3.3.1. Rationale

The Aegean region and Anatolia are underlain by orogenically destabilized continental lithosphere (Papanikolaou et al., 2004; Stephenson et al., 2004). Along the Hellenic and Cyprus arc-trench system oceanic lithosphere of the Eastern Mediterranean, representing the last remnant of the Neotethys, is presently subducted northward beneath the Aegean and Anatolian regions. By contrast, East-Anatolia is located in the collision zone between the Arabian craton and the Taurides (Okay and Tüysüz, 1999).

The present-day Hellenic arc-trench system was activated during the latest Miocene (Papanikolaou et al., 2004) after the consumption of the external Hellenic block that formed part of the Apulia plate. The Erathostenes seamount collided during the Messinian with the Cyprus arc-trench system that was activated during the early Miocene (Robertson, 2000; Stephenson et al., 2004). Arabia collided during the Senonian with the intra-oceanic Semail arc-trench system. The remnant Semail Ocean was closed during the Eocene-Oligocene, resulting in collision of Arabia with the amalgamated Taurides elements (Yilmaz, 1993; Gilmour and Mäkel, 1996; Robertson, 2000; Stampfli and Borel, 2004; Agard et al., 2005). In the eastern Taurides crustal shortening persisted during the Miocene (Yilmaz, 1993; Gilmour and Mäkel, 1996; Ziegler et al., 2002) whilst in the eastern Pontides (Transcaucasus) and Caucasus major crustal shortening ended prior to the late Miocene (Yilmaz et al., 2000b; Nikishin et al., 2001). Possibly owing slab break-off, orogenic over-thickening of the lithosphere in the domain of the eastern Pontides and Taurides and continued convergence of the Arabian craton with Eurasia, the dextral North Anatolian Fault Zone (NAFZ) was activated during middle to late Miocene times (Burchfiel et al., 2000; Yilmaz et al., 2000b; Nikishin et al., 2001) whereas the sinistral East Anatolian Fault Zone (EAFZ) was activated during the latest Miocene (Robertson, 2000). This facilitated westward escape of the rigid Anatolian block. During the late Miocene the sinistral Levant (Dead Sea) transform fault system was activated (Mart et al., 2005). With this, the Arabian indenter became decoupled from the African plate of which the continental Sinai-Levant and the oceanic East-Mediterranean domains form part.

Geodetic data indicate that Arabia currently moves northward at rates of 20-24 mm/y whereas the African plate converges with Europe in a counter clock-wise mode at rates increasing from 3.3 mm/y in the West to 10 mm/y near Arabia (Le Pichon et al., 1995; McClusky et al., 2000). On the other hand, the entire Anatolian-Aegean region moves in a counter clockwise rotational mode W- and SSW-ward at rates of 20 mm/y in central Anatolia, increasing to 30 mm/y near the Hellenic trench (Figs. 6, 48) (Jiménez-Munt et al., 2003).

At present the Aegean-West-Anatolian region is subjected to SSW-directed extension whilst the Hellenic arc is affected by arc-parallel extension (Hatzfeld, 1999) and frontal accretion persists along the Hellenic-Cyprus arc-trench system. Seismic tomography images a continuous, deep-reaching subduction slab that extends from the Ionian Islands via Crete to Rhodes, and dips beneath the North-Aegean region, penetrating the 410 km and 660 km discontinuities (Fig. 11). A separate subduction slab appears to be associated with the western part of the Cyprus arc while there is clear evidence for a detached slab beneath eastern Anatolia (Faccenna et al., 2005). On the other hand, the subduction slab of the Dinarides was apparently detached from the lithosphere, although at depth it may still be connected with the Hellenic slab (Wortel and Spakman, 2000). The Dinaridic subduction slab was apparently detached from the lithosphere at the Eocene-Oligocene transition, as evidenced by a widespread high-K calc-alkaline and shoshonitic magmatism (Pamic et al., 2002). The question arises whether the weight of the Dinaridic and/or the Anatolian slab causes propagation of slab-detachment and contributes to the progressive sinking of the Hellenic slab into the deep mantle (Wortel and Spakman, 2000; Faccenna et al., 2005), thus facilitating the rapid SSW advance of the Hellenic arc-trench system and the associated extension in its Aegean and West-Anatolian back-arc domain.

In the northern parts of the Aegean region, back-arc extension commenced during the Late Eocene, and in time progressed southward while stacking of nappes derived from the External Hellenic platform continued along the convergent front (Jolivet and Patriat, 1999; Papanikolaou et al., 2004). Tensional subsidence of the Thrace Basin commenced during the Late Eocene (Turgut et al., 1991), whereas the North Aegean and Marmara troughs, which are closely associated with the NAFZ, began to subside during the late Burdigalian (17 Ma; Lybéris, 1984; Georgakopoulos et al., 1995; Görür et al., 2000). Subsequently, back-arc extensional tectonics propagated southward into the central and southern Aegean domains, reaching the latter during the Tortonian (10 Ma; Papanikolaou et al., 2004).

Westward movement of the Anatolian block probably commenced around 17 Ma and was followed by a 40° clockwise rotation of the West-Aegean domain between 15-13 to 8 Ma (Van Hinsbergen et al., 2005b) that was accompanied by increased subduction rates along the South Aegean arc (Jolivet and Patriat, 1999). By latest Miocene-earliest Pliocene times (± 5 Ma), the present-day Hellenic arc-trench system was activated (Papanikolaou et al., 2004), implying that subduction of the oceanic East-Mediterranean Neotethyan lithosphere commenced. At the same time the essentially thin-skinned External Hellenic nappe stack, forming part of the Hellenic orogenic wedge, became inactive. With this, the Aegean – West-Anatolian back-arc domain was subjected to a new extensional phase that overprinted all earlier formed grabens (Papanikolaou et al., 2004), and that was accompanied by a further 10° clockwise rotation of the West-Aegean domain during the last 4 My (Van Hinsbergen et al., 2005b).

At the Miocene-Pliocene transition subduction prograded from the Pindos-Cylades zone to the southern margin of the External Hellenic block, presumably owing to subduction resistance of the latter (Papanikolaou et al., 2004), In the process of this, compressional stresses were exerted onto the African passive margin, causing inversion of the Jabal al Akhdar Basin in northern Libya (El-Havat and Shelmani, 1993; Ziegler et al., 1998; Papanikolaou et al., 2004). The configuration of the tomographically imaged deep-reaching Hellenic subduction slab (Wortel and Spakman, 2000; Faccenna et al., 2003) suggests that subduction progradation did not involve detachment of earlier formed slabs but that these form now the middle and lower parts of the present-day deep-reaching slab (Papanikolaou et al., 2004; Van Hinsbergen et al., 2005b).

West-Anatolia was affected by E-W extension during Early–Middle Miocene times whilst the thrust front of West-Taurides Lycian nappes continued to advance southward. Following their emplacement on the Antalya Basin, N-S extension affected West-Anatolia during the Late Miocene. After a remission at the Miocene-Pliocene transition, N-S extension resumed and persisted to the Present. The observed N-S extension is presumably closely related to westward escape of Anatolia that may have commenced already during the Middle Miocene and accelerated during Late Miocene-Pliocene times (Yilmaz et al., 2000b).

Roll back and steepening of the Hellenic subduction slab (Spakman and Wortel, 2004) has been proposed as the controlling mechanism for extension of the Aegean-West-Anatolian region (Le Pichon and Angelier, 1979; McKenzie and Yilmaz, 1991). However, as the Hellenic slab is anchored in the deep mantle beneath the North Aegean domain (Wortel and Spakman, 2000), such a mechanism cannot be implied. Conversely, numerical modelling of geodetic and seismologic data suggests that slab-pull forces exerted on the African lower plate and trench-suction forces exerted onto the Aegean upper plate by the gravitationally sinking Hellenic slab, combined with lateral escape of Anatolia in response to the impact of the Arabian indenter, are the primary driving mechanisms controlling the Pliocene-Quaternary evolution of the Aegean-West-Anatolian region (Heidbach and Drewes, 2003). Africa-Europe convergence and possible slab tearing are thought to play secondary roles (Jiménez-Munt et al., 2003).

In the Central Mediterranean, southeast- and eastward roll-back of the subducting slab underlies the opening of back-arc basins such as the Ligurian-Provencal, the Algerian and the Tyrrhenian basin (Faccenna et al., 2001a; Spakman and Wortel, 2004). At present, traces of this large subduction system are seismologically recognised only beneath the Calabrian arc. In Calabria, the Wadati-Benioff plane and tomographic images allow to define a narrow subducting slab (less than 300 km wide) that dips at high-angle toward the northwest and presently undergoes in-plane compression (Selvaggi and Chiarabba, 1995). While there is general agreement on the key role played by subduction processes in the evolution of the Mediterranean domain (Malinverno and Ryan, 1986; Patacca et al., 1990; Doglioni et al., 2001; Faccenna et al., 2004), the recent tectonic activity of the slab as well as the causes for its present-day narrow shape are uncertain. The Calabrian subduction zone has retreated during the last 10 My by about 300-400 km (Malinverno and Ryan, 1986; Patacca et al., 1990). In the course of this rapid retreat, the subducting slab was progressively deformed and its width reduced to its present configuration. Such a mechanism is characteristic for orogenic arcs in the Alpine-Mediterranean domain, such as the Gibraltar and the Carpathian arcs (Royden, 1993; Lonergan and White, 1997; Wortel and Spakman, 2000; Faccenna et al., 2004; Faccenna et al., 2005). High-resolution tomographic analyses have recently stimulated studies on deformation processes affecting the central Mediterranean subduction slab (Carminati et al., 1998; Wortel and Spakman, 2000; Gvirtzman and Nur, 2001; Faccenna et al., 2004; Faccenna et al., 2005). Geochemical data on volcanic rocks and tomographic images suggest that progressive narrowing of the active subduction front is related to the opening of a slab-window at deep levels, as evident in the southern Apennines and in the Sicily Channel. As a result, the Calabrian slab was progressively separated from the adjacent continental lithosphere whilst north-westward subduction of Ionian oceanic lithosphere continued. Geochemical data show that mantle material originally located beneath this slab moved upward through subduction windows. This suggests the existence of complex 3-D mantle flows, which are poorly constrained. The development of toroidal flows at the edges of the slab could in fact have caused a temperature increase, both in the orogenic arc and in the subducting slab, thus causing thermal erosion (Kincaid and Griffiths, 2003; Davaille and Lees, 2004; Funiciello et al., 2004) and accelerated rollback of the slab (Dvorkin et al., 1993), giving rise to dynamic uplift of the orogenic belt. Geodetic data (Hollenstein et al., 2003; D'Agostino and Selvaggi, 2004) have recently shown that extensional tectonics in the Tyrrhenian Basin are presently quiescent and that in Calabria the present-day convergence rate is only a few millimetres per year, whereas it amounted to at least 3-4 cm/y during the Pliocene. Moreover, seismological data show that none of the recent earthquakes are related to on-going subduction processes, several aspects of which are still unclear. Most importantly, it is still unclear whether subduction beneath the Calabrian-Apennine region is still active and what process controls wholesale uplift of the Italian peninsula.