Tectonic evolution of the Mediterranean: a damewith four husbands

A. M. C. ŞENGÖR*

İTÜ Avrasya Yerbilimleri Enstitüsü, Ayaza a Istanbul 34469, Turkey.

*e-mail: sengor@itu.edu.tr

Abstract: The tectonics of the Mediterranean Region since the end of the Hercynian orogeny inEurope has been dominated successively by four major developments largely outside it, whose effectsinteracted in the Mediterranean to give it its extremely complicated geology. The first influence wasthat of the consumption of the Paleo-Tethys under Gondwanaland in regions east of the present-dayBulgaria. A second concurrent influence was the right-lateral transpression along the Scythides as farwest as the North Dobrudja: this led to the Triassic extension and partial ocean-floor generation in theCarpathians, the Alps and the Pyrenees. These two influences were still active when the CentralAtlantic began opening, adding a third influence. This rifted new oceans in the Betics, the AtlasMountain complex, the Apennines and the Alps. When Gondwanaland began rifting in Late Jurassic,its extensional tectonics began influencing places as far into the Mediterranean as the northern Africancontinental margin east of Libya and parts of the Arabian Peninsula, adding a fourth influence. TheAptian-Albian opening in the South Atlantic finally halted the extension in these areas, but it imposedon Africa a northerly drift component with respect to Laurasia. This led to the generation of new sub-duction zones and the obduction of giant ophiolite nappes. In the northern African continental mar-gin east of Libya, aborted ophiolite obduction led to the generation of the Syrian Arcs orogen, a fore-land fold/thrust system with no corresponding internides. Major continental collisions began in theEocene, but the final pinching of the Mediterranean basin at both ends was a Miocene affair.

Keywords: Mediterranean, Scythide shear zone, Paleo-Tethys, Central Atlantic Ocean, South AtlanticOcean, Gondwanaland, Syrian Arcs orogen.

The Mediterranean area is tectonically one of the mostcomplicated regions in the world. It includes today theseismically most active region in the world and its pastseems to have been equally stirring. With the advent ofplate tectonics it became clear that the opening of theAtlantic Ocean had been the controlling factor of theoverall kinematic framework of Mediterranean tecton-ics. On the basis of this recognition, Hsü (1971),Smith (1971) and Dewey et al. (1973) provided syn-theses of the Mediterranean tectonic developmentimmediately after the widespread recognition of the

plate structure and the great horizontal mobility of thelithosphere. Common to all these syntheses was a cen-tral kinematic problem: how to maintain extension onboth sides of Argand’s African promontory duringTriassic and Liassic? The usual answer given was toassume subduction somewhere farther east, where thegeology was less known than in any west Europeancountry. Only in 1979 did the discovery of the Paleo-Tethyan suture (Şengör, 1979) put flesh on the bonesof this hypothesis. However, it was later recognised thatthe Paleo-Tethyan subduction was highly oblique and

Trabajos de Geología, Universidad de Oviedo, 29 : 45-50 (2009)

that a serious component of strike-slip characterisedthe tectonic evolution of its northern margin well intothe Mesozoic (Natal’in and engör, 2005). This discov-ery made it clear that not one, but two Paleo-Tethyansubduction zones were necessary: one dipping underLaurasia and the other under Gondwanaland.

The question then became how many such strike-slip convergent motions were absorbed within thePaleo-Tethys and how much was transferred fartherwest into the Mediterranean realm. If any was trans-ferred into the Mediterranean, how did it interferewith the Atlantic opening kinematics? Figure 1

Figure 1. Tentative kinematic model showing the relationships between different plate boundaries.

Figure 2. Tectonic map of theMediterranean region during theLate Triassic showing the locationof the main faults.

A. M. C. ŞENGÖR46

shows a tentative kinematic model to answer thesequestions. It shows the northern margin of thePaleo-Tethys as a pure strike-slip system (akeirogen), although there was also much subduc-

tion. I drew it here as a pure strike-slip to emphasisethat it is the strike-slip component that is of signifi-cance for the Mediterranean tectonics farther west.The diagram also shows that there was much dis-

Figure 3. Tectonic map of theMediterranean region during theEarly Jurassic showing the loca-tion of the main faults.

Figure 4. Tectonic map of theMediterranean region during theKimmeridgian-Tithonian (LateJurassic) showing the location ofthe main faults.

TECTONIC EVOLUTION OF THE MEDITERRANEAN: A DAME WITH FOUR HUSBANDS 47

placement and strain partitioning amongst variousplate boundaries and plate boundary zones.

The basic tenet of this model is to assume that notall of the strike-slip along the northern margin of thePaleo-Tethys (along the Scythides) was consumed bythe subduction zone, but a very insignificant partwas partitioned to impose extension along the futureCarpathians, Alps and the Pyrenees. The presence ofstrike-slip in the Scythides is based entirely on geo-logical data as no part of Paleo-Tethyan ocean flooris now extant. This accounts for the Triassic exten-sional phases of extension in the Mediterraneanmountain ranges that were independent of the laterJurassic extension. Figure 2 shows what theMediterranean area may have looked like during theLate Triassic.

As early as during the Hettangian/Sinemurian, theMediterranean area was feeling the effects of theCentral Atlantic opening. This is the time of themajor rifting in the future Mediterraenan Tethysidechains (Fig. 3) and the time of the opening of themain Pennine Ocean in the Alps. InKimmeridgian/Tithonian times, ocean openingcontinued, except for the Meliata Ocean in theeastern Alps and the Carpathians, that was con-sumed (Fig. 4).

During the Hauterivian, the Mediterranean area wasalready beginning to feel the influence of a third play-er outside its area, namely the initial rifting in theSouth Atlantic. The South Atlantic rifting was a partof a much larger area of the inner-Gondwanaland rift-ing field that extended from the present-day Gulf ofGuinea all the way into southeastern Turkey and theZagros in Iran. It created the major basins within theChad basement and the ones farther to the northwestsuch as the Marmarica taphrogen in Libya and Egypt(Fig. 5).

In the Aptian-Albian, a number of subduction zoneswere already active in the Mediterranean area.However, a period of rifting characterised the periph-ery of the Arabian promontory and the Sirte Basin inLibya.

In the Late Cretaceous, major ophiolite obductioncharacterised the eastern part of the Mediterranean.Until now it was always assumed that the Turkishophiolites were the westernmost representatives ofthis episode of widespread obduction. Recently, how-ever, it was recognised that a double thickness ocean-ic crust characterises the oceanic basement just off-shore of Cyrenaica, whereas onshore Cyrenaica ischaracterised by intense, foreland-type folding andthrusting of Senonian age. This episode of deforma-

Figure 5. Tectonic map of theMediterranean region during theAptian-Albian showing the loca-tion of the main faults.

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tion had long been known, but its cause had remainedenigmatic. It now seems as if the westernmost LateCretaceous obduction event was actually in Libya, butit was aborted before the ophiolite nappe could fullyoverthrust the continental margin. This attemptedobduction put the entire northern margin of Africaunder compression and the effects of it were felt as faraway as the Benue aulacogen in Nigeria, which at thistime experienced shortening. By contrast, the Sirtebasin continued expanding.

A string of granite intrusions cut through the obduct-ed ophiolite nappes in Turkey, close to their rootzones. This has been interpreted as a result of decom-pression melting, but it is possible that a subductionzone flip may also have occurred and created a short-lived continental margin arc system north of theMenderes-Taurus block in Turkey.

Since the Cretaceous, the history of theMediterranean has been one of continuous orogeny(Fig. 6). Major collisions happened mainly in theEocene, but both to the east (in SE Turkey) and to thewest (Gibraltar arc) collisions were delayed as late asMiocene (Fig. 7).

As early as in the Oligocene, major extensional sub-duction zones began creating oceanic basins, aprocess that continues today. It is unclear how

much of the extension in the Aegean is a result oforogenic collapse (Suess’ Rule) and how much of ita consequence of roll-back of subduction hinge. Itis also unclear when the subduction under theHellenic arc may have commenced. It seems clearfrom the available kinematic constraints thatspreading in the eastern Mediterranean must eitherhave been uncommonly slow, or it must have beenaccommodated by corresponding subduction underthe Hellenic/Tauric system or was episodic withlong intervals of repose.

The final act of the Mediterranean orogeny was theclosure of the Gibraltar arc and the collision insoutheastern Turkey. The collisional area in south-eastern Turkey is very peculiar, because the highplateau there is underlain by a continental crustonly 40 km thick! It seems that it consists entirelyof subduction-accretion material and that once thesubducting oceanic slab was lost, the accretionarycomplex was underplated by hot asthenosphericmaterial. This led to rapid uplift and very wide-spread alkalic volcanism.

The tectonic evolution of the Mediterranean wasthus the result of four factors: firstly, the subduc-tive removal of the Paleo-Tethys under Pangeaalong future eastern Mediterranean mountainbelts; secondly, the opening of the Central Atlantic;

Figure 6. Tectonic map of theMediterranean region during theLate Cretaceous showing thelocation of the main faults.

TECTONIC EVOLUTION OF THE MEDITERRANEAN: A DAME WITH FOUR HUSBANDS 49

thirdly, the opening of the South Atlantic and,finally, the opening of the North Atlantic. It looksas if the Mediterranean was a dame with four suc-cesive husbands who dictated her life and in theprocess managed to give birth to possibly the mostbeautiful sea and its surroundings in the world.Little wonder that it also was the cradle of humancivilisation and grew hand in hand with the idea ofhuman freedom.

Acknowledgements

I am grateful to the organisers of the YORSGET-08 Symposium forinviting me for a keynote address in one of the most beautiful andculturally pivotal places in the world. No honour can be higherthan being valued by one’s younger colleagues and hardly any placemore exciting for an Earth scientist than Spain, where the revival ofcuriosity about our beautiful planet began more than half a millen-nium ago that has given us our modern scientific society.

Figure 7. Tectonic map of theMediterranean region during theLate Eocene showing the locationof the main faults.

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