The Red Sea Arabia-Nubia tectonic jigsaw

What is it about?

The fitting of the coasts of Arabia and Nubia was one of those great ideas of the plate-tectonic revolution. I remember an Open University programme in the early morning on the BBC from the 1970s showing (I think) Ron Girdler demonstrating how the tectonic fragments of Arabia and Nubia (Africa) have drifted apart from one-another. How much Nubia and Arabia have separated is important for working out the type of crust within the Red Sea. Apparently, the type of crust can affect whether the sediments there host petroleum fluids. If oceanic crust lies there, the sediments would have been too hot to form oils (though this idea may now be more nuanced, as continental rifts can have high heat flow also). The fragmented Arabian-Nubian continental shield comprises many shear zones and ancient sutures. The sutures formed originally when collisions between continental fragments created the shield. To relate the structures on either side accurately so that we can study that early history, we need to know how the Arabia and Nubia have drifted apart in order to reverse the effect of that drift. in Mitchell et al. (2025), we have used observed lineaments in gravity anomalies (Augustin et al., 2021) to work out the direction of Arabia-Nubia movement (Fig. 1). This was possible because the lineaments were formed by melting in Earth's mantle beneath the Red Sea and therefore record the plate movements relative to those melting points. Using them to "play back" the Arabia-Nubia movement to around 10 million years before present, the structures on either side can be almost brought together again, allowing a better comparison. The results support a new association of structures and generally how the structures somehow affected the segmentation of new oceanic crust (Izzeldin and Mitchell, 2024; Delaunay & Jonsson, 2025).

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Why is it important?

The type of crust in the northern Red Sea has been controversial - either it is strongly extended continental crust or it is oceanic. An earlier plate-tectonic reconstruction pole has been constructed using known movements on the Aqaba/Dead Sea transform fault and across the Suez Rift (Joffe & Garfunkel, 1987), so it should be useful for addressing this controversy. The pole also brings together the same structures in the central Red Sea as Izzeldin and I have found. In the northern Red Sea, applying it to Nubia brings the two sides close together but with a significant gap remaining (Fig. 2). This implies that the crust is mostly continental, not oceanic. That the type of crust has been controversial is not surprising. The Red Sea is floored by kilometres-thick evaporitic and other sediments, obscuring the crust beneath. The Arabia-Nubia separation has been "ultra-slow", hence any oceanic spreading would not have been expected to produce recognizable seafloor spreading magnetic anomalies. If we look at work on other continental margins, researchers looking at different datasets often put the ocean-continent boundary in different places (Eagles et al., 2015) so working out crustal type is tricky. In the Red Sea crustal type debate, the arguments made by each side can mostly be explained away by the other side, hence they are not conclusive. However, for me, the two pieces of evidence remaining as tricky to explain are the cross-sea trends in gravity anomalies, which continue into the northern Red Sea (Augustin et al.) and the amount of Arabia-Nubia separation. It seems to me the amount of separation is not enough for oceanic crust to have been produced widely. Hence, other explanations may be needed for the gravity lineaments.

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