Transcript East Africa`s Great Rift Valley: A Complex Rift System

East Africa's Great Rift
Valley: A Complex Rift
System
Structural Geology
Steffanie Mahusay
Rosfel Bernal
Ivan Fritz Esguerra
The East African Rift System
• Is one of the geologic wonders of the world.
• The Nubian Plate makes up most of Africa, while the smaller
plate that is pulling away has been named the Somalian Plate.
• These two plates are moving away form each other and also
away from the Arabian plate to the north.
• The point where these three plates meet in the Afar region of
Ethiopia forms what is called a triple-junction.
The East African Rift System
What is the East African Rift System?
• The oldest and best defined rift occurs in the Afar region of
Ethiopia and this rift is usually referred to as the Ethiopian Rift.
• Further to the South a series of rifts occur which include a
Western branch, the "Lake Albert Rift" or "Albertine Rift" which
contains the East African Great Lakes, and an Eastern branch
that roughly bisects Kenya north-to-south on a line slightly
west of Nairobi.
• These two branches together have been termed the East
African Rift (EAR), while parts of the Eastern branch have
been variously termed the Kenya Rift or the Gregory Rift.
• The two EAR branches are often grouped with the Ethiopian
Rift to form the East Africa Rift System (EARS).
• The complete rift system therefore extends 1000's of
kilometers in Africa alone and several 1000 more if we include
the Red Sea and Gulf of Aden as extensions.
How did these rifts form?
• One popular model for the EARS assumes that elevated heat
flow from the mantle is causing a pair of thermal "bulges" in
central Kenya and the Afar region of north-central Ethiopia.
• Bulges are initiated by mantle plumes under the continent
heating the overlying crust and causing it to expand and
fracture.
• The stretching process associated with rift formation is often preceded by
huge volcanic eruptions which flow over large areas and are usually
preserved/exposed on the flanks of the rift.
The East African Rift
• The rifting of East Africa is complicated by the fact that two
branches have developed, one to the west which hosts the
African Great Lakes and another nearly parallel rift about 600
km to the east which nearly bisects Kenya north-to-south.
• The East African Rift is referred to as “continental rifting” – a
rifting process that occurs at continental setting.
• The Eastern and Western branches were developed by the
same processes but they have very different characters.
• These two branches are generally following old sutures
between ancient continental masses that collided billion
years ago to form the African craton.
• The split around the Lake Victoria region occurred due to
the presence of a small core of ancient metamorphic rock,
the Tanzania craton, that was too hard for the rift to tear
through.
• It instead diverged around it leading to the two branches
that can be seen today.
• The Eastern Branch is characterized by greater volcanic
activity while the Western Branch is characterized by much
deeper basins that contain large lakes and lots of sediment.
• Recently, basalt eruptions and active crevice formation have
been observed in the Ethiopian Rift which permits us to
directly observe the initial formation of ocean basins on land.
• Most rifts in other parts of the world have progressed to the
point that they are now either under water or have been filled
in with sediments and are thus hard to study directly.
Triple Junction in the Afar region of Ethiopia. Image shows areas of stretched and oceanic crust as
well as areas of exposed flood basalts that preceded rifting. Areas unshaded or covered by flood
basalts represent normal continental crust.
Afar East African Rift System
This image shows several fault scarps that are progressively farther away. Edges of several horst
blocks and a graben that contains Lake Baringo.
The gorge was carved
by water, and an
igneous dike cutting
through the wall of the
canyon.
Rift Valley
Jordan Rift Valley
Jordan Rift Valley
• Is an elongated depression located in modernday Israel and Jordan.
• Includes the Jordan River, Jordan Valley, Hula
Valley, Lake Tiberias, and the Dead Sea.
Jordan rift Valley
Origins and Physical Features
• The Jordan Rift Valley was formed many million
years ago in the Miocene epoch (23.8 - 5.3 Myr ago)
when the Arabian tectonic plate moved northward
and then eastward away from Africa.
• The lowest point in the Jordan Rift Valley is at the
shores of the Dead Sea, which is also the lowest
point (on land) on the surface of the earth at 400
meters below sea level.
A 2003 satellite image
of the region showing
the Jordan Rift Valley.
• Rising sharply to almost 1,000 meters in the
west, and similarly in the east, the rift is a
significant topographic feature over which few
narrow paved roads and difficult mountain tracks
lead.
Northern section of the Great
Rift Valley. The Sinai Peninsula
is in the center and the Dead
Sea and Jordan River valley
above.
Jordan river is a river in
Southwest Asia flowing
to the Dead Sea
Panorama of Jordan Valley
The Dead Sea Transform
• The plate boundary which extends through the valley
is variously called the Dead Sea Transform or Dead
Sea Rift. The boundary separates the Arabian
plate from the African plate, connecting the
divergent plate boundary in the Red Sea (the Red
Sea Rift) to the East Anatolian fault in Turkey.
• The interpretation of the tectonic regime that led to
the development of the Dead Sea Transform is highly
contested.
Anatolian Plate
• Some consider it as a transform fault that accommodates a
105 km northwards displacement of the Arabian plate.
• Others presume that the Rift is an incipient oceanic spreading
center, the northern extension of the Red Sea Rift, and the
displacement along it is oblique, with approximately 10–15 km
of extension in addition to the more substantial left lateral
(sinistral) strike-slip.
• The evolution of the rift, according to this latter model, started
in the late Miocene with the linear series of basins that
propagated gradually along their axes to form the present rift
valley.
Dead Sea