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IDC-6, February 2010
Ethiopian Flood Basalt Province: 2. The Ogaden Dyke Swarm
(1 : Dyke swarms of northwestern Ethiopia, poster)
Daniel MEGE
Planetology and Geodynamics Lab, Nantes, France
Peter PURCELL
P & R Geological Consultants, Scarborough, Australia
Pexco (East Africa) N.V. Ethiopia Manager until 2009
with first radiochronology results obtained by
Fred JOURDAN
Western Australia Argon Isotope Facility, Curtin University of Technology, Perth, Australia
Ethiopian Flood Basalt Province
Mège and Korme, 2004
Ethiopian Flood Basalt Province
Holden and Vogt, 1977
Mantle plume theory
Mège and Korme, 2004
Ethiopian Flood Basalt Province
Holden and Vogt, 1977
Mantle plume theory
Gurnis et al., 2000
Mège and Korme, 2004
Low density rocks have been traced
down to the core-mantle boundary.
The Ogaden region of eastern Ethiopia
YEMEN
SUDAN
ETHIOPIA
KENYA
SOMALIA
The Ogaden region of eastern Ethiopia
YEMEN
SUDAN
ETHIOPIA
OGADEN
KENYA
SOMALIA
Surface geology of the study area
EOCENE
AURADU Fm
limestone
PALEOCENE
JESSOMA Fm
sandstone
CENOZOIC
BASALT
EARTH
MARS
HiRISE
25 cm/pixel
GeoEye, 74 cm/pixel
Spot 5, 2.5 m/pixel
SRTM, 5 m/pixel (v)
0.6 km
14 m
MOLA, 37.5 cm/pixel (v)
1 km
13 m
1.4 km
30 m
SW
0.8 km
25 m
NE
1000 km
+
Mège and Masson, 1996 + IDC-3 (1995)
Such linear troughs have been interpreted on Mars as possible surface consequence
of non-emergent dyke emplacement by several research groups.
flyby and landing sites
study area
Pexco concession boundary
ETHIOPIA
SOMALIA
Pexco Exploration (East Africa) N.V.
Geological reconnaissance survey
September 2008
site 8
densely vegetated margin
margin
trough
Dyke exposure
at the SE end of the
studied trough
Dyke exposure
at the SE end of the
studied trough
Aeromagnetic data
Aeromagnetic data
Aeromagnetic data:
back to the 70s
Whitestone Ethiopia Petroleum
Harar aeromagnetic survey (1976)
The Marda Fault Zone
MARDA FAULT ZONE
1976 aeromagnetic survey
2008 aeromagnetic survey
The Marda Fault Zone
MARDA FAULT ZONE
former suggestions of Marda
fault zone continuation to the SE
(Wood, 1979; Boccaletti et al., 1991)
1976 aeromagnetic survey
magnetic evidence
2008 aeromagnetic survey
AFAR
Gravity data
• The dyke trend may be followed
along the Marda Fault Zone from the
Afar margin to the Somalia boundary
New Bouguer gravity data
(Pexco, February-March 2008)
AFAR
Gravity data
very
weak to no
magnetic
data
coverage
1976 mag data coverage
no magnetic
data coverage
• The dyke trend may be followed
along the Marda Fault Zone from the
Afar margin to the Somalia boundary
• The dyke trend can be traced
across the magnetic data gaps
2008 mag data coverage
Dyke ages
~30 Ma
Preliminary Ar-ages obtained at
Western Australia Argon Isotope Facility
basalt samples from emergent dyke site
(similar composition and structure)
Bosworth et al., 2005
Dyke swarm size
dyke swarms
of NW Ethiopia
~ 100 km
400 km
31 Ma
TRAP SERIES
minimum dyke extent
from aeromagnetic data
Dyke swarm size
dyke swarms
of NW Ethiopia
~ 100 km
+ 200 km
suggested by Bouguer anomaly
31 Ma
TRAP SERIES
minimum dyke extent
400 km
Regional implications
dyke swarms
of NW Ethiopia
~ 100 km
+?
+ 200 km
suggested by Bouguer anomaly
31 Ma
400 km
minimum dyke extent
+?
TRAP SERIES
old Esso aeromagnetic
data being re-analysed
Dyke trough formation
Classical model: graben
Mastin and Pollard, 1988
Dyke trough formation
Observed topography (SRTM90)
Classical model: graben
Mastin and Pollard, 1988
The closed depressions are karstic.
One of the main troughs follows both a negative magnetic anomaly and closed depressions.
site 33
Karstifiction probably plays a role
in linear trough formation.
topography + magnetic anomaly
Host rock (Eocene limestone) has been modified on both sides of the troughs!
2 km
Proposed origin of linear troughs
dyke emplacement
karstification
damage zone at dyke tip
dissolution in damage zone
hydrothermal flow along dyke margins
(Delaney, 1982)
gradual surface collapse
compositionally modified host rock
topographic surface
Eocene limestone
(+ gypsum?)
Paleocene sandstone
(+ gypsum?)
red sand infill
Conclusions
• The Ogaden dyke swarm (an IDC-6 scoop) is by far the longest swarm
feeding the Ethiopian LIP identified to date.
• Identified length is currently 400 km, it is probably 600 km or more.
• The swarm is consequently one of the major tectonic and magmatic
elements of the African Horn evolution during the Cenozoic. Its
orientation may have been guided by a stress field imposed by the
geometry of the Zagros subduction at 30 Ma.
• Its surface expression sheds light on mechanisms of linear trough
formation above dykes in planetary crusts.
• Acquisition of magnetic data, partnership with other petroleum
companies, and re-analysis of old data are planned in order to
determine the total length of the swarm.
Where are the magma sources?
No reservoir or chamber
has been identified to date.
man for
scale
modified after Woldetinsae, 2005
residual gravity
(isostatic regional field – Bouguer anomaly)
White dots: Trap Series
9.5 m-thick dolerite dyke in western Ethiopia
Old aeromagnetic data
field work area
One of the main troughs follows both a negative magnetic anomaly and closed depressions.
The closed depressions are karstic.
site 33
along-strike SRTM topographic profile
topography
topography +
magnetic anomaly
Host rock (Eocene limestone and chert) has been modified around the troughs!
It displays laterite crust instead of
red sand.
2 km
Soil leaching suggests
paleotopography higher than average.