THE FINAL ROUND: ALPINE BELTS AND THE MEDITERRANEAN

The final round began effectively 40 Ma aga after North America had finally parted from Europe with the growth of the Atlantic Ocean between Greenland and Norway. With that influence gone, Europe from then on has been dominated by the stress regime created from its interaction with Africa. At around this time, the convergence between Africa and Europe set off a new Eocene-Miocene phase of Alpine collision and major mountain uplift, nappe emplacement and development of the Molasse foreland basin through crustal flexure, recently modelled by Sinclair et ai. (1991). At around this time, the eastward translation of Africa relative to Europe ceased and began to reverse so that dextral strike-slip beg an to develop, resulting in a concentration of compression and crustal shortening in the Western Alps and the onset of transtension and subsidence of the Pannonian Basin to the east (Figure 2-6d). Dextral transpressive movement within the Pyrenees slid Spain west relative to France and rotated it slightly clockwise in the final stage ofthe orogeny. Transtensional stresses in western Europe permitted the development of the Rhone-Rhine-Eger rift system from Late Eocene to Oligocene time.

Within the Mediterranean region, the collage of micropiates that had evolved through the Jurassic and Cretaceous had created a complexly looped chain of arcuate structures. Where the western arc of the Alps meets the Mediterranean, it switches polarity and continues, facing eastward, in the Apennine along the Italian peninsula, and, in an east-facing loop, across the straits ofMessina and Tunis. In Maghrebide North Africa, the south-facing orogen continues with the Tell and the Sahara Atlas. This chain contains no more ophiolites, and the red desert sandstones in its African foothills show that it was formed in the shoaling west end of the Tethyan embayment. Finally, in the High Atlas, it reaches stable Africa where, for lack of sediment, its sole thrusts are within, not above the basement. From the Rif of Morocco

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Figure 2-7. Sketch map reconstructing the position ofthe Adriatic microplate in the Late Messinian (5.2 Ma), the anticlockwise rotation ofCorsica and Sardinia and the opening and closing of oceanic basins, after Patacca and Scandone (1990).

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across the Straits of Gibraltar and into the Betic Cordillera of Andalusia, the orogenie chain loops once more around the Alboran sea, again with ophiolites and Alpine-Iooking units of sedimentary carbonate. On the fringe ofthis loop, the Mediterranean orogen has dumped the chaotic terrane of the pre-Rifian and pre-Betic thrust sheets out into the Atlantic. Along the Iberian south coast, the north-facing Betic Cordillera can be followed into the Balearic islands, and may possibly loop through Corsica back into the Alps.

It is not surprising that such a complex of tectonic units should act as a system which is internally more or less independent of its surroundings. Caught between Africa and Europe, Eocene-Oligocene N-S convergence resulted in the closure ofthe Ligurian-Alboran Sea and movement between the SW margin of Iberia and the Corsica-Sardinia block. When in the Miocene this convergence became directed NW -SE and dextral movement increased, crustal shortening became concentrated on the Appenines and the western and Southern Alps. The Corsica-Sardinia block rotated anticlockwise, crustal shortening occurred in the Betic Cordillera of SE Spain and a rapid opening of small oceanic basins ensued, including the West Mediterranean and Tyrrhennian basins. This is seen in Figure 2-7 in relation to the situation for the Adriatic micropIate in the Late Messinian. Movement of Africa and Arabia relative to Europe continues to the present day, exemplified by dextral movement along the Anatolian

A CONTINENT REVEALED

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fault system in Turkey, rifting ofthe Red Sea and of grabens transeeting the Calabrian are to the south and east of Sieily. Indeed, as diseussed in later ehapters, strike-slip faulting may weB be eurrently aetive in the Sardinia Channel.

Extensional basins in the stage of initial subsidenee are a key element of Mediterranean tectonics. They oeeur nested within the orogenie loops as oblong or eireular sites of backare spreading. In the Alpine-Zagros ehain, we count the Pannonian, Aegean, and Menderes extensional sites. In the Apennine-Betic ehain, there are the Proveneal-Balearie and the Tyrrhenian sites. Extensional basins in the stage of thermal subsidenee oeeur on the pre-Alpine eratons of Afriea and Europe, essentiall y outside of the Mediterranean provinee. They also surround the early Mesozoie oeeanie erust of the East-Mediterranean basin. The Proveneal-Balearie site of baekare spreading eontains an undisturbed and deep layer of Messinian salt whieh signals that the site had reaehed extensional stability by late Mioeene time.

The interplay of plate eonvergenee, indentation, baekare spreading, and tightening of orogenie loops is kinematically possible only by involving the asthenosphere. It mushrooms above subdueted slabs of lithosphere, and, at shaBow depth, the forees of its eontrasting rheology and density are eapable of powering the movements of mountains. We examine these implieations further in Chapter 7. This teetonie style mayaiso have been alive when some of the older provinees of Europe eonsolidated, but here in the Mediterranean, as we show in Chapters 4 and 5, we ean measure these movements and estimate the stress fields.

With this insight, perhaps the Mediterranean may give us the key to understanding lithosophere dynamies.