New
generation UK equipment in the Ethiopian Rift is shedding new
light on the break-up of continents
“The East African Rift System” says
Peter Maguire, Professor of Geophysics at Leicester
University, speaking at the BA today, “is the result of
embryonic continental separation”. This is a
continent tearing itself apart, probably above one or more
mantle plumes rising from dfeep in the Earth and hiting the
base of the lithosphere.
The lithosphere under Africa is old
and complex – comprising Archaean cratons and Proterozoic
orogenic belts, and affected by extension in the Late
Palaeozoic (Karroo basins) and in the Cretaceous to Late
Palaeogene (the Central African Rift System). The whole gamut
of lithospheric response to separation can be seen there -
from the initial stages (when mechanical processes largely
control along-axis rift segmentation) to the final stages –
just prior to the growth of a mid-ocean ridge system like that
of the Atlantic Ocean. All in all, a fascinating
laboratory for the tectonicist and geophysicist to learn about
how these processes initiate and develop.
EAGLE, the Ethiopia Afar Geoscientific Lithospheric
Experiment, is a current international geophysical and
geological project to study the transition between continental
and oceanic rifting in the northern part of the East African
Rift System. EAGLE’s aims are to image the crust and upper
mantle just prior to break-up of the magmatic continental rift
in northern Ethiopia in the southwest corner of
Afar.
What got EAGLE started was the UK’s
purchase of a new generation of low power, high capacity,
light weight seismic recorders, managed by the SEIS-UK
(Seismic Equipment Infra-Structure in the UK) consortium at
Leicester University. The main part of the project comprises a
15 month deployment phase (from October 2001) of a large
aperture 2-D seismic array to study deep mantle processes
beneath the Ethiopian Rift, followed by a four-month
deployment of a denser 2-D seismic array from October
2002. Then, a high-resolution controlled source
experiment will be undertaken in January 2003,
firing two 400km refraction profiles across and
along the rift axis. This will enable the
researchers to image the crustal (and immediate upper mantle)
structure beneath the chosen transitional rift
segment.
The East African Rift System
(stretching 3500km from the Zambesi in the south to its
junction with the Red Sea and Gulf of Aden spreading centres
beneath Afar in the north) has Eastern and Western branches.
These bifurcate around the elevated Nyanza Archaean craton.
The Eastern branch is a volcanic system comprising the
Ethiopian Rift over the elevated Ethiopian Plateau, and the
Kenya Rift on the eastern slopes of the East African Plateau.
They are separated by the Turkana depression in northern
Kenya.
The evidence suggests that the Eastern branch was
initiated partly in the early Paleogene, but mostly in the
early Miocene. The Western branch initiated later during the
late Miocene and is associated with much less volcanism than
the Eastern branch. Both are seismically and volcanically
active today. In both segments volcanism has generally
preceded or been contemporaneous with major rift faulting,
strongly suggesting extension is occurring above one or more
mantle plumes.
Building an integrated
model for the origin and evolution of this Rift System
requires information on crustal and upper mantle structure if
geologists are to understand the various ways in which the
lithosphere seems to respond to extension. The principal work
has been undertaken in Kenya (the Kenya Rift International
Seismic Project (KRISP)) with a lesser amount in Ethiopia and
even less in the Western Rift.
“KRISP was the catalyst for a huge
increase in research activity in Kenya through the 1990s” says
Maguire. “The principal result of that research has
been the demonstration of extreme along-axis crustal thinning
beneath northern Kenya, with most of this thinning occurring
in the lowest crustal layer, itself interpreted as underplated
or intruded material. Cross-rift profiles demonstrate that
crustal thinning and anomalously low seismic P-wave velocity
(7.5km.s-1) sub-Moho material underlie the characteristic
surface asymmetric half-grabens.” To Maguire, this
suggests upper crust simple shear (controlled by major
bounding faults within the brittle part of the crust) above a
lower crust / upper mantle pure shear mechanism of extension
(controlled by ductile stretching).
The seismic results, supported by geochemical
studies, suggest that melting begins to occur at depths of
~50km, providing up to 3-5% partial melt in the mantle under
the rift itself, and lending support to the notion that the
rift is underlain by a mantle plume, possibly rising from
beneath the Archaean Nyanza craton.
Opinion has always varied over the
number and location of mantle plumes hitting the lithosphere
beneath the East African Rift System. Maguire
believes that high resolution studies are presently being
undertaken over the Ethiopian and northern Kenya Rifts should
throw new light on the debate.
The real question is – how does a
rift become an ocean? How does rift become
ridge? Says Maguire: “The Kenya Rift provides an
example of the very early stages in the break-up of a
continental rift, in which, at least in the south, the first
order structure is controlled by faulting. However, the first
order structure of oceanic rifts is controlled by the supply
of magma. Thus asthenospheric processes controlling magma
supply must come to dominate over lithospheric processes as
rifting proceeds to seafloor spreading. Current models of
continental break-up and the initation of oceanic rift
segmentation predict profound differences in 3-D geometry of
the crust and upper mantle during break-up, but existing data
are inadequate to distinguish between these models. There is
clear need to study this transitional stage in an active
rift.
“The northern Ethiopian Rift is a
region experiencing slow extension (4 mm/yr) in which the rift
basins are asymmetric and bounded by steep border faults
showing more than 3km throw. New ~60km long magmatic segments
within the rift are arranged en echelon and show little
correlation with the older border fault pattern. Geodetic data
show that ~80% of the strain across the rift is accommodated
over a < 30km wide zone of magmatic construction, although
seismicity attests to some deformation outside this zone. It
is a region apparently demonstrating the exact transition from
continental to oceanic rift processes.
“Impressive work undertaken in the 1970s provided
near 1-D models of the northern Ethiopian Rift, the plateau
margin and Afar, demonstrating major modification of the crust
associated with rifting. These results, together with MT data,
and anomalously low S-wave velocities from surface wave
studies all suggest the presence of high temperatures and
partial melt within the upper mantle and pervasive magmatic
modification of the crust beneath Afar and the northern
Ethiopian Rift. However, to resolve current continental
break-up models, increased resolution seismic images of the
crust and upper mantle across a transitional rift segment are
required.
Which brings us to EAGLE and
prospects for the future. “EAGLE is to study the
transition from continental to oceanic rifting beneath the
Northern Ethiopian Rift and the as yet poorly understood
processes associated with lithospheric break-up; for example,
underplating and lower crustal intrusion, basement control on
the rifting process, the formation of thick igneous crust
beneath resultant volcanic margins, mantle layering. The
project aims to image 3D variations in crustal thickness and
upper mantle structure to characterise the distribution of
strain and magmatism across a typical transitional rift sector
and to map upper mantle anisotropy and thus the flow of mantle
material beneath the rift, thereby providing a snapshot of the
lithosphere immediately prior to separation.”
- The work is being undertaken by
the Universities of Leicester, Leeds and Royal Holloway,
London, the University of Addis Ababa, and Stanford
University and the University of Texas, El Paso in the
United States.
Previous
story
