The protracted development of the continent–ocean transition in Afar

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Nature Geoscience
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Continental breakup and the transition to seafloor spreading is characterized by extensional faulting, thinning of the lithosphere and, at magmatic margins, voluminous intrusive and extrusive magmatism1, 2, 3, 4. It is difficult to discriminate between different mechanisms of extension and magmatism at ancient continental margins because the continent–ocean transition is buried beneath thick layers of volcanic and sedimentary rocks5, 6 and the tectonic activity that characterized breakup has ceased. Instead, the timing of these mechanisms is inferred from theoretical models or from the geological record preserved at the fully developed, ancient rifted margins1, 5, 7, 8. Ongoing rifting in Ethiopia offers a unique opportunity to address these problems because it exposes subaerially the transition between continental rifting towards the south and seafloor spreading further northward. Here we synthesize constraints on the spatial and temporal evolution of magmatism and extension in Ethiopia. We show that although intrusion of magma maintains crustal thickness during the early stages of the continent–ocean transition, subsidence of the margin below sea level, and eruption of voluminous basalt flows, is initiated by late-stage thinning of the heavily intruded, weakened plate just before the onset of seafloor spreading. We thus conclude that faulting, stretching and magma intrusion are each important, but at different times during breakup.

At a glance


  1. Topography and bathymetry in the Horn of Africa.
    Figure 1: Topography and bathymetry in the Horn of Africa.

    Black lines (X–X′) are the locations of seismic wide-angle experiments used to constrain crustal thicknesses shown in Fig. 3. The white triangles are volcanoes. Areas shaded red are Quaternary–Recent basaltic volcanism. The arrow shows the motion of the Arabian plate relative to Africa. Dashed red lines are seafloor spreading centres in the Red Sea and Gulf of Aden.

  2. Rift structure and volcanism in Ethiopia.
    Figure 2: Rift structure and volcanism in Ethiopia.

    Variations in Quaternary–Recent basaltic volcanism (see for example, refs 21, 22 and Fig. 1) exposed every 0.1° latitude along the profile (a), elevation (b) and crustal thickness12, 13 (c), with latitude in Ethiopia along the profile X–X′ (Fig. 1). Velocities on (c) are in kms−1. Note the abrupt thinning of the crust in northern Afar (>13°N), which coincides with subsidence of the margin below sea level and a marked pulse in Quaternary–Recent volcanism.


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  1. Department of Earth Sciences, University of Bristol, Bristol, BS8 4RJ, UK

    • Ian D. Bastow
  2. School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK

    • Derek Keir
  3. Present address: National Oceanography Centre, University of Southampton, Southampton, SO14 3ZH, UK

    • Derek Keir


I.D.B. and D.K. both developed the scientific concepts presented herein, and jointly wrote the paper. I.D.B. created the figures.

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The authors declare no competing financial interests.

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