Tectonics : the Mediterranean Sea [1]

Tectonique : la Méditerranée [1]

The Earth : The Mediterranean Sea - A jigsaw puzzle [Genista]

By Nicolle Mathé, 'Genista Informations' n° 300, January-February, 2004 (Plate tectonics)

At first sight, the Mediterranean Sea is a sea which is open onto the Atlantic Ocean through the Gibraltas Strait, onto the Black Sea through the lowlands of the Bosphorus and the Dardanels, onto the Red Sea through the Suez Canal.

In fact, this is a double sea !

Two basins

They are separated by the Sicily-Tunisia shoal, from Cape Bon to Marsala.

The West basin, not deeper than 12,130 ft (3700 m), was made by a collapsing of the South border of the European plate with the Algeria-Provence basin avec le bassin algéro-provençal which continues the Rhine-Rhone basin, and small seas.
The Alboran Sea forms a transitional corridor before reaching Gibraltar.

The East basinl, a lot deeper, with a maximum of 16,822 ft (5131 m), is what remains of the ancient Tethys Sea. It continues towards the North-West as the Adriatic Sea depression, and towards the North-East as the collapsed basin of the Egean Sea, which is divided up by troughs surrounding archipelagoes.

A domino of giant plates...

The monumental Eurasian plate has moved steadily towards the South while continents were getting isolated from the Gondwanaland — the single continental block linked with the Antarctic plate — while travelling towards the North.
The ocean floor of the Tethys, a sea which separated the two blocks, disappeared by subduction under the Eurasian plate accompanied with earthquakes and volcanic eruptions, island arcs and, too, at last, the collision of some continents, which generated collision ranges and numerous faults.

The dismantled continents are then clustered together along the great fault which materialises the common limit of the pieces of the jigsaw puzzle.

This fault runs from the Azores, on the medio-Atlantic ridge, beyond Turkey towards the Sultanate of Oman along a winding line through North Africa, Sicily, the Apennines, Piemonte, the East coast of the Adriatic Sea, Greece, Cyprus.

... and microplates !

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general geological map of the Mediterranean Sea [carte géologique générale de la Méditerranée]

The Iberian peninsula has been detached from the West of Eurasia while performing a rotation to the West (anticlockwise) with the opening of the Bay of Biscay, a collision with the Southern point of the European continent and the forming of the collision range of the Pyrenees, sliding along a North-Pyrenean fault.

The Adriatic plate is a spur of the African plate containing the Ragusa table land in Sicily, South and East Italy with the region of Puglie, that of Ancona and the floor of the Adriatic Sea. Because it is pushed by the African plate, it collides with the European plate and forms the collision range of the Alps.

The Corsica-Sardinia plate is a piece of the European continent which has probably got retached during a 30° left around a pole located near Genova, at the time when the Western Mediterranean Sea was formed by the collapsing of the continental floor. The Ligurian and Tyrrhenian Seas were then open in a double system of faults, and Corsida rose through that system of faults.

The Egean plate derives from a hypocollision of large plates.

A ballet of plates !...

Without a choreographer, there comes anarchy !

The problem lies in the fact that the friction in the rock layers blocks the movement of the plates but when the displacement force is bigger than rock resistance, a sudden breaking takes place.

The magnitude of the earthquake depends on the size of the fault which is moving : it is 5 for a 3·1-mile (5 km) fault, 6 for 18·6 miles (30 km), 7 for 62 to 93 miles (100 to 150 km). In France, some of the faults have a length of some 10 to 60 miles (10-20 to about 100 km).

the Iberian peninsula with the Pyrenees [la péninsule ibérique avec les Pyrénées]

the Iberian peninsula with the Pyrenees [la péninsule ibérique avec les Pyrénées]

... The Iberian plate

In the Pyrenees the seismic centres are not deeper than 15·5 miles (20 km) (Arette, August 1967 : 5·7) and it is necessary to know that the Pyrenees, which are in line with the Cantabric range, are themselves in live with a granitic range to the East, under the Lion Gulf, South of Camargue and in Provence (the Toulon massifs, the Maures and the Tanneron).

The folds of Languedoc, of chalk Provence (the Sainte Baume) and the very numerous faults have a link with the movements of the Iberian plate. As it was sliding, it has caused the forming of the Iberian range which links the Cantabric Mounts to the alpine Betic range which has been laid onto the European continent before bending to pass under the Catalan cordillera.

The Iberian peninsula is pushed southwards by Europe, and goes towards a meeting with Africa which is going northwards. This bringing closer of the two continents has a speed of ³/₈ inch (1 cm) every year, and causes the Gibraltar Strait — 8·9 miles wide (14·3 km) — to get inexorably narrower.

The Alboran volcano is located on a strike-slip fault which links Almería to Melilla. The seismic centres are deep there.

the meeting of the European plate and the African plate [rencontre le la plaque européenne et de la plaque africaine]

... The African plate

The African plate moves towards the North and the East, pushed by the activity of the rifts in the Indian and Atlantic oceans, but it is blocked by the activity of the recent Red Sea rift which opposes a force in the opposite direction, and it is moving in a rotating movement around a pole located at Tangiers.

The alpine ranges which are on the North margin of the African continent are pushed onto this continent and they have a set of large faults, normal inverted or strike-slip faults, with a compression along an axis oriented NNW-SSE, and the seismic risksare important (El Asnam in October, 1980 : 7·3 and Boumerdes in May, 2003 : 6·8).

The contact with the Eurasian plate is located between Sousse and Port Said, and the Tunisian Atlas moves northwards towards Cape Bon and the Ragusa table land, to reach the Apennines then on to Libya. The seafloor of the Ionian Sea, which is a margin of the former tethysian ocean, gets under that part of the European plate which is covered by the Tyrrhenian Sea. The Ionian trough is therefore formed, and it is dominated by the vertical escarpment, full of faults and located under water — Malta or Sicily. These movements are visible through volcanic activity with chalk-alcaline volcanoes like the Vesuvius, the chimney of which has been blocked since 1944, Campi Flegrei and Ischia, the Lipari Islands with the Stromboli, active for the past 200 years, and Vulcano, which has had a cyclical activity. The Etna, a basalte volcano, has shown paroxysmic phases since 1966.

seismic activity in Provence, in France [la sismicité en Provence, en France]

...The Adriatic plate

Under the pressure of the African plate, the movement of the Adriatic plate is hindered by the European plate, thus causing the closure of the Tethys Sea, and, because of the collision, the forming of the Alps and the Apennines.
This folding continues, at a speed of ³/₈ inch to ⁶/₈ inch (1 to 2 cm) every year, causing earthquakes (Assisi in September, 1997 : 5·9).

This push has a repercussion in Provence, by moving the system of inverted or strike-slip faults, and creating new faults.

The epicentres of earthquakes are aligned along the Briançon arc, the middle valley of the River Durance and the River Ubaye, the Rhone Valley fromù Valmence to Arles, from Chamonix to Valence, the valley of the Tinée and the River Vesubie.

The depth of seismic centres is over 12·4 miles (20 km) and the magnitudes are smaller than 6.

What lies in the future for the West basin ?

The closing Gibraltar Strait, the African plate moving towards the North, pushed from all parts, the aggressive Adriatic plate and the seafloor, in the Ionian Sea, which is sliding under this basin — will all these elements finally get the better of that part of the Mediterranean Sea ?

Tectonics - In this series

TECTONICS : THE EARTH

(1) Our planet is living : its quakes

(2) Drift away, plates : the drift

(3) It folds or it breaks : an active planet

TECTONICS : THE MEDITERRANEAN SEA

(4) The Mediterranean Sea : a jigsaw puzzle (The present Page)

(5) In the Mediterranean Sea : meditate...

Top of Page • [Haut de Page]

Cette Page en français • [This Page in French]

Part 2 : In the Mediterranean Sea... Meditate • [Deuxième Partie : En Méditerranée... Méditer]

Contents : Nature & Environment • [Sommaire Nature & Environnement]