Geoscience Reference
In-Depth Information
3.3.2 The Atlantic Ocean
The magnetic-anomaly map of the Atlantic Ocean is by far the simplest for the
three major oceans (Fig. 3.19). The history of continental splitting and seafloor
spreading is almost completely preserved because, apart from the short lengths
of the Puerto Rico Trench and the South Sandwich Trench, there are no subduc-
tion zones. The Mid-Atlantic Ridge is the plate boundary between the Eurasian,
African and North and South American plates. Despite the changes in pairs
of plates, the poles and instantaneous rotation rates are such that the spread-
ing rate of the Mid-Atlantic Ridge does not vary greatly along its length. The
boundary between the North American and the South American plates is best
described as a 'diffuse plate boundary' between the Mid-Atlantic Ridge and the
Caribbean.
The oldest identified anomaly in the South Atlantic is M11, which occurs just
off the west coast of South Africa. Thus, Africa and South America must have
started separating shortly before this time (135 Ma). The oldest anomaly in the
central Atlantic is M25, identified off the east coast of North America and the
northwest coast of Africa. Africa and North America therefore started to sepa-
rate during the mid Jurassic, probably soon after 180 Ma. This motion resulted in
considerable faulting and folding in the Mediterranean region because the early
stage of rifting between Eurasia and North America did not begin until much later
(
120-140 Ma). Rifting then proceeded northwards in stages. The Reykjanes
Ridge between Greenland and Eurasia started spreading at about 55 Ma. The
northwest-southeast anomalies in the Labrador Sea between Canada and Green-
land (anomalies 27-19) and extending northwards into Baffin Bay (anomalies
24-13) indicate that there was also an active ridge there from about 60 Ma
until 43 or 35 Ma. Since this time, Greenland has not moved independently
and has been part of the North American plate: the spreading that started with
anomaly 24 has continued only along the Reykjanes Ridge. North of Iceland
the Arctic ridge system includes the Kolbeinsey, Mohns, Knipovitch and Gakkel
Ridges (Fig. 2.2).
∼
3.3.3 The Indian Ocean
The magnetic-anomaly map of the Indian Ocean (Fig. 3.20)isconsiderably
more complex than that of the Atlantic Ocean. The three present-day mid-ocean
ridges - the Central Indian Ridge, Southwest Indian Ridge and Southeast Indian
Ridge - intersect at the Indian Ocean (or Rodriguez) Triple Junction (Fig. 2.2),
an RRR triple junction. The Southeast Indian Ridge is spreading fairly fast
(3 cm yr
−
1
half-rate) and has smooth topography, whereas the Southwest Indian
Ridge is spreading very slowly (0.6-0.8 cm yr
−
1
half-rate) and has rough topog-
raphy and many long fracture zones. The Carlsberg Ridge starts in the Gulf of
Aden and trends southeast. At the equator it is intersected and offset by many
transform faults, so the net strike of the plate boundary between Africa and
