Ethiopian Fl ood B asalt P rovince: 2. The Ogaden Dyke Swarm

1. 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

2. Ethiopian Flood Basalt Province Mège and Korme, 2004

3. Holden and Vogt, 1977 Mantle plume theory Ethiopian Flood Basalt Province Mège and Korme, 2004

4. Holden and Vogt, 1977 Mantle plume theory Gurnis et al., 2000 Low density rocks have been traced down to the core-mantle boundary. Ethiopian Flood Basalt Province Mège and Korme, 2004

5. The Ogaden region of eastern Ethiopia YEMEN SUDAN S O M A L I P L A T E A U ETHIOPIA SOMALIA KENYA

6. OGADEN The Ogaden region of eastern Ethiopia YEMEN SUDAN S O M A L I P L A T E A U ETHIOPIA SOMALIA KENYA

7. Surface geology of the study area EOCENE AURADU Fm limestone PALEOCENE JESSOMA Fm sandstone CENOZOIC BASALT

8. MARS HiRISE 25 cm/pixel MOLA, 37.5 cm/pixel (v) 0.6 km 1 km 0.8 km 1.4 km 30 m 25 m 14 m 13 m NE SW EARTH GeoEye, 74 cm/pixel Spot 5, 2.5 m/pixel SRTM, 5 m/pixel (v)

9. 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.

10. flyby and landing sites study area Pexco concession boundary ETHIOPIA SOMALIA Pexco Exploration (East Africa) N.V. Geological reconnaissance survey September 2008

11. densely vegetated margin site 8 margin trough

12. Dyke exposure at the SE end of the studied trough t r o u g h t r e n d

13. Dyke exposure at the SE end of the studied trough t r o u g h t r e n d

14. Aeromagnetic data

15. Aeromagnetic data

16. Aeromagnetic data: back to the 70s Whitestone Ethiopia Petroleum Harar aeromagnetic survey (1976)

17. The Marda Fault Zone MARDA FAULT ZONE 1976 aeromagnetic survey 2008 aeromagnetic survey

18. magnetic evidence former suggestions of Marda fault zone continuation to the SE (Wood, 1979; Boccaletti et al., 1991) The Marda Fault Zone MARDA FAULT ZONE 1976 aeromagnetic survey 2008 aeromagnetic survey

19. 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)

20. very weak to no magnetic data coverage 1976 mag data coverage no magnetic data coverage • The dyke trend can be traced • across the magnetic data gaps 2008 mag data coverage AFAR Gravity data • The dyke trend may be followed • along the Marda Fault Zone from the • Afar margin to the Somalia boundary

21. Bosworth et al., 2005 Dyke ages ~30 Ma Preliminary Ar-ages obtained at Western Australia Argon Isotope Facility basalt samples from emergent dyke site (similar composition and structure)

22. dyke swarms of NW Ethiopia Dyke swarm size ~ 100 km 400 km minimum dyke extent from aeromagnetic data 31 Ma TRAP SERIES

23. dyke swarms of NW Ethiopia + 200 km suggested by Bouguer anomaly Dyke swarm size ~ 100 km 400 km minimum dyke extent 31 Ma TRAP SERIES

24. dyke swarms of NW Ethiopia + ? + 200 km suggested by Bouguer anomaly + ? old Esso aeromagnetic data being re-analysed Regional implications ~ 100 km 400 km minimum dyke extent 31 Ma TRAP SERIES

25. Mastin and Pollard, 1988 Dyke trough formation Classical model: graben

26. Observed topography (SRTM90) Mastin and Pollard, 1988 Dyke trough formation Classical model: graben The closed depressions are karstic.

27. site 33 One of the main troughs follows both a negative magnetic anomaly and closed depressions. Karstifiction probably plays a role in linear trough formation. topography + magnetic anomaly

28. 2 km Host rock (Eocene limestone) has been modified on both sides of the troughs!

29. 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 red sand infill topographic surface Eocene limestone (+ gypsum?) Paleocene sandstone (+ gypsum?)

30. 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.

32. man for scale 9.5 m-thick dolerite dyke in western Ethiopia Where are the magma sources? No reservoir or chamber has been identified to date. modified after Woldetinsae, 2005 residual gravity (isostatic regional field – Bouguer anomaly) White dots: Trap Series

33. Old aeromagnetic data field work area

34. site 33 along-strike SRTM topographic profile topography + magnetic anomaly One of the main troughs follows both a negative magnetic anomaly and closed depressions. The closed depressions are karstic. topography

35. It displays laterite crust instead of red sand. 2 km Soil leaching suggests paleotopography higher than average. Host rock (Eocene limestone and chert) has been modified around the troughs!