Ethiopian Flood Basalt Province: 2. The Ogaden Dyke Swarm

(1 : Dyke swarms of northwestern Ethiopia, poster)

Daniel MEGEPlanetology and Geodynamics Lab, Nantes, France

Peter PURCELLP & R Geological Consultants, Scarborough, AustraliaPexco (East Africa) N.V. Ethiopia Manager until 2009

with first radiochronology results obtained by

Fred JOURDANWestern Australia Argon Isotope Facility, Curtin University of Technology, Perth, Australia

IDC-6, February 2010

Ethiopian Flood Basalt Province

Mège and Korme, 2004

Ethiopian Flood Basalt Province

Mège and Korme, 2004

Mantle plume theory

Holden and Vogt, 1977

Ethiopian Flood Basalt Province

Mège and Korme, 2004

Low density rocks have been traceddown to the core-mantle boundary.

Gurnis et al., 2000

Mantle plume theory

Holden and Vogt, 1977

The Ogaden region of eastern Ethiopia

S O M A L IP L A T E A U

ETHIOPIA

KENYA

SUDAN

SOMALIA

YEMEN

S O M A L IP L A T E A U

The Ogaden region of eastern Ethiopia

OGADEN

ETHIOPIA

KENYA

SUDAN

SOMALIA

YEMEN

Surface geology of the study area

PALEOCENEJESSOMA Fm

sandstone

EOCENEAURADU Fm

limestone

CENOZOICBASALT

GeoEye, 74 cm/pixel

Spot 5, 2.5 m/pixel

EARTH

MOLA, 37.5 cm/pixel (v)

HiRISE25 cm/pixel

SW NE30 m 25 m

1.4 km0.8 km

MARS

13 m14 m

0.6 km 1 km

SRTM, 5 m/pixel (v)

Mège and Masson, 1996 + IDC-3 (1995)

+1000 km

Such linear troughs have been interpreted on Mars as possible surface consequenceof non-emergent dyke emplacement by several research groups.

flyby and landing sites

Pexco Exploration (East Africa) N.V.Geological reconnaissance surveySeptember 2008

Pexco concession boundary

study area

ETHIOPIA

SOMALIA

site 8 densely vegetated margin

margin

trough

t r o u g h t r e n d

Dyke exposure at the SE end of the

studied trough

t r o u g h t r e n d

Dyke exposure at the SE end of the

studied trough

Aeromagnetic data

Aeromagnetic data

Aeromagnetic data:back to the 70s

Whitestone Ethiopia PetroleumHarar aeromagnetic survey (1976)

The Marda Fault Zone

1976 aeromagnetic survey

2008 aeromagnetic survey

MARDA FAULT ZONE

The Marda Fault Zone

1976 aeromagnetic survey

2008 aeromagnetic survey

MARDA FAULT ZONE

former suggestions of Mardafault zone continuation to the SE

(Wood, 1979; Boccaletti et al., 1991)

magnetic evidence

Gravity data

New Bouguer gravity data (Pexco, February-March 2008)

• The dyke trend may be followed along the Marda Fault Zone from the Afar margin to the Somalia boundary

AFAR

Gravity dataAFAR

1976 mag data coverage

2008 mag data coverage

• The dyke trend can be traced across the magnetic data gaps

no magneticdata coverage

veryweak to nomagneticdatacoverage • The dyke trend may be followed

along the Marda Fault Zone from the Afar margin to the Somalia boundary

Preliminary Ar-ages obtained atWestern Australia Argon Isotope Facility

Dyke ages ~30 Ma

Bosworth et al., 2005

basalt samples from emergent dyke site(similar composition and structure)

Dyke swarm size

minimum dyke extentfrom aeromagnetic data

400 km

31 Ma

dyke swarmsof NW Ethiopia

TRAP SERIES

~ 100 km

minimum dyke extent400 km

suggested by Bouguer anomaly + 200 km

31 Ma

dyke swarmsof NW Ethiopia

TRAP SERIES

~ 100 km

Dyke swarm size

Regional implications

minimum dyke extent400 km

suggested by Bouguer anomaly + 200 km

31 Ma

dyke swarmsof NW Ethiopia

TRAP SERIES+ ?

+ ?

old Esso aeromagneticdata being re-analysed

~ 100 km

Classical model: graben

Mastin and Pollard, 1988

Dyke trough formation

Dyke trough formationObserved topography (SRTM90)

The closed depressions are karstic.

Classical model: graben

Mastin and Pollard, 1988

One of the main troughs follows both a negative magnetic anomaly and closed depressions.

topography + magnetic anomaly

site 33

Karstifiction probably plays a rolein linear trough formation.

2 km

Host rock (Eocene limestone) has been modified on both sides of the troughs!

damage zone at dyke tip

Proposed origin of linear troughs

dissolution in damage zone

compositionally modified host rock

hydrothermal flow along dyke margins(Delaney, 1982)

karstification

red sand infill

dyke emplacement

gradual surface collapse

topographic surface

Eocene limestone(+ gypsum?)

Paleocene sandstone(+ gypsum?)

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.

No reservoir or chamberhas been identified to date.

Where are the magma sources?

residual gravity(isostatic regional field – Bouguer anomaly)

modified after Woldetinsae, 2005

White dots: Trap Series

9.5 m-thick dolerite dyke in western Ethiopia

man forscale

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.

along-strike SRTM topographic profile

topography +magnetic anomaly

topography

site 33

Host rock (Eocene limestone and chert) has been modified around the troughs!

2 km

It displays laterite crust instead ofred sand.

Soil leaching suggests paleotopography higher than average.