Download Why is the drainage network of Southern Africa so asymmetrical?

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ng
Kafu
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Lo be
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Za
Zambezi
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ge
deep mantle
plume
rift valley
plume
ARABIA
rift valley
Red Sea
rift valley
Gulf of Aden
AFRICA
Rising mantle plumes cause the crust to bulge and split into three rifts,
which may widen into oceans, such as the Red Sea-Gulf of Aden-East
African Rift Valley split.
Parana-Etendeka plume
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Za
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Cu
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Lebombo-Limpopo plume
Limp
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A FR I CA
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Proto-Atlantic Ocean
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plume head
TOP VIEW
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Cuit
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THE WORLD AROUND US
crust and
upper mantle
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140
SIDE VIEW
SO
he drainage of southern Africa is very
southern africa’s rivers today
asymmetrical. The Vaal and Orange rivers
and 100 million years ago
rise almost on the eastern escarpment, and
then flow westward across the entire country to
discharge into the Atlantic Ocean. By contrast,
the Zambezi River system rises near the Atlantic
Kafu
e
coast and flows eastwards across the continent
Cunene
Ok
av
to the Indian Ocean. Almost all other easterly
an
go
flowing rivers are much shorter, however, with
the exception of the Limpopo. This asymmetry
has its roots deep in the past, and to understand
e
Sav
Makgadikgadi
o
it, we need to delve back into the ancient history
p
Pans
po
m
of these rivers.
Li
The Limpopo River of today is but a small
vestige of what it used to be. In the Cretaceous
al
Va
Period over 65 million years ago, its tributaries
included the upper Zambezi, Kafue and
Okavango rivers, and it provided the main
drainage for southern central Africa. It is for
Orange
this reason that the Limpopo River Delta,
present
which extends from Maputo to Beira, is the
largest on the African continent. At that time,
the ancestral Orange-Vaal River, known as the
Karoo River, discharged into the Atlantic Ocean
much further south than it does today, probably
Kw
via the Olifants River; and what is today the
an
do
lower Orange River was part of a separate river
system known as the Kalahari River. There were
thus three major drainage systems in southern
Li
Limpopo
Africa. These were the Karoo (Orange-Vaal) and
m
po
Delta
po
Kalahari rivers in the south, rising in the east
and flowing west to the Atlantic; and further
north, the ancestral Limpopo, rising almost on
i
the west coast and flowing eastwards across the
ar
lah
continent to the Indian Ocean.
Ka
oo
Kar
This drainage pattern is believed to have been
escarpment
imprinted on the region when the supercontinent
Gondwana broke up. Africa formed the core
100 million years ago
of Gondwana and was flanked in the east by
Antarctica and in the west by South America.
The major drainages of southern Africa as they are today
Breakup was caused by plumes of hot material
(top) and as they were during the Cretaceous Period about
100 million years ago (above).
that rose from deep in the Earth’s mantle and
then spread sideways like a mushroom beneath
the cooler upper mantle and crust, causing them
to bulge upwards. Plume bulges are huge, rising hundreds of metres and spreading to 2 000 km in diameter.
Under such forces, the crust above the plume tears and slides apart under the influence of gravity, usually
forming three rift valleys arranged at about 120° to each other and known as a triple junction. Such rift valleys
usually widen and eventually become oceans when inundated. Sometimes, one of the rift branches may fail
and does not progress beyond the valley stage, as is the situation around the plume at the southern end of
the Red Sea. Here, two branches have opened to the ocean to create the Gulf of Aden and the Red Sea, while
e
T
Africa was separated
from Gondwana by
the Lebombo-Limpopo
plume, which
opened the
Indian Ocean;
and the ParanaEtendeka plume,
which opened
the Atlantic Ocean.
Ka
fu
Why IS THE DRAINAGE NETWORK OF
SouthERN Africa SO ASYMMETRICAL?
mantle plumes and the breakup of gondwana
one branch is probably failing and remains
in the rift valley stage, forming the East
African Rift Valley. Later, and because the
bulge remains elevated after rifting, radial
river drainage patterns become established.
The Lebombo-Limpopo plume that
initiated opening of the Indian Ocean was
centred at the northern end of the Lebombo
Mountains near Pafuri, where rifting started
about 140 million years ago. Two branches
opened that separated Antarctica from
Africa, but the third branch lying along the
current Limpopo valley never progressed
beyond the rift valley stage. Elevated
ground on the southwestern flank of the
plume gave rise to westerly flowing rivers,
which became the Karoo River system.
But at that time, a high mountain range
lay along the southern margin of South
Africa, of which the Cape Mountains are
a remnant, so the Karoo River skirted its
northern flank and collected its tributaries,
before emptying into the Atlantic.
The centre of the Parana-Etendeka plume
that initiated opening of the Atlantic lay
close to the current Cunene River mouth,
where rifting commenced about 120 million
years ago. Here, only two branches
developed, along which South America
and Africa separated. The associated
bulge gave rise to easterly flowing rivers
including the Omatako, Cubango, Cuito,
Kwando and the western tributaries of the
Zambezi. These rivers flowed eastwards
into the rift valley formed by the Limpopo
branch of the Lebombo-Limpopo plume.
The Kalahari River drained the southern
margin of the Parana-Etendeka plume and
is today represented by the lower Orange,
Kainab, Fish and Hartbees rivers and the
Molopo River and its tributaries such as the
Auob and Nossob.
These ancient river systems have
undergone substantial adjustments
since the Cretaceous Period as a result
of erosion and geological processes that
buckled and continue to rift and fracture
the crust (see page 86), but the original
asymmetry implanted at the time of
breakup of Gondwana is still evident. Most
notable is the southward propagation of
the East African Rift Valley system, which
has disrupted the upper tributaries of the
ancestral Limpopo River and diverted
some into the lower Zambezi River.
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Ka
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Fal
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Ag Fra
Proto-Indian Ocean
141
The mantle plumes that caused the opening of the Indian and Atlantic
oceans also caused southern Africa’s asymmetric drainage pattern.