Geoscience Reference
In-Depth Information
India becomes almost north-south. This part of the plate boundary is called the
Central Indian Ridge. The Southwest Indian Ridge, which extends from the
Bouvet Triple Junction in the South Atlantic to the Indian Ocean Triple Junction,
is the boundary between the African and Antarctic plates. The position of the
present-day rotation pole for Africa relative to Antarctica at about 6
◦
N, 39
◦
W
(Table 2.1) means that this ridge is offset by a series of very long transform
faults. One of the main fault complexes, the Andrew Bain Fracture Zone, offsets
the ridge axis by some 500 km. The half-spreading rate in this region is about
0.8 cm yr
−
1
. The African is subdivided into the Nubian and Somalian plates (Sect.
10.4.2). The triple junction between the Nubian, Somalian and Antarctic plates is
located within the Andrew Bain Fracture zone at
30
◦
E. The other major bathy-
metric features of the Indian Ocean are the Ninety-East Ridge and the Chagos-
Maldive-Laccadive Ridge system. Both of these linear north-south submarine
mountain chains are hotspot tracks - of the Kerguelen and Reunion hotspots,
respectively.
The oldest magnetic anomalies in the Indian Ocean occur in the Wharton basin
(M9-M25), which lies between Australia and the Java Trench, west of Western
Australia (M0-M22), between Madagascar and east Africa (M2-M22), in the
Somali basin (M13-M21) and north of Antarctica at 20
◦
E (M1-M16). These
anomalies indicate that the separation of Africa and Antarctica began by the time
of anomaly M21 (150 Ma). At this time, east Antarctica and Madagascar (then
joined) moved southwards away from Africa (Fig. 3.21(a)). In so doing, they
generated the symmetrical anomalies in the Somali basin, as well as those off the
east Antarctic coast and west of Madagascar, which appear to be the northern
and southern halves of a symmetrical pattern. At about the time of anomaly
M0, seafloor spreading in the Somali basin north of Madagascar stopped and a
new ridge was initiated between Madagascar and Antarctica: the proto-Southwest
Indian Ridge formed, separating Antarctica from Africa, Madagascar and India.
From the time of anomaly 34 (
∼
90 Ma) until the present, spreading along this
ridge appears to have been fairly constant, but magnetic-anomaly data from this
ridge are very scarce.
The anomalies north of Australia, to the north and east of the Exmouth Plateau,
M9-M25, are only the southern half of a symmetrical pattern; the northern part
has been subducted by the Java Trench, which is currently active there. The oldest
anomaly close to the continental margin north of Australia, M25, gives an estimate
of the date of opening of this ocean as 155 Ma. A northeast-southwest pattern
of anomalies (M11-M0) is present in the Bay of Bengal and west of Western
Australia there is a symmetrical pattern of anomalies M0-M10. This indicates
that a ridge was active here by M10 time (134 Ma). This ridge was the plate
boundary between the Indian plate and the Australia-Antarctic plate. Australia
and Antarctica were still joined at this stage, though continental extension had
been taking place since the separation of India (Fig. 3.21(b)).
At about 96 Ma a major change in the plate motions occurred; and the major
part of the Indian Ocean has been created since then. At this time (or shortly
∼
