian and African plates. The second trace (heavy dashed and dotted line) represents the same triple junction path determined by calculating the relative motion between the Indian and African plates via the plate circuit
Africa-Antarctica-India. The greater the coincidence between the two traces, the better the reconstruction. Tectonic development of the Indian Oceun General comments At first gtance, the most striking pattern observed in Fig. 4 is the change at anomaly 1X time from a period of rapid, northward seafloor spreading to a period of slower, northeasterly directed spreading. This change in spreading direction, attributed to the collision of India with Eurasia, has
60
-50
+
70
+
Fig. 12. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 22.
60
226
been previously noted by numerous workers (e.g., McKenzie and Sclater, 1971; Schiich, 1982). The detailed set of reconstructions presented in this study, however, permits us to take a closer look at the tectonic development of the Indian Ocean, Upon closer inspection, a more complex scenario emerges due chiefly to a better understanding of the SWIR history. Indeed, the change in the direction of the triple junction path, which is the boundary between the crust created at two different ridges, is the best record of change in the relative motions of the three plates. According to this criterion, we recognize no fewer than five
distinct periods of seafloor spreading: (I) fatest Cretaceous-Early Paleocene (A29-A27), (21 Middle Paleocene-Early Eocene (A27-AZ], (3) Middle Eocene-Late Eocene (A22-AIX), (4) Late Eocene-Early Miocene (A18-A8), and (5) Early Miocene-Recent (AGA5). in the following section we will outline the characteristics that render each tectonic epoch distinct. Epoch AD-A27 (Pigs. 5- 7) At A29 time the Africa-Indian plate boundary rapidly lengthened, growing eastward from the Mascarene Basin into the Madagascar Basin at the
-IO+
-40,
Fig. 13. Reamstruction of the CentraE hc#im Chaa
with respect to Africa
at ,anOmaty 21.
expense of the SEIR. This resulted in the reorganization of the Africa-India-Antarctic Triple Junction and the formation of the modern CNIR. This period is characterized by the highest opening rate (about 20 cm/yr) in the Indian Ocean and by a pause in the shortening of the SEIR. As illustrated in Figs. 5-7, the traces of the Central Indian Ocean Triple Junction on the African and Antarctic plates cross-cut one other (Figs. 5-7) indicating that the SWIR was undergoing complex tectonic modifications at this time, with probable subsequent ridge jumps. This was
40
30 0
i
also the time of creation of Gallieni Fracture Zone. The NW-SE spreading direction on this ridge does not appear to be recorded in the topographic fabric. The poor fit between the three plates (Figs. 5 and 6) indicates that the evolution during this period is not yet well understood. Epoch A27-A22 (Figs. 7-12) By anomaly 27 time seafloor spreading between Madagascar and the Seychelles Islands had ceased and the spreading center in the Mascarene Basin jumped northward to a position between the west-
50
i
//I
10 +
-2o*
-50
+
+ Fig. 14. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 20.
228
em coast of India and the Seychelles Plateau (Schlich, 1982). During this tectonic epoch, rates of seafloor spreading were high (15 cm/yr) along the CNIR and the SEIR, and the SWIR lengthened substantially due to the evolution of the Central Indian Ocean Triple Junction (Patriat, 1987). However, the rate of spreading along the CNER began to slow down after anomaly 23 (Fig. 12), and there was a minor ridge jump at this time (Fig. 12) along the ridge in order to preserve the edge-edge-edge mode at the triple junction. A NNW-SSE spreading direction prevailed on the
0
30 +
-20
l
_40,
/
SWIR up to A24 time (Figs. 4 and IO). hut again no indication of this direction is found in the topographic fabric. At this time the spreading direction turned to the more N-S present-day o~entation and the Atlantis II Fracture Zone had just been created. During this same tectonic epoch, unexplained tectonic events were taking place along the SEIR. Although anomaly 29 is present along the entire length of the SEIR, east of the 60” meridian anomalies 28-25 are missing. Well-defined magnetic lineations do not reappear in this region until anomaly 24 time. Although we can only
”
/
(
0 .JO+
f Fig, 15. Reconstruction of the Centrd hdiaa oaken with mpcct to Africa at anomaly 18.
229
speculate as to the reason for the lack of well-defined linear magnetic anomalies, the evolution of a miniplate in this region, similar to the Easter Island plate might be one possible solution (note also that in this region spreading has been symmetric between anomalies 29 and 24 (Fig. 10)). The finite rotation parameters of the A22 Africa-India reconstruction are very different to the corresponding ones for A23 and A21 (Table 2) leading to “instantaneous poles” for the A23-A22 and A22-A21 periods located very close to the CNIR. This peculiarity is illustrated by the overlap of the Laccadive Ridge and Saya de Malha
30 ,
0
40 i
50 *
and by the non-alignment of the arrows on both sides of the ridge (Figs. 12-13), indicating a very rapid change in spreading directions. This difference between the rotation parameters is due to the fanning of anomaly 22 (Fig. 13). A means of achieving more homogeneous rotation parameters would be to remove the constraint of realigning La Boussole Fracture Zone. Epoch A22-A18
(Figs. 12-15’)
Between A22 and A21 times, spreading rates decreased dramatically along the Central and Southeast Indian ridges. Additionally, the spread-
60
70
///'
-io+
-50
+
+
Fig. 16. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 13.
80
230
ing shifted to a more northeasterly direction along the CNIR (Fig. 13). The most striking change in plate motion during this tectonic epoch was the 20” easterly shift in spreading directions along the Central and Southeast Indian ridges during the period A20-A18 (Fig. 15). Note the various aspects of the reorganization of the SEIR during this period: ridge segments merging (Fig. 13), ridge jump (Fig. 14), and obliq~ty variation (Fig. 15). These changes were accompanied by a decrease in spreading rate along the CNIR and an increase in spreading rate along the SWIR. The similarity
0
in the magnitude of these spreading rates led to a dramatic change in the evolution of the triple junction. During this period the small amount of lengthening of the SWIR and the growth of the Melville Fracture Zone show that a ridge-ridge-fault or ridge-fault-fault mode should have prevailed. At A18 time one observes the fargest discrepancy in the fit between the three plates. Without additional data on the northern flank of the SEIR, it is dif~~ult to know whether there is an error in the reconstruction or a problem of unknown origin.
+
to +
-00
+
+ Fig. 17. lkcunstmtion
+-
c
+
.
“i
of theCentral Inckan Ocean with wpect to Africa at anomaly 8.
+
231
These plate boundary response
changes
to the continued
were probably
collision
of India
in and
along the CNIR CNIR,
Eurasia.
east-
and there was a progressive
ward shift in the direction
of spreading
and to a lesser extent
along
along the SEIR.
It was also at the end of this tectonic that
Epoch Al&A8 (Figs 15-17) During this tectonic epoch, the plate boundaries in the Indian aspect. motion
Ocean
began
It was during ceased
Australia
along
drifted
Between
Al8
0
to assume
this interval
and Al3
away
Ridge,
the development
and
hotspot
and was transferred
to the African
a modern
of the Reunion
from
spreading
Reunion
that strike-slip
the Ninety-East
rapidly
CNIR
the
hotspot
from the Indian
resulted
of the volcanic
Bank (Fig. 18).
-10,
of the Central
Indian
Ocean with respect
the plate
in the ending
30 *
Fig. 18. Reconstruction
epoch
beneath
of the Chagos-Laccadive
form the Nazareth
rates decreased
crossed
plate (Figs. 16 and 17). The transfer
pelago and the eruption
Antarctica.
the
to Africa at anomaly
6
of
Archirocks that
232
Epoch A8-AI
(Figs. I 7-20)
Although simifar in most respects to the previous tectonic epoch, during this interval there was a significant change in the direction of spreading
0
along the CNIR. In comparison to previous times, spreading rates along all three ridges were very slow.
30 +
-20
-30
-40
Fig. 19. Reconstruction
of the Central Indiaa Ekean with respect to Africa at anomaly 5.
233
0
00
60
50
30 +
-1o+
-20
-30
-40
+
_.‘. --
Fig. 20. Reconstruction
of the Central
Indian
Ocean with respect
to Africa at anomaly
1.
234
Conclusions
The sixteen reconstructions presented in this paper provide a detailed and comprehensive description of the tectonic evolution of the Indian Ocean. We have also demonstrated that while it is important that any reconstruction should result in the correct alignment of fracture zones, it is not necessary to use fracture-zone trends to obtain coherent reconstructions. Despite our overall success, there are imperfections in the proposed reconstructions that are difficult to explain (see Figs. 13 and 14). Because of the high quality and abundance of available data, it is unlikely that all these discrepancies will be resolved by the collection and analysis of additional data. It is therefore possible that some of these discrepancies may have a more fundamental, geological origin. At present, we can only speculate as to what unknown factors might be involved: regional plate deformation, a slight change in the radius of the Earth, or possibly a difference in the mechanism by which magnetic anomalies are produced on fast and slow spreading ridges. Besides shedding considerable light on the kinematic development of the Indian Ocean, this model ultimately may provide insights into the processes that occur at accreting plate boundaries. We believe that only through the careful analysis of detailed tectonic data sets, such as the one presented here, will we eventually be able to answer questions such as: how do ridge axes evolve?, how often and under what circumstances do ridge jumps take place?, what conditions lead to oblique spreading?, what are the causes of plate boundary reorganization?, and, are the tectonic epochs we recognize in the Indian -Ocean of local or global extent? This brief review has only begun to address some of these questions.
reviews, and C.R. Scotese for his help in revising the original manuscript. This work were supported by INSU (ATP GGO 0981036) and IFREMER (contract 85241059). Most of the data used for this work were collected during rhe Marion Dufresne cruises organized and supported by the Terres Australes et Antarctiques Franqaises (TAAF).
References
Duncan,
R.A.,
absolute
continents. McKenzie, Indian
1981.
The authors would like to thank J.Y. Royer and D.B. Rowley for their careful and constructive
in the southern
Tectonophysics,
for motion
oceans-an
of the Gondwana
74: 29-42.
D. and Sclater, J.G., 1971. The evolution Ocean
since
the late Cretaceous.
of the
Geophys.
J.R.
Astron. Sot., 25: 437-528. Morgan, W.J., 1983. Hot spot tracks and the early rifting of the Atlantic. Testonophysics,
94: 123-139.
Patriat, P., 1987. Reconstitution
de I’&olution
du systeme de
dorsales de I’Gcean Indien par les methodes
de la Cidma-
tique des Plaques. Terres Aust. Antarct. Fr. (Mission Rech.), Paris, pp. 308. Patriat,
P., S&goufin, J., Goslin,
Relative
positions
Cretaceous:
J. and Beuzart,
of Africa and Antarctica
evidence
for non-stationary
P., 1985.
in the Upper
behatiour
of frac-
ture zones. Barth Planet. Sci. Lett., 75: 204-214. Royer, J.Y., Patriat, Evolution
P., Bergh, H. and Scotese,
of the Southwest
Cretaceous
(anomaly
Indian
Ridge
34) to the Middle
CR.,
and Cerozoic
Plate Reconstructions.
1988.
from the Late
Eocene (anomaly
20). In: C.R. Scotese and W.W. Sager (Editors),
Mesozoic
Tectonophysics,
155
(this issue): 235-260 Schlich, R., 1982. The Indian Ocean: aseismic ridges, spreading centers
and oceanic
Stehli (Editors),
basins.
The Ocean
In: A.E.M.
Naim
and F.G.
Basins and Margins.
Vol. 6,
The Indian Ocean. Plenum, New York, pp. 51-147. Segoufin,
J., 1981.
Mozambique. bourg, 236 pp.
Acknowledgements
Hotapot
frame of reference
Morphologic
et structure
du Canal
de
Th&se Doct. d’Etat, Univ. L. Pasteur, Stras-
Tectonoph_wics, 155 (1988) 211-234 Elsevier Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
reconstruction of the Central Indian Ocean PHILIPPE
PATRIAT and JACQUES
SEGOUFIN
Institut de Physique du Globe de Paris, Laboratoire de GtGophysiqueMurine, CNRS U.A. 729, 4 place Jumeu. 75252 Paris Cedex 05 (France) (Received
September
5,1987;
revised version
accepted
of the Central
Indian
November
20,1987)
Abstract
Patriat,
P. and Stgoufin,
(E.ditors).
Mesozoic
J., 1988. Reconstruction and Cenozoic
Sixteen plate tectonic the latest Cretaceous into
five distinct
Eocene
tectonic
Early Miocene-Recent rates of spreading motion
occurred
reconstructions
(anomaly
(A27-A22),
Plate Reconstructions.
epochs:
(3) Middle (A8-AS). along
between
are presented
29) to the present. (1) latest
Eocene-Late During
the Central,
illustrating
The history
spreading
Paleocene
(A22-AU),
Scotese
of the Central in the Indian
(A29-A27),
and W.W.
Sager
epochs,
and Southwest
Indian
A21 and Al8 time due to the collision
subtle changes ridges.
Indian
Ocean
Miocene
Paleocene-Early (Al8-A@,
and (5)
took place in the directions
The most
from
Ocean can be divided
(2) Middle
(4) Late Eocene-Early
each of these tectonic
Southeast
In: C.R.
the development
of seafloor
Cretaceous-Early Eocene
Ocean.
Tectonophysics, 155: 211-234.
dramatic
changes
and
in plate
of India and Eurasia.
Introduetion
The Indian Ocean is the result of seafloor spreading between the African, Antarctic, and Indian plates. Identifiable magnetic anomalies of Late Cretaceous and Cenozoic age have been mapped on both sides of the Southwest Indian Ridge (SWIR), Southeast Indian Ridge (SEIR) and Central Indian Ridge (CNIR). Using these magnetic anomalies, we have produced plate tectonic reconstructions describing the tectonic evolution of each ridge (Segoufin, 1981; Patriat, 1987; Royer et al., this issue). This paper puts together these sets of reconstructions in order to illustrate the tectonic development of the Central Indian Ocean around the Central Indian Triple Junction. Anomaly 29 is the oldest identifiable magnetic anomaly mapped in the western Central Indian Basin. It is only from this epoch onwards that 004%1951/88/$03.50
0 1988 Elsevier Science Publishers
B.V.
complete sequences of magnetic anomalies exist on both sides of the three ridges. As a consequence we are able to present sixteen plate tectonic reconstructions which describe, in detail, the plate tectonic evolution of the Central Indian Ocean from the Late Cretaceous (A29) to the Present (Fig. 1). Of particular interest are the reconstructions that describe the change in plate motions that took place between anomaiy 22 and 18 as a result of the progressive collision of India with Eurasia. This paper represents the first synthesis of magnetic anomaly data from the Southwest, Southeast and Central Indian ridges. We will explain the method that was used to produce the plate reconstructions (Figs. S-20), we will discuss the most important constraints, and we will describe, in a step-by-step fashion, the tectonic development of the Central Indian Ocean.
212
_
--
-7
Fig. 1. Magnetic anomalks and isccbrons for the Central Indian Ocean.
---
213
TABLE 2
Methods used to produce the reconstructions
Finite rotation poles. India-Africa
General considerations
Epoch
Lat. (+,
At
any
boundary
given is made
segments
that
the spreading in this paper
history
were produced
is based
described
by Patriat
on the minimization
7.42
A6
16.2
44.2
11.09
chrons
are listed were then
drawn
1-3.
about
46.0
15.40
11.3
51.6
21.31
Al8
14.9
49.9
23.23
A20
17.0
48.1
24.09
A21
12.6
48.3
28.34
A22
23.8
38.4
23.53
magnetic
A23
14.9
41.9
29.93
A24
15.9
38.6
31.11
A25
17.6
33.9
32.70
A26
17.6
31.9
34.63
A27
14.0
33.4
40.22
A28
15.3
31.0
40.50
A29
13.1
31.4
45.65
(1987) which scalar
Magnetic
by linking
14.8
presented
of the triple
in Tables
A8 Al3
spread-
product (McKenzie and Sclater, 1971). The finite poles of rotation that were calculated using this method
(“)
of ridge
anomaly picks from each side of the spreading axis. To determine the goodness of fit, we have used the method
-1 “W)
51.4
disposed
by refitting
“E;
8.2
floor as a series of magnetic
are symmetrically
(+,
A5
will be pre-
axis. The reconstructions
OS)
plate
faults. As seafloor
the spreading
served in the ocean anomalies
accretionary
up of a combination
and transform
ing progresses,
an
time,
Angle
Long. ON; -,
together
iso-
From Patriat (1987).
ob-
served or rotated magnetic anomalies of the same age. As a consequence, in this paper the isochrons
picks at the beginning of the normal polarity sequence, with the exception of anomalies 13, 25,
are the result of the reconstructions and are then identical on both sides of each ridge.
27 and polarity
In order to facilitate comparisons with other studies. we have defined
28, for which the end of the normal sequence was chosen (Fig. 2). Due to
of our result the anomaly TABLE 3 Finite rotation poles, Africa-Antarctica
TABLE 1
Epoch
Finite rotation poles, India-Antarctica Epoch
Lat. (+,
Long. ON; -,
“S)
(+,
“E; -,
Lat. (+.
Angle “W)
(“)
Angle
Long. ON; -,
OS)
(+.
“E;
-1 “W)
(“)
A5 (1)
9.1
-41.6
1.49
A6 (1)
13.4
-55.1
2.79 4.42
A5
9.2
40.7
7.13
14.6
- 52.2
A6
14.7
32.1
11.97
Al3 *
15.1
-51.0
5.86
A8
16.2
30.6
15.65
Al8 (1)
15.3
- 50.4
6.92 7.84
A8
l
Al3
17.0
30.5
19.62
A20 (1)
10.2
- 42.8
A18
19.2
21.4
22.88
A21 (1)
7.9
- 39.6
8.86
A20
16.4
28.8
25.41
A22 *
9.3
-41.7
9.31
A21
12.8
28.5
28.67
A23 *
10.4
- 43.4
9.12
A22
14.5
24.6
29.56
A24 (1)
11.3
- 44.9
10.11 10.38
A23
13.4
23.3
32.17
A25 *
6.9
-41.8
A24
12.8
21.9
34.48
A26 (2)
3.8
- 39.7
10.63
A25
12.4
19.3
37.52
A27 *
1.7
- 39.4
11.07
A26
12.4
17.5
39.45
A28 (2)
0.6
- 39.2
11.32
A27
10.1
18.0
43.40
A29 (2)
- 0.4
- 39.4
11.59
-41.6
11.84
- 41.4
13.47
A28
11.1
15.4
44.24
A31 (2)
1.1
A29
11.2
13.0
47.44
A32 (2)
- 1.8
A31
9.4
13.7
51.59
A32
9.8
10.6
55.31
From Patriat (1987).
(1) From Patriat (1987). (2) From Royer et al. (this issue). *
Interpolated.
214
-0Ma
40 Ma
’
pi
17
2
1=3
t8
PA
H
IS 20
21
4A
E3 E
-
0
D cc
$zJ
5
- DMa
25 “I
continuing controversies concerning the correlation between the magnetic reversal time scale and the absolute time scale. we have chosen to describe our results only in terms of the magnetic reversal time scale. It is important to point out, that in our calculations, as often as possible, only magnetic anomaly picks were used to determine the best-fit rotation parameters. The shape of the fracture zone between magnetic anomalies was not used because, in general, the shape of the fracture zone is a poor predictor of instantaneous plate motion. The geometry of a fracture zone is the result of the evolution of a series of active transform faults. The larger the offset between magnetic anomalies, the more likely it is that the fracture-zone trend is a composite feature made up of the superposition
PRESENT DAY CONFIGURATION
26 -1
CNIR
SE
f3
_ 2OMa
15ochron t _
8OMo
CNlR
-
6 6A
27 CrJ
= :
RECONSTWCT~ON EPOCH f
- 4oMa
- 6OMa
Fig. 2. Magnetic reversal chrom&g. ~y~ti~~~ drawn. Black points refer to the point where the mpsaetic anomalies were picked.
Fig. 3. A method for rcconst~~ctiott of the SoutbwM Indian Ridge (SWIR), using previously rcwnstructed iswbrom (heavy lines) on the Southeast Indian IGdgz (SEIR) md Cu@ti Indian Ridge (CNIR). The large dot is the trace of the Ittd& Ocean Triple Junction on the thne plates. The magnetic anomaly picks to be rfmscm bled are indicated by &cles (African plate) and triangles (Antarctic p&e).
215
216
L
ll
217
--I i ; LI I
’
;?
-1
218
of successive trends of the active transform boundary. Due to the composite nature of fracture-zone trends their use to estimate relative plate motions may produce inaccurate results (Patriat et al., 1985).
When there are numerous magnetic anomalies observed on either side of a ridge axis, it is possible to calculate the best-fitting rotation parameters using magnetic anomaly data only. A unique fit of magnetic anomalies is possible because each
60
a
-20
R 0
+
-30
-40
+
c
Fig. 5. Reconstructions
of the Central Indian Ocean with respect to Africa at anomaly
29 time. The Seychelles
has been rotated by 5.62” around a pole at 18.4O N, 16.8” E to allow for the dying Mascarene apply for this figure and for Figs. 6-20.
Heavy continuous
location of axis, if spreading had been symmetrical (see text for explanation);
Location
of magnetic
at the time of the reconstruction;
and geometry unchanged
heavy dashed line-tripie
the Indian plate rotated to Africa via Antart&; de La Foumaise).
line-axis
junction fine lines-0
anomalies:
Circle-
strutted isochrons;
since the last step; arrow-direction
and 2500 m isobaths; asterisk-location triangle-Africa;
dashed line-interpolated
and spreading rate
Triple Junction trace on
of La Reunion hotspot (Piton
square-India;
isochrons.
symbols
heavy dotted line-predicted
traces; heavy dotted and dashed line-Indian Antarctica;
Plateau (dashed line)
Basin Ridge. The following
continuous
iine-recon-
219
ridge segment has a specific trend. In these cases, a visual check is made to be certain that the major fracture zones are realigned when the plates are reconstructed. However, in cases where there are few magnetic anomalies, or where the ridge length is small, the relative spacing of fracture zones is taken into account. In these cases, the calculations require that the end points of one or two conjugate fracture zones line up when the plates are reconstructed. The calculations do not require, however, that the trends of the fracture zones .I 0
superimpose. In this way, the location of the fracture zones is taken into consideration, but no importance is placed on the trend. Special constraints
One of the most important constraints for reconstructing the Central Indian Ocean is reassembly of the fracture-zone segments that bound anomalies 20-29. One of these fracture-zone segments (La Boussole) lies on either side of the
50
b0
0
Fig. 6. Reconstruction
of the Central
been rotated
Indian
by 2.53O around
Ocean
with respect
to Africa
at anomaly
70
2X. The Seychelles
a pole at 18.4O N, 16.8” E to allow for the dying Mascarene
8 0
Plateau
(dashed
Basin Ridge.
line) has
220
CNIR; it is located southeast of Reunion and its conjugate member southeast of the Chagos-Laccadive Islands (Fig. 1). The other fracture-zone segments (L’Astrolabe) lie on either side of the SEIR. The southernmost part of the L’Astrolabe Fracture Zone extends northward from the eastem edge of Crozet; its northern counterpart lies halfway between the Chagos-Laccadive Islands and the Ninety-East Ridge (Fig. 1). The trace of the Indian Ocean Triple Junction in the Central Indian Basin is located between the northern extensions of the La Boussoie and L’Astrolabe frac-
30 +
0
40
50
+
b
ture zones since their conjugate part?; lie on both sides of the SWIR. The unique geometry of the La Boussole and L’Astrolabe fracture zones was used to reconstruct the CNIR and SEIR, and subsequently the trace of the Indian Ocean Triple Junction in the Central Indian Ocean Basin, between anomalies 20 and 29. This was done by realigning the southern sections of these fracture zones with their northern counterparts when magnetic anomaly data was sparse. Moreover, the remonstrated isochrons of the SEIR and CNIR were used to reassemble the 60 +
70 +
//
-10,
-20
-30
-40
.a0
+
+
+
Fig. 7. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 27.
80
221
magnetic
anomalies
along
the SWIR
Fig. 3. In this way, although each ridge were performed tions
were required
the Central
The rotation magnetic 1-3.
parameters along
Indian
The confidence
parameters
varies
on the special
the Southeast,
of the
Central
we place
in these
rotation
from ridge to ridge depending
characteristics
of each
spreading
In general,
ridge between
the rotations because
magnetic
defined
fracture-zone
The rotation of the CNIR
Ocean
Fracture
anomaly
data
Junction abunwell-
offsets.
parameters
for the reconstruction
are well constrained
due to the very
simple ridge geometry
before Al8 time and due to
the different
of the successive
ments
obliquity
after Al8
Ocean with respect
ridge seg-
time (Fig. I). The exceptions
50
Indian
Triple
of the
and numerous
50
of the Central
length
Zone provides
&
Fig. 8. Reconstruction
for the SEIR are
the great
the Indian
and the Amsterdam dant
are listed in Tables
30 I
system.
best constrained
Junction.
used to reconstruct
Ridges
in of
all rota-
the coherency
Ocean Triple
isochrons
and Southwest
independently,
to maintain
Indian
as shown
the reconstructions
to Africa
at anomaly
26.
are
at A29 where the data are too sparse and at A5 where the great dispersion of the reassembled picks is unexpected and unexplained. Despite its slow spreading rate, the reconstructions of the SWIR are fairly well constrained due to the great length of the ridge. However, because it has lengthened considerably since the Early Tertiary, it is difficult to make accurate reconstructions prior to anomaly 26 time. As a consequence the rotations describing the reconstruction of the ridge for anomalies 5-24 are taken from Patriat (1987) while the rotation parameters for anomalies 26-29 are based on the work of Royer
30 +
0
Explanation of the figures
In Figs. 5-20, plate tectonic reconstructions of the Central Indian Ocean are represented for anomalies 5, 6, 8, 13, and 18 and for anomalies 20-29. The ridge axis in each figure has been drawn by reassembling the magnetic anomaly picks
50
SO
40
et al. (this issue), which is based on additional data between the Bouvet Triple Junction and the Gallieni Fracture Zone.
70
f
I /I’ /
- 10
/’
+
-40,
0
JO *
1)
+
Fig. 9. Reconstruction of the Central Indian Ocean with respectto Africa at anomdy 25.
80
223
from each pair of conjugate plates. Triangles, squares and circles represent magnetic anomaly picks from the African, Indian and Antarctic plates, respectively. On successively younger reconstructions the reconstructed ridge axes become symmetric magnetic isochrons. Magnetic isochrons have been drawn for each anomaly epoch. with the exception of anomalies 8, 13, 22 and 23. and 25-27 along the SWIR (Fig. 1), where no reassembly of magnetic picks has been performed. In each figure, spreading directions and rates are indicated by vectors whose absolute lengths
represent the amount of oceanic crust generated in 5 million years. Two sets of vectors have been plotted. In addition to vectors at the ridge axes, the direction and rate of spreading during the last anomaly epoch are also indicated. These two sets of vectors help to highlight times during which there were major changes in spreading direction or rate (e.g. Fig. 15). The fine dotted line plotted at the active spreading center represents the previous configuration of the ridge axis. The difference in the shape and location of the ridge between the dotted line (previous configuration) and the active 60 ,
r
-20
40
-50,
‘
+
Fig. 10. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 24.
224
spreading center may indicate ridge jumps, periods of asymmetric spreading, or errors in our interpretation of the ridge shape. The continents, aseismic ridges, magnetic isochrons and plate boundaries have been reconstructed in a reference framework in which Africa has been held fixed. The Mascarene Plateau and the Saya de Malha Bank, as well as the Chagos-Laccadive Ridge are assumed to have been produced by movement of the African and Indian plates over the Reunion hotspot (Duncan, 1981; Morgan, 1983). As a consequence on succes-
sively older reconstructions, parts of the Mascarene Bank and Chagos-Laccadive Ridge have been removed according to the age progression suggested by the Reunion hotspot track. In a similar manner, the older parts of the Ninety-East Ridge have been removed to avoid overlap with the Kerguelen Plateau. The goodness of the fit can be estimated by the alignment, along the ridge axes, of the symbols that represent magnetic anomaly picks from the African, Indian and Antarctic plates (Figs. 5-20). In general, the magnetic anomaly picks are colin-
10,
-20
-30
-50
+
+
Fig. 11. Reconstructionof the Canti
Indian Ocean with respect to Africa at anon&y 23.
22s
ear and the transform offsets predicted from the magnetic data coincide with the location of known fracture zones. As an additional estimate of the goodness of fit, the trace of the Indian Ocean Triple Junction on the Indian plate has been drawn twice, using two independent data sets. The first trace (heavy dashed line) represents the path of the triple junction rotated by the relative motion between the Indian and African plates. The second trace (heavy dashed and dotted line) represents the same triple junction path determined by calculating the relative motion between the Indian and African plates via the plate circuit
Africa-Antarctica-India. The greater the coincidence between the two traces, the better the reconstruction. Tectonic development of the Indian Oceun General comments At first gtance, the most striking pattern observed in Fig. 4 is the change at anomaly 1X time from a period of rapid, northward seafloor spreading to a period of slower, northeasterly directed spreading. This change in spreading direction, attributed to the collision of India with Eurasia, has
60
-50
+
70
+
Fig. 12. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 22.
60
226
been previously noted by numerous workers (e.g., McKenzie and Sclater, 1971; Schiich, 1982). The detailed set of reconstructions presented in this study, however, permits us to take a closer look at the tectonic development of the Indian Ocean, Upon closer inspection, a more complex scenario emerges due chiefly to a better understanding of the SWIR history. Indeed, the change in the direction of the triple junction path, which is the boundary between the crust created at two different ridges, is the best record of change in the relative motions of the three plates. According to this criterion, we recognize no fewer than five
distinct periods of seafloor spreading: (I) fatest Cretaceous-Early Paleocene (A29-A27), (21 Middle Paleocene-Early Eocene (A27-AZ], (3) Middle Eocene-Late Eocene (A22-AIX), (4) Late Eocene-Early Miocene (A18-A8), and (5) Early Miocene-Recent (AGA5). in the following section we will outline the characteristics that render each tectonic epoch distinct. Epoch AD-A27 (Pigs. 5- 7) At A29 time the Africa-Indian plate boundary rapidly lengthened, growing eastward from the Mascarene Basin into the Madagascar Basin at the
-IO+
-40,
Fig. 13. Reamstruction of the CentraE hc#im Chaa
with respect to Africa
at ,anOmaty 21.
expense of the SEIR. This resulted in the reorganization of the Africa-India-Antarctic Triple Junction and the formation of the modern CNIR. This period is characterized by the highest opening rate (about 20 cm/yr) in the Indian Ocean and by a pause in the shortening of the SEIR. As illustrated in Figs. 5-7, the traces of the Central Indian Ocean Triple Junction on the African and Antarctic plates cross-cut one other (Figs. 5-7) indicating that the SWIR was undergoing complex tectonic modifications at this time, with probable subsequent ridge jumps. This was
40
30 0
i
also the time of creation of Gallieni Fracture Zone. The NW-SE spreading direction on this ridge does not appear to be recorded in the topographic fabric. The poor fit between the three plates (Figs. 5 and 6) indicates that the evolution during this period is not yet well understood. Epoch A27-A22 (Figs. 7-12) By anomaly 27 time seafloor spreading between Madagascar and the Seychelles Islands had ceased and the spreading center in the Mascarene Basin jumped northward to a position between the west-
50
i
//I
10 +
-2o*
-50
+
+ Fig. 14. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 20.
228
em coast of India and the Seychelles Plateau (Schlich, 1982). During this tectonic epoch, rates of seafloor spreading were high (15 cm/yr) along the CNIR and the SEIR, and the SWIR lengthened substantially due to the evolution of the Central Indian Ocean Triple Junction (Patriat, 1987). However, the rate of spreading along the CNER began to slow down after anomaly 23 (Fig. 12), and there was a minor ridge jump at this time (Fig. 12) along the ridge in order to preserve the edge-edge-edge mode at the triple junction. A NNW-SSE spreading direction prevailed on the
0
30 +
-20
l
_40,
/
SWIR up to A24 time (Figs. 4 and IO). hut again no indication of this direction is found in the topographic fabric. At this time the spreading direction turned to the more N-S present-day o~entation and the Atlantis II Fracture Zone had just been created. During this same tectonic epoch, unexplained tectonic events were taking place along the SEIR. Although anomaly 29 is present along the entire length of the SEIR, east of the 60” meridian anomalies 28-25 are missing. Well-defined magnetic lineations do not reappear in this region until anomaly 24 time. Although we can only
”
/
(
0 .JO+
f Fig, 15. Reconstruction of the Centrd hdiaa oaken with mpcct to Africa at anomaly 18.
229
speculate as to the reason for the lack of well-defined linear magnetic anomalies, the evolution of a miniplate in this region, similar to the Easter Island plate might be one possible solution (note also that in this region spreading has been symmetric between anomalies 29 and 24 (Fig. 10)). The finite rotation parameters of the A22 Africa-India reconstruction are very different to the corresponding ones for A23 and A21 (Table 2) leading to “instantaneous poles” for the A23-A22 and A22-A21 periods located very close to the CNIR. This peculiarity is illustrated by the overlap of the Laccadive Ridge and Saya de Malha
30 ,
0
40 i
50 *
and by the non-alignment of the arrows on both sides of the ridge (Figs. 12-13), indicating a very rapid change in spreading directions. This difference between the rotation parameters is due to the fanning of anomaly 22 (Fig. 13). A means of achieving more homogeneous rotation parameters would be to remove the constraint of realigning La Boussole Fracture Zone. Epoch A22-A18
(Figs. 12-15’)
Between A22 and A21 times, spreading rates decreased dramatically along the Central and Southeast Indian ridges. Additionally, the spread-
60
70
///'
-io+
-50
+
+
Fig. 16. Reconstruction of the Central Indian Ocean with respect to Africa at anomaly 13.
80
230
ing shifted to a more northeasterly direction along the CNIR (Fig. 13). The most striking change in plate motion during this tectonic epoch was the 20” easterly shift in spreading directions along the Central and Southeast Indian ridges during the period A20-A18 (Fig. 15). Note the various aspects of the reorganization of the SEIR during this period: ridge segments merging (Fig. 13), ridge jump (Fig. 14), and obliq~ty variation (Fig. 15). These changes were accompanied by a decrease in spreading rate along the CNIR and an increase in spreading rate along the SWIR. The similarity
0
in the magnitude of these spreading rates led to a dramatic change in the evolution of the triple junction. During this period the small amount of lengthening of the SWIR and the growth of the Melville Fracture Zone show that a ridge-ridge-fault or ridge-fault-fault mode should have prevailed. At A18 time one observes the fargest discrepancy in the fit between the three plates. Without additional data on the northern flank of the SEIR, it is dif~~ult to know whether there is an error in the reconstruction or a problem of unknown origin.
+
to +
-00
+
+ Fig. 17. lkcunstmtion
+-
c
+
.
“i
of theCentral Inckan Ocean with wpect to Africa at anomaly 8.
+
231
These plate boundary response
changes
to the continued
were probably
collision
of India
in and
along the CNIR CNIR,
Eurasia.
east-
and there was a progressive
ward shift in the direction
of spreading
and to a lesser extent
along
along the SEIR.
It was also at the end of this tectonic that
Epoch Al&A8 (Figs 15-17) During this tectonic epoch, the plate boundaries in the Indian aspect. motion
Ocean
began
It was during ceased
Australia
along
drifted
Between
Al8
0
to assume
this interval
and Al3
away
Ridge,
the development
and
hotspot
and was transferred
to the African
a modern
of the Reunion
from
spreading
Reunion
that strike-slip
the Ninety-East
rapidly
CNIR
the
hotspot
from the Indian
resulted
of the volcanic
Bank (Fig. 18).
-10,
of the Central
Indian
Ocean with respect
the plate
in the ending
30 *
Fig. 18. Reconstruction
epoch
beneath
of the Chagos-Laccadive
form the Nazareth
rates decreased
crossed
plate (Figs. 16 and 17). The transfer
pelago and the eruption
Antarctica.
the
to Africa at anomaly
6
of
Archirocks that
232
Epoch A8-AI
(Figs. I 7-20)
Although simifar in most respects to the previous tectonic epoch, during this interval there was a significant change in the direction of spreading
0
along the CNIR. In comparison to previous times, spreading rates along all three ridges were very slow.
30 +
-20
-30
-40
Fig. 19. Reconstruction
of the Central Indiaa Ekean with respect to Africa at anomaly 5.
233
0
00
60
50
30 +
-1o+
-20
-30
-40
+
_.‘. --
Fig. 20. Reconstruction
of the Central
Indian
Ocean with respect
to Africa at anomaly
1.
234
Conclusions
The sixteen reconstructions presented in this paper provide a detailed and comprehensive description of the tectonic evolution of the Indian Ocean. We have also demonstrated that while it is important that any reconstruction should result in the correct alignment of fracture zones, it is not necessary to use fracture-zone trends to obtain coherent reconstructions. Despite our overall success, there are imperfections in the proposed reconstructions that are difficult to explain (see Figs. 13 and 14). Because of the high quality and abundance of available data, it is unlikely that all these discrepancies will be resolved by the collection and analysis of additional data. It is therefore possible that some of these discrepancies may have a more fundamental, geological origin. At present, we can only speculate as to what unknown factors might be involved: regional plate deformation, a slight change in the radius of the Earth, or possibly a difference in the mechanism by which magnetic anomalies are produced on fast and slow spreading ridges. Besides shedding considerable light on the kinematic development of the Indian Ocean, this model ultimately may provide insights into the processes that occur at accreting plate boundaries. We believe that only through the careful analysis of detailed tectonic data sets, such as the one presented here, will we eventually be able to answer questions such as: how do ridge axes evolve?, how often and under what circumstances do ridge jumps take place?, what conditions lead to oblique spreading?, what are the causes of plate boundary reorganization?, and, are the tectonic epochs we recognize in the Indian -Ocean of local or global extent? This brief review has only begun to address some of these questions.
reviews, and C.R. Scotese for his help in revising the original manuscript. This work were supported by INSU (ATP GGO 0981036) and IFREMER (contract 85241059). Most of the data used for this work were collected during rhe Marion Dufresne cruises organized and supported by the Terres Australes et Antarctiques Franqaises (TAAF).
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Acknowledgements
Hotapot
frame of reference
Morphologic
et structure
du Canal
de
Th&se Doct. d’Etat, Univ. L. Pasteur, Stras-