For the first time, a group of European researchers led by the University of Lisbon, with the participation of the University of Trieste, has identified a ‘sinking’ piece of plate, in the lower part of the lithosphere (the outermost layer of the Earth) in an area of the Atlantic Ocean.

This geological phenomenon is called ‘lithosphere delamination’ and is known to cause ‘seismic events. However, the phenomenon had previously only been documented on continents.

The discovery, published in Nature Geoscience, therefore opens up new perspectives on seismic risk prediction in Europe.

Using sophisticated seismic tomography techniques, which employ seismic waves from earthquakes to understand the structure of the Earth at great depths, researchers have detected a high-velocity anomaly up to 250 km deep beneath the Horseshoe Abyssal Plain, in the south-western region of the Iberian Peninsula, between the African and Eurasian plates, an area known for its intense seismic activity, the scene of historic earthquakes such as the Lisbon earthquake of 1755 (M8.5 – 8.7) and the San Vincenzo earthquake of 1969 (M7.9).

In this region, despite the lack of obvious surface signs like twisted landscapes or underwater mountains, a portion of the oceanic lithosphere is sinking into the Earth's mantle, creating new faults.

This phenomenon, enhanced by the presence of a thick serpentinised layer (which acts as a weak layer), could explain the origin of historical earthquakes of great magnitude.

Chiara Civiero, geophysicist and researcher at the Department of Mathematics, Computer Science and Geosciences at the University of Trieste, co-author of the study and head of the tomographic analyses, emphasises: 'This discovery opens up new avenues for understanding the evolution of the very early stages of oceanic subduction with important implications for plate tectonics. If even areas without obvious surface faults, such as the Horseshoe Abyssal Plain, can be subject to strong earthquakes, there is a need to revise seismic hazard models to include deep processes and structures that cannot be mapped using traditional methods.’

Alongside the subduction of the oceanic lithosphere, a process whereby one plate can slide under another if compression occurs along the contact margin, the delamination of the continental lithosphere is the main mechanism through which the Earth's surface is recycled into the mantle. Tectonic plates, i.e. the rigid lithospheric plates and the most rigid outer part of planet Earth, are in fact constantly moving in slow motion, with some edges separating and others colliding. 

Numerical models developed by researchers suggest that delamination could evolve into a fully fledged subduction zone, contributing to a tectonic reorganisation of the Atlantic. This process could lead to the formation of a new subduction system that would connect to the Gibraltar Arc, with long-term implications for the closure of the Atlantic Ocean.

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Study published in Nature Geoscience

Seismic evidence for oceanic plate delamination offshore Southwest Iberia

João C. Duarte*1,2, Nicolas Riel3, Chiara Civiero4, Sónia Silva1, Filipe M. Rosas1, Wouter P. Schellart5, Jaime Almeida6,7, Pedro Terrinha8, António Ribeiro1

1. IDL - Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016, Lisboa, Portugal
2. UNIARQ, Centro de Arqueologia da Universidade de Lisboa, 1600-214, Lisboa, Portugal
3. Institute of Geosciences, Johannes Gutenberg University, Mainz, J.-J.-Becher-Weg 21, D-55128 Mainz, Germany
4. Department of Mathematics, Computer Science and Geosciences, University of Trieste, Trieste, Italy
5. Department of Earth Sciences, Vrije Universiteit Amsterdam, Amsterdam, Netherlands
6. SEGAL, Departamento de Informática (UBI), Rua Marquês d’Ávila e Bolama, Covilhã, 6201-0012, Portugal
7. IDL - Instituto Dom Luiz, Universidade da Beira Interior, Covilhã, 6201-0012, Portugal
8. Instituto Português do Mar e da Atmosfera (IPMA), Lisboa, Portugal