1. 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 IDC-6, February 2010

2. Ethiopian Flood Basalt Province Mège and Korme, 2004

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

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

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

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

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

8. GeoEye, 74 cm/pixel Spot 5, 2.5 m/pixel EARTH MOLA, 37.5 cm/pixel (v) HiRISE 25 cm/pixel SW NE 30 m 25 m 1.4 km 0.8 km MARS 13 m14 m 0.6 km1 km SRTM, 5 m/pixel (v)

9. Mège and Masson, 1996 + IDC-3 (1995) + 1000 km 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 Pexco Exploration (East Africa) N.V. Geological reconnaissance survey September 2008 Pexco concession boundary study area ETHIOPIA SOMALIA

11. site 8 densely vegetated margin margin trough

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

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

14. Aeromagnetic data

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

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

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

19. 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

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

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

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

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

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

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

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

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

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

29. 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?)

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. site 13 3 observation sites along one of the linear troughs

33. site 7 trough margin

34. Aeromagnetic data: back to the 70s MARDA VOLCANIC LINE North Whitestone Ethiopia Petroleum, Harar aeromagnetic survey (1976) Second vertical derivative map

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

36. Regional implications minimum dyke extent from aeromagnetic data 400 km suggested by Bouguer anomaly + 200 km 31 Ma Red Sea 24 Ma 19 Ma Owen FZ Sheba Ridge dyke swarms of NW Ethiopia TRAP SERIES + ? old Esso aeromagnetic data being re-analysed ~ 100 km

37. Regional implications minimum dyke extent from aeromagnetic data 400 km suggested by Bouguer anomaly + 200 km 31 Ma Red Sea 24 Ma Erta 'Ale segment 5 Ma 19 Ma Owen FZ Sheba Ridge dyke swarms of NW Ethiopia TRAP SERIES + ? old Esso aeromagnetic data being re-analysed ~ 100 km

38. Regional implications minimum dyke extent from aeromagnetic data 400 km suggested by Bouguer anomaly + 200 km 31 Ma Red Sea 24 Ma Erta 'Ale segment 5 Ma 19 Ma Owen FZ Sheba Ridge 3 Ma dyke swarms of NW Ethiopia TRAP SERIES + ? old Esso aeromagnetic data being re-analysed ~ 100 km

39. Hydrothermal flow following dyke emplacement Delaney, 1982! short-term heat transfer thermal pressurization flow along dyke margin long-term heat transfer inverted pressure gradient buoyancy flow along dyke margin

40. No reservoir or chamber has 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 for scale

41. Old aeromagnetic data field work area

42. 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

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