As continental rift
zones evolve to seafloor spreading they do so through progressive episodes of
lithospheric stretching, heating and magmatism, yet the actual process of
continental break-up is poorly understood. The East African rift system in
north-eastern Ethiopia is central
to our understanding of this process as it lies at the transition between
continental and oceanic rifting (Ebinger
and Casey, 2001). We are exploring the kinematics and dynamics of
continental breakup through the Ethiopia Afar Geoscientific Lithospheric
Experiment (EAGLE), which aims to probe the crust and upper mantle structure
between the Main Ethiopian (continental) and Afar (ocean spreading) rifts, a
region providing an ideal laboratory to examine the process of breakup as it is
occurring.
EAGLE is a
multi-disciplinary study centered around the most advanced seismic project yet
undertaken in Africa (Figure
1). Our study follows the Kenya
Rift International Seismic Project (e.g., KRISP Working Group, 1995), and
capitalizes on the IRIS/PASSCAL broadband seismic array (Nyblade and Langston, 2002), providing a
telescoping view of the East African rift within the suspected plume province.
The three-dimensional
structure of oceanic rifts is primarily controlled by the supply of magma (e.g.
Phipps-Morgan and Chen, 1993),
whereas that of youthful continental rifts is controlled by the spatial
arrangement of large displacement border faults (e.g. Hayward and Ebinger, 1996). Thus, magmatic processes increase in
importance as rifting proceeds to seafloor spreading, but there is no consensus
as to when or how this transition occurs.
The volume of melt produced and its seismic velocity structure provide
critical constraints on mantle dynamics as continental breakup proceeds to
seafloor spreading, but there remain fundamental questions regarding the 3-D
distribution of strain and melt as continents rift apart.
Our approach in EAGLE
is to examine the nature of crust and upper mantle along a highly extended,
magmatically active continental rift prior to the modifying effects of post-rift
sedimentation, erosion of the uplifted rift flanks, and thermal decay. In the
Ethiopian rift we can: 1) trace the evolution from broadly distributed to
focused strain during rift development; and 2) study the active processes of
continental break-up associated with a mantle plume (or other upper-mantle
convective upwelling), while avoiding interactions between subducted slabs and
asthenospheric flow; the region has been tectonically stable since 600
Ma.
There is general
agreement that the broad uplifted Ethiopia-Yemen plateau and Oligocene flood
basalt province has been affected by one or more Cenozoic plumes (e.g. Nyblade and Langston, 2002). A synthesis of
40Ar/39Ar data shows that flood basalts were erupted
across an ~1000 km diameter region at ~31 Ma, presumably coincident with plume
head contact with Afro–Arabian lithosphere (e.g. Hofmann et al., 1997). Previous geophysical studies show
crustal thinning northward into the Afar depression. Refraction profiles in Afar, interpreted
as near 1D structures due to the very small number of shots and receivers, suggest thinned 25km-thick crust
underlain by a 10 km thick layer with anomalously low upper mantle P-wave
velocities above apparently normal mantle (Berkhemer et al., 1972).
Young (<10 Ma)
continental rifts, as in southern Kenya, commonly comprise asymmetric rift
basins bounded by steep border faults that accommodate most of the strain across
the rift. The older, more evolved
Asal rift in Djibouti illustrates a much narrower zone of magmatic accretion and
faulting immediately after the onset of seafloor spreading. Project EAGLE
focuses on the transitional rift sector at the northern end of the Main
Ethiopian Rift where strain and magmatism have migrated from the border faults
to a narrow zone within the rift valley (Ebinger and Casey, 2001). The rift valley shows two stages of
along-axis segmentation marking distinct differences in strain
partitioning. During the early
stages, strain localized along large-offset border faults bounding broad
basins. During the last 2 Ma,
strain and magmatism have localized to narrow zones of aligned volcanoes in the
central rift valley, termed magmatic segments. The en echelon arrangement of the
‘new’ magmatic segments shows little correlation with the older border-fault
segmentation (Figure
2). Geodetic data show that
~80% of the strain across the rift is accommodated across the magmatic segment,
although teleseismic activity attests to some deformation outside this zone (Bilham et al., 1999). To first order,
geochemical data from the magmatic segments indicate that upwelling plume
material sampled in central Ethiopia incorporates depleted mantle during ascent
beneath the more highly extended portions of the rift (Furman et al.,
2003).
Project EAGLE aims to
define the anatomy of a rift immediately prior to break-up. We carried out a 3-phase seismic
experiment (Figure
1), followed by a
magnetotelluric project and a variety of supporting efforts that collectively
provide a diversity of data for integrated analysis:
Phase I consisted of deployment of 30 SEIS-UK (Maguire et al., 2002) broadband seismometers for a period of 16 months over a 250x250 km2 area of the rift valley and its uplifted flanks. The primary aim is to help define the shape of the Moho and to improve images of the uppermost mantle across the underlying mantle plume province. Receiver functions and body-wave analysis will provide images of deep earth structure while S-wave polarization analysis will be used to assess the nature of seismic anisotropy and its interpretation in terms of mantle dynamics, tectonic fabric and stress regime.
Phase II consisted of the deployment of a further 50 SEIS-UK 3-component broadband instruments for a period of 4 months over a 200x100 km2 area encompassing 4 magmatic segments. The array will be used to locate local earthquakes, to analyze fault plane solutions, and to conduct receiver function analyses. The aims of Phase II are to estimate strain partitioning and lithospheric strength variations in 3-D, and to identify the distribution of magma reservoirs beneath the narrow magmatic segments.
Phase III consisted of the deployment of a further 1100 seismic instruments during a controlled source seismic project involving 20 seismic charges being fired into one 450km cross-rift profile (Profile 1), one 450km axial profile (Profile 2), and a dense 2-D array of instruments in a 150km diameter circle around the profiles’ intersection (Profile 3), all centered on the magmatically active Nazret region (Figure 2). The crust and upper mantle velocity models to be derived will be used to provide estimates of total crustal thinning across the rift, to assess the role of basement in the location of major faults and magmatic segments, and to determine whether significant underplating takes place during the syn-rift stage. The high-density array around the intersection of the two profiles was designed to image any magma bodies beneath the Boset magmatic segment.
Phase IV featured the recording of a magnetotelluric profile along the central ~200km of profile I. The electrical conductivity structure of the crust and immediate upper mantle should provide additional constraints on the nature and distribution of crustal heterogeneity, and image melt accumulation zones. New gravity and geodetic information have also been acquired.
Phase I, II and III
were completed in January / February 2003.
Phase IV continued until March 2003.
Phase
I: Preliminary results from SKS splitting
analyses show delay times varying from ~1.6 seconds on the Ethiopian plateau
(Nubia plate) in the west, to ~2 seconds towards the eastern flank (Somalia
plate). Within the rift valley
there is a consistent south-to-north increase in delay times, which increase
from 1.0 second in the south to 2.1 seconds in the north (Figure
2). The increased delays
towards the Somalia plate may arise from stretched / fractured lithosphere, or
alternatively represent a different tectonic domain. The increased splitting times northwards
towards Afar correlates with an increase in strain which is accommodated both by
faults and dikes as well as with lower crustal residence times for erupted lavas
(Furman et al., 2003). Outside the
rift, the polarizations of the fast shear wave lie on a NE-SW rift parallel
trend. Within the rift the
orientations swing to more northerly azimuths parallel to the volcanic centers
and perpendicular to the geodetically determined opening direction.
Initial tomographic
inversions of relative teleseismic residuals show low velocities underlying the
Ethiopian Rift down to depths of at least 300 km. The anomaly appears to be tabular in
shape beneath the continental part of the rift in the SW region of our
deployment. Towards Afar the
anomaly is less focused and more triangular in shape. In one region the low
velocity anomaly extends perpendicular to the rift axis and correlates with
off-rift volcanoes south of Addis Ababa (I. Bastow, pers
comm).
Phase
II: Data from the 50-element array is at
present being organized into event-network files at SEIS-UK. Station spacing was kept at 15-20
km based on the distribution of schools, state farms, game parks, and secure
buildings within the region. Initial analyses of the local event data from Phase
I and Phase II suggest a concentration of seismicity along the western margin of
Afar (Figure
2). Relocations of explosive
shots show position accuracies of ~250 m (D. Keir, pers comm).
Phase
III:
Preparations for the
controlled source effort began in April 2001 and 91 broadband instruments were
first deployed at a nominal 5km interval along the 450km cross rift profile in
November – December 2002. The controlled experiment took place in January of
2003 and involved 18 sources in boreholes (typically 1 mt dynamite at 50 m
depth) and 2 sources in lakes (Shalla and Arenguade) and the deployment of ~1000
short-period recorders. An environmental impact assessment required by the
Ethiopian government incorporating a full report on the lake flora, fauna, and
chemistry both before and after the shots were detonated, should provide data of
immense value for future seismic studies elsewhere in the
world.
Examples of the record
sections from two of the borehole shots are shown in Figure 3 (a,
b,
c). These sections show good energy
propagation along the entire length of the profiles, but with an increase in
ambient noise at the southern end of Profile 2. As well as wide coverage of Pg (the
diving wave traveling in the uppermost crust), PmP (the reflection from the
Moho) and Pn (a diving wave in the uppermost mantle) at least two reflectors
from intracrustal interfaces can be observed prior to the PmP arrival on a
number of the sections. The PmP
arrival is observed with a critical offset of ~180km in the south and ~150km to
the north suggesting a crustal thinning from c. 35 to c. 30 km northwards (Figure
3a). Crustal thinning is also
observed beneath the rift along Profile 1 based on the early arrival of PmP from
shotpoint 12 on the rift flanks (Figure
3b) and an increase in this critical offset from SP25 east of the rift (Figure
3c).
Analysis of the bulk
of the EAGLE data is just beginning. Our results should have broad implications
for our understanding of processes at work during the breakup of continents, but
will be of value not just to the international earth science community. Ethiopia is a country beset by
humanitarian difficulties, but also demonstrably capable of supporting a major
international scientific effort.
EAGLE served as a catalyst to unite academic, government, and private
geoscientists who are now combining efforts in other collaborative
projects. The results of EAGLE have
longer-term implications for hydrocarbon and geothermal energy assessment and
hazard reduction within Ethiopia, providing a framework for prioritization of
exploration targets, and provide much valued information for the country’s long
term development: on the distribution of Mesozoic sediments beneath the Tertiary
flood basalts on the Ethiopian plateau, of interest to the Petroleum Operations
Department of the Ethiopian Ministry of Mines; on the distribution of magma
intrusions within the crust, of relevance to the Geothermal Division of the
Ethiopian Geological Survey; on the local seismicity, of importance for both
seismic and volcanic hazard studies in the Ethiopian Rift; and even, despite our
limited information, from the water depths identified in the seismic shot holes,
being of relevance to the country’s hydrogeological
development.
Acknowledgements
We gratefully
acknowledge the contribution of the Commissioner of the Ethiopian Science and
Technology Commission, Mulugeta Amha, together with I. Bastow, D. Keir, D.
Cornwell, K. Keranen, K. Tadesse, E. Gashaw-Beza, and S. Harder and many others
to the EAGLE project. Participants
in the EAGLE project came from:
Ethiopia: Addis Ababa
University, Ethiopian Geological Survey, Ethiopian Petroleum Operations
Department, Ethiopian Commission for Science and Technology, Oromia
Council.
USA: Stanford
University, University of Texas at El Paso, Penn State University, US Geological
Survey, University of Colorado
Europe: University of
Leicester, University of London, Royal Holloway, University of Leeds, SEIS-UK,
University of Edinburgh, University of Copenhagen, Dublin Institute for Advanced
Studies, University of Vienna, University of Stuttgart.
Funding for the EAGLE
project was provided by the NERC, the National Science Foundation Continental
Dynamics program, the Texas Higher Education Coordinating Board, the Royal
Society and the University of Leicester.
Instrumentation was provided by SEIS-UK, IRIS-PASSCAL, the NERC
Geophysical Equipment Pool, and the University of
Copenhagen.
Authors
P.K.H. Maguire, C.J. Ebinger, G.W. Stuart, G.D. Mackenzie, K.A. Whaler, J-M. Kendall, M.A. Khan, C.M.R. Fowler (UK)
S.L. Klemperer, G.R. Keller, T. Furman, and K. Mickus (USA)
Laike Asfaw, Atalay Ayele, Bekele Abebe (Ethiopia)
For additional information contact P.K.H. Maguire, Dept of Geology, University of Leicester, Leicester LE1 7RH, UK. Email: pkm@le.ac.uk.
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Figure Captions
Figure 1: Topographic map of the central Ethiopian Rift showing the locations of the EAGLE Phase I – III instruments. MT soundings (Phase IV) were taken along the central 200 km of Profile 1. Also indicated are the locations of the Penn State array (Nyblade & Langston, 2002).
Figure
2: Tectonic setting of the
EAGLE study area (from Wolfenden,
2003) showing border faults, magmatic segments and seismicity in the zone of
intersection between the Northern Main Ethiopian and Southern Red Sea Rift
basins. Crosses are epicentres from
EAGLE Phase I instruments, and stars are combined Phase I and Phase II
instruments (Figure 1). Also indicated are SKS shear-wave splitting results for
an event on 12/02/2001, origin time 13:01:54, Mb 6.1. The orientation of the arrows shows the
fast splitting direction; their length illustrates the amount of splitting
delay. The absolute plate motion is
shown by the arrow marked APM.
Figure 3: Example seismic record sections from (a) the central SP25 into Profile 2 (b) SP12 on the northwestern rift flank into Profile 1 and (c) SP25 into the eastern end of Profile 1. Sections have been reduced at 6 kms-1 and bandpass filtered. Pg - crustal diving wave; PmP - Moho reflected phase; Pn - mantle diving wave; PiP - inter crustal phases.
