New revelations in Strait of Gibraltar tectonics

Utrecht University researchers have gained a remarkable new understanding of what causes earthquakes in the area around the Strait of Gibraltar. What they have discovered can be applied to many other areas of the world where tectonic plates are disappearing into the earth's mantle. The results of the research are published in Nature Geoscience on 19 February.

The scientists from Utrecht have been conducting geophysical and geological research in the Gibraltar region for several years. They are delighted that they have now finally gained a better understanding of this complex tectonic system. “The geological history of this area is quite enigmatic and has been the subject of intense scientific debate for decades”, says Wim Spakman, Professor of Geophysics at Utrecht University and lead author of the publication. “Thanks to what we have discovered, the pieces of the tectonic jigsaw puzzle are finally falling into place.”

Only partially broken

One of the important discoveries is that the plate boundary between Africa and Europe is only partially broken. “In this region, the African plate is actually still largely connected to the European plate. It is not really possible to identify a plate boundary”, says Spakman. “This explains why the many earthquakes are spread across a wide area rather than being concentrated in several important fault zones. Examples include the huge, mysterious earthquake of 1755 that happened in the Atlantic ocean to the west of Gibraltar, killing tens of thousands of people and devastating the whole of Lisbon and the Algarve. That event is the largest documented earthquake in the European-African area. It will not be the last occurrence of such events.”

CAT scan of the earth's mantle

The researchers used a seismological CAT scan to map the earth's mantle under the area. It appears that below the Strait of Gibraltar,  a tectonic plate dives steeply to the east. This tectonic ‘slab’ reaches a depth of almost 700 km below the sea area between Africa and Spain, east of Gibraltar.

Computer simulation of subduction

The plate subduction process appears to be far more complex than previously thought. Computer simulations show that at the same time as the slab is diving away to the east, the entire slab is simultaneously being pushed to the north by the northward movement of the African plate.

“The slab is being dragged along by the African plate during millions of years of subduction”, explains Spakman. “This ‘slab dragging’ causes friction with the surrounding mantle. Compare it to the friction your hand (‘the slab’) experiences when you move your arm (‘the plate’) through water.”

Consequences for the earth's surface

“The deep friction between the slab and the mantle that is due to slab dragging explains how the North-Moroccan seaways between the Atlantic Ocean and the Mediterranean Sea were closed around six million years ago”, says Douwe van Hinsbergen, geologist at Utrecht University and co-author. “This caused the Mediterranean Sea area to dry out, creating thick layers of rock salt and gypsum. This phenomenon is known as the Messinian Salinity Crisis, an event that attracts a lot of scientific interest internationally and of which we can now finally explain the cause.”

The friction between the slab and the surrounding mantle also leads to regional fault systems in the earth's crust, the cause of which had previously been unknown. Van Hinsbergen: “These faults are very seismically active. Now that we have a much better understanding of the deep cause of the movement and cracks in the crust across the whole area, we can start to develop research focusing on where earthquakes will happen.”

Rest of the world

The researchers expect that the newly-discovered slab-dragging process caused by tectonic plate movements also happens in many other regions. “This will result in many useful new insights for reconstructing geological history”, says Van Hinsbergen. Spakman adds: “It is actually surprising that this important process of slab dragging has been almost completely overlooked since the discovery of plate tectonics around 50 years ago.”

Publication

W. Spakman, M. V. Chertova, A. P. van den Berg, and D J. J. van Hinsbergen, Puzzling features of western Mediterranean tectonics explained by slab dragging, Nature Geoscience