Supplementary material 1 (see File 2). Geological map of the Marda Fault Zone and surroundings. The map has been prepared from satellite imagery (Landsat ETM+, and sub-metre resolution satellite images) and reinterpretation of earlier maps (Tasfai 1975; BEICIP; 1985; Tefera et al. 1996; Earth Satellite Corporation 2004; Yihune and Haro 2010). The reported fold hinge lines and dip angles have been redrawn from Tasfai (1975), who used stereoscopic analysis of aerial photographs for geological interpretation. The NW-SE fracture lines are parallel to the Precambrian fabric. The latter is frequently subparallel to the maximum topographic slope, resulting in unclear tectonic control of Ogaden rivers flowing to the SE. The topographic contours (separation 200 m) have been extracted from METI/NASA/ASTER Global DEM.

Supplementary material 2a. Argon dating analytical protocol.
The plagioclase crystals were separated using a Frantz magnetic separator, and then carefully hand-picked under a binocular microscope. The selected groundmass and plagioclase were further leached in diluted HF for one minute and then thoroughly rinsed with distilled water in an ultrasonic cleaner.
The samples were irradiated in 2009 (irradiation I10t2h) and again in 2011 (irradiation I12t2h). The samples were loaded into individual large wells of 1.9 cm diameter and 0.3 cm depth aluminum discs. These wells were bracketed by small wells that included Fish Canyon sanidine (FCs) used as a neutron fluence monitor for which an age of 28.305 ± 0.036 Ma (1σ) was adopted (Renne et al. 2010). For both irradiations, the discs were Cd-shielded (to minimize undesirable nuclear interference reactions) and irradiated for 2 hours in the Hamilton McMaster University nuclear reactor (Canada) in position 5C. The mean J-values were computed from standard grains within the small pits and determined as the average and standard deviation of J-values of the small wells for each irradiation disc. Mass discrimination was monitored using an automated air pipette and calibrated relative to an air ratio of 298.56 ± 0.31 (Lee et al. 2006). The J-value and mass discrimination value are given for each sample in the raw analytical data (Appendix 2b). The correction factors for interfering isotopes were (39Ar/37Ar)Ca = 7.30x10-4 (± 11%), (36Ar/37Ar)Ca = 2.82x10-4 (± 1%) and (40Ar/39Ar)K = 6.76x10-4 (± 32%). 
The 40Ar/39Ar analyses were performed at the Western Australian Argon Isotope Facility at Curtin University. The sample was step-heated in a double vacuum high-frequency Pond Engineering© furnace. The gas was purified in a stainless steel extraction line using two AP10 and one GP50 SAES getters and a liquid nitrogen condensation trap. Ar isotopes were measured in static mode using a MAP 215-50 mass spectrometer (resolution of ~400; sensitivity of 4x10-14 mol/V) with a Balzers SEV 217 electron multiplier mostly using 9 to 10 cycles of peak-hopping.
Data acquisition used an Argus program written by M. O. McWilliams and ran under a LabView environment. The raw data were processed using the ArArCALC software (Koppers 2002) and the ages have been calculated using the decay constants recommended by Renne et al. (2010). Furnace blanks were monitored every 3 samples and typical 40Ar blanks range from 1 x 10-16 to 2 x 10-16 mol. Ar isotopic data corrected for blank, mass discrimination and radioactive decay are given in Appendix 2b. Individual errors in Appendix 2b are given at the 1σ level. Our criteria for the determination of a plateau require that they must include at least 70% of 39Ar, and are distributed over a minimum of 3 consecutive steps agreeing at 95% confidence level and satisfying a probability of fit (P) of at least 0.05. Plateau ages (Table 2 and Fig. 7) are given at the 2σ level and are calculated using the mean of all the plateau steps, each weighted by the inverse variance of their individual analytical error. Mini-plateaus are defined similarly except that they include between 50% and 70% of 39Ar and are given much less significance compared to a plateau. Inverse isochrons include the maximum number of steps with a probability of fit ≥ 0.05. All sources of uncertainties are included in the calculation.

Supplementary material 2b (see File 3). Ar isotopic data corrected from baseline, background, mass discrimination interference corrections and radioactive decay. Values in red indicate negative values. Negative values are expected when measured values are around background level. Step ages calculated after Renne et al. (2010).

Supplementary material 3a (see File 4). Petrographic description of representative samples of the Ogaden Dyke Swarm.

Supplementary material 3b. Analytical protocol used for geochemistry.
Each sample was carefully prepared to extract only the freshest parts of the rocks prior powdering. Major and trace elements were analysed respectively by ICP-OES (Thermo I-CAP) and ICP-MS (Bruker 820MS) at the Laboratoire de Planétologie et Géodynamique in Nantes University (France). Major elements were analysed on 125 mg aliquot of samples following the procedure described by Cotten et al. (1995), and trace element were analysed following the Bézos et al. (2009) method. The results are reported in Table 3. Appendix 4 provides results for geo-referenced materials BCR-2 recalculated with calibration curves obtained with standards AGV62, BE-N, BHVO-2, W-2, JB-2, BIR-1, and BCR-2 for both major and trace elements. Also reported in this table are results for sample MQR8 that was analyzed during each analytical session in order to monitor data processing reproducibility.

Supplementary material 4 (see File 5). Major and trace analyses of georeferenced materials BCR-2 for the Ogaden Dyke Swarm samples.

References
BEICIP 1985. Geological map of the Ogaden and surrounding area. Addis Ababa, Ethiopia, 1:1,000,000. Ministry of Mines and Energy, Addis Ababa, Ethiopia.
Bézos, A., Escrig, S., Langmuir, C. H., Michael, P. J. & Asimow, P. D. 2009. Origins of chemical diversity of back-arc basin basalts: A segment-scale study Eastern Lau Spreading Center. Journal of Geophysical Research, 114, B06212, doi:10.1029/2008JB005924.
Cotten, J., Le Dez, A., Bau, M. et al. 1995. Origin of anomalous rare-earth element and yttrium enrichments in subaerially exposed basalts: Evidence from French Polynesia. Chemical Geology, 119, 115–138.
Earth Satellite Corporation, 2004. Photogeologic and hydrocarbon microseepage mapping, Ogaden Basin, Ethiopia. Petronas Caligari SDN BHD, Mapsheet 2.
Koppers, A. A. P. 2002. ArArCALC-software for 40Ar/39Ar age calculations. Computers & Geosciences, 28, 605–619.
Lee, J.-Y., Marti, K., Severinghaus, J. P., Kawamura, K., Yoo, H.-S., Lee, J. B. & Kim, J. S. 2006. A redetermination of the isotopic abundance of atmospheric Ar. Geochimica et Cosmochimica Acta, 70, 4507–4512.
Renne, P. R. Mundil, R. Balco, G., Min, K. & Ludwig, K. R. 2010. Joint determination of 40K decay constants and 40Ar*/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology. Geochimica et Cosmochimica Acta, 74, 5349–5367. 
Tasfai, 1975. Photogeologic interpretation, OC-3 & EL-8, scale 1:420,000. Whitestone Ethiopia Petroleum Co., 1 map.
Tefera, M., Chernet, T. & Haro, W. 1996. Geological map of Ethiopia, 1:2,000,000, second edition. Addis Ababa, Ethiopia, Geological Survey of Ethiopia.
Yihune, T. & Haro, W. 2010. Geological map of Harar area, NC 38-9. Ethiopian Mapping Agency, 1:250,000 Regional Geologic Map Series.