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Plate tectonics beneath the Mediterranean: evidence from deep faults

The weak to moderate seismicity of the Central Mediterranean area is the consequence of the very slow convergence (a few mm/yr) of African and Eurasian tectonic plates. The Mediterranean Basin underwent a very complex history, and in its central part only the most recent part (less than 30-35 million years) is well understood. The Ionian Sea is located between Libya, Italy and Greece; beneath the northern part of the Ionian Abyssal Plain lies the limit between the African plate and the sediments accumulated during its downward motion under the Eurasian plate. Prior to this period, the tectonic history of this area (< 200 million years) is still unclear.


The Ionian Sea is located the seismologically active regions of Italy, Greece (circle size proportional to earthquake magnitude) and Libya. The white square shows study area (Ionian Abyssal Plain)


The data collected below the Ionian Abyssal Plain during two oceanographic surveys provided new insights on the processes and timing of the geological evolution of Central Mediterranean.

The scientists used "seismic reflection", a technique where very low frequency sound waves are emitted in surface waters by an airgun (sudden release of pressurized air); hydrophones placed at regular intervals along 2.5 km-long floating lines pulled by a ship record the echoes of these waves, reflected by the bottom and the underlying interfaces. After crossing the water layer, the waves enter the sediment where they are partly reflected by the discontinuities between geological strata of different densities; this reflected part is recorded by the hydrophones. After a very complex processing, the analysis of these echoed signals lead to vertical profiles where the limits between sedimentary layers and the faults can be identified and their depth localized. This study is based on 36 such profiles, covering a total length of more than 6000 km, from the water-sediment interface to a depth of about 10 km below the surface.


The principle of seismic reflection
(© Fondation d'entreprise ALCEN pour la connaissance des énergies)

These profiles allowed the identification of sedimentary layers present below the Ionian Abyssal Plain and the study of their deformation under two compressive tectonic processes differing by their origin and their age. The most recent one is posterior to the "Messinian salinity crisis", a period where the Mediterranean dried out during about 600,000 years, more then 6 million years ago; it affected only the post-Messinian sedimentary cover and generated faults oriented SE-NW. The oldest one occurred prior to the Messinian salt deposits and is characterized by a network of SW-NE faults, 10 to 20 km spaced and about 100 km long, which concern the crust and the entire pre-Messinian sedimentary cover.


Part of one of the seismic profiles (top) and its geological interpretation (bottom). Note the orientation of the faults (F) and the vertical offset of crust top (blue) and sedimentary layers.
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The presence of these faults provides precious informations on the history of this part of the Mediterranean Basin.

They are "reverse faults" which were activated by a horizontal compression of the crust, since the part situated above the fault plane moved upwards; their fault plane is steep (60 à 65°) and the resulting vertical offset can reach 1.5 km. The whole set of these parallel SW-NE faults led to a relative shortening of 3.5 to 5 %.

Among the three possible hypotheses to explain their formation, the most consistent with the field observations and the previous sources of information is that of the reactivation of pre-existing faults.

The oceanic nature of Ionian Basin crust, the spacing between faults and the slope of fault planes show that the initially formed during the late Trias-early Jurassic opening of the basin, about 200 million years ago. In this extension context, these faults were "normal", with a downward movement of the part situated above the fault plane. But the type of fault can change, from normal to reverse (or vice-versa) if the nature of the tectonics changes from extensive to compressive (or vice-versa). This is what occurred under the Ionian Abyssal Plain about 7 million years ago, thus after a very long period of fault inactivity. This reactivation of former normal faults as reverse faults has also been observed in areas of diffuse intraplate deformation, like the Central Indian Ocean.

It is likely that this tectonic inversion occurred during a reorganization phase of the Mediterranean, when the Calabrian block rotated and the Tyrrhenian Sea opened; however a current intraplate deformation of an Ionian microplate cannot be excluded.


The paper

Gallais F., Gutscher M.-A., Graindorge D., Chamot-Rooke N., Klaeschen D., 2011. A Miocene tectonic inversion in the Ionian Sea (central Mediterranean): Evidence from multichannel seismic data. Journal of Geophysical Research, 116, B12108, doi:10.1029/2011JB008505.
See the first page


The authors

This study was conducted by three members of the Domaines océaniques laboratory of IUEM, in collaboration with two researchers from Ecole Normale Supérieure (Paris) and Kiel University (Germany).


The journal

Journal of Geophysical Research is the flagship journal of the American Geophysical Union. Over its 115 years of continual publication, it has adapted to meet the needs of multidisciplinary science. It now has seven disciplinary sections, one of which is dedicated to the Earth. JGR-Solid Earth focuses on the physics and chemistry of the solid Earth and the liquid core of the Earth, geomagnetism, paleomagnetism, marine geology/geophysics, chemistry and physics of minerals, rocks, volcanology, seismology, geodesy, gravity, and tectonophysics.



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