EGU2020-2620, updated on 12 Jun 2020
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Multiphase rotational extension and marginal flexure along a developing passive margin: the Western Afar Margin, East Africa

Frank Zwaan1, Giacomo Corti2, Derek Keir1,3, Federico Sani1, Ameha Muluneh4, Finnigan Illsley-Kemp5, and Mauro Papini1
Frank Zwaan et al.
  • 1Università degli Studi di Firenze, Dipartimento di Scienze della Terra, Florence, Italy (
  • 2Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, Florence, Italy
  • 3University of Southampton, School of Ocean and Earth Science, Southampton, United Kingdom
  • 4Addis Ababa University, School of Earth Sciences, Addis Ababa, Ethiopia
  • 5Victoria University of Wellington, School of Geography, Environment and Earth Sciences, Wellington, New Zealand

This multidisciplinary study focuses on the tectonics of the Western Afar Margin (WAM), which is situated between the Ethiopian Plateau and Afar Depression in East Africa. The WAM represents a developing passive margin in a highly volcanic setting, thus offering unique opportunities for the study of rifting and (magma-rich) continental break-up, and our results have both regional and global implications.

Earthquake analysis shows that the margin is still deforming under a ca. E-W extension regime (a result also obtained by analysis on fault measurements from recent field campaigns), whereas Afar itself undergoes a more SW-NE extension. Together with GPS data, we see Afar currently opening in a rotational fashion. This opening is however a relatively recent and local phenomenon, due to the rotation of the Danakil microcontinent modifying the regional stress field (since 11 Ma). Regional tectonics is otherwise dominated by the rotation of Arabia since 25 Ma and should cause SW-NE (oblique) extension along the WAM. This oblique motion is indeed recorded in the large-scale en echelon fault patterns along the margin, which were reactivated in the current E-W extension regime. We thus have good evidence of a multiphase rotational history of the WAM and Afar.

Furthermore, analysis of the margin’s structural architecture reveals large-scale flexure towards Afar, likely representing the developing seaward-dipping reflectors that are typical for magma-rich margins. Detailed fault mapping and earthquake analysis show that recent faulting is dominantly antithetic (dipping away from the rift), bounding remarkable marginal grabens, although a large but older synthetic escarpment fault system is present as well. By means of analogue modelling efforts we find that marginal flexure indeed initially develops a large escarpment, whereas the currently active structures only form after significant flexure. Moreover, these models show that marginal grabens do not develop under oblique extension conditions. Instead, the latter model boundary conditions create the large-scale en echelon fault arrangement typical of the WAM. We derive that the recent structures of the margin could have developed only after a shift to local orthogonal extension. These modeling results support the multiphase extension scenario as described above.

Altogether, our findings are highly relevant for our understanding of the structural evolution of (magma-rich) passive margins. Indeed, seismic sections of such margins show very similar structures to those of the WAM. However, the general lack of marginal grabens, which are so obvious along the WAM, can be explained by the fact that most rift systems undergo or have undergone oblique extension, often in multiple phases during which structures from older phases control subsequent deformation.

How to cite: Zwaan, F., Corti, G., Keir, D., Sani, F., Muluneh, A., Illsley-Kemp, F., and Papini, M.: Multiphase rotational extension and marginal flexure along a developing passive margin: the Western Afar Margin, East Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2620,, 2020

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  • CC1: Comment on EGU2020-2620, Neil Mitchell, 06 May 2020

    Hi Frank, thanks for your presentation, which is very interesting.  I have more a general question that your work provokes.  People suggest that the seawater that ultimately formed the Miocene evaporites were all supplied via the Mediterranean.  This is based on faunal provenance studies.  However, as far as I know, those studies were based on fairly limited samples from the central and northern Red Sea.  I wonder if some of the water might have been supplied through the Danikil Depression.  This was an idea put to me by Philip Ball.  Thanks again for posting your presentation and good luck coming out of the virus lockdown!  cheers, Neil.

    • AC1: Reply to CC1, Frank Zwaan, 06 May 2020

      Hi Neil,

      Thanks for your question. This is not something I looked into. As far as I know the fauna in the Danakil Depression is indeed of the Red Sea (Mediterranean?) type. The question would then be if there is any influence from the Gulf of Aden (Indian Ocean?). Yet it would be indeed much easier to bring in water from the north (given the topography) and there are potential seaways with very recent corals (now subaereal, probably due to some local uplift curring them off), supporting this idea. I know that colleagues from Fribourg (Switzerland) are working on this problem at the moment. You could contact Anneleen Foubert and Valentin Rime. They were in Afar this winter to take samples. Perhaps they can give you some more info.



      • CC2: Reply to AC1, Neil Mitchell, 06 May 2020

        Hi Frank, thanks for replying and for the suggestion.  cheers, Neil.