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Subduction zone evolution and deep slab structure in the Mediterranean Michaela Christine Biela (319913) E-Mail: [email protected] Abstract The evolution of the Mediterranean subduction zones and their deep slab structure started during the Late Cretaceous and is a result of the relative movement of the African and European plate including the independent motion of five microplates (Adria, Iberia, Alcapia, and Tiszia), which caused subduction zones consuming the Tethys Ocean – a Mesozoic Ocean preserved in the Alps. This subduction of the Alpine Tethys since Late Cretaceous caused a very complex plate-boundaryreorganization between seven (micro-) plates. Since the last 45 Ma until today the Alpine Tethys is still consumed by four main still active subduction zones, on which will mainly be focused in this paper: The Alps-Betics or Alpine-Betic and the Dinarides-Hellenides-Taurides subduction zones with an eastwards- or north-eastwards-direction and the Apennines-Maghrebides and Carpathian subduction zones with a westward-direction. The term “Alpine-Tethys” include remnants of the two JurassicCretaceous Valais and Piemont-Liguria Oceans, while the term “Neotethys” describes the creation of a Late Paleozoic-Mesozoic Ocean due to the breakoff of Pangea. Introduction et al., 2004). These subduction zones The Mediterranean is tectonically one of consumed the previously formed Tethys the most complex and seismically most Ocean. active regions in the world (Sengör, 2009). Mediterranean was dominated by the The main controlling factors are the Alps-Betics subduction of the Paleo-Tethys under Apennines-Maghrebides, the Carpathian Pangaea and the opening of the Central, and South and North Atlantic Ocean. Therefore subduction zones (after Carminati et al., the tectonic of the Mediterranean is a 2004). The Dinarides, Hellenides, and result Taurides of rifting of the African and the During the or Cenozoic Alpine-Betic, the the Dinarides-Hellenides-Taurides are “a polyphase orogeny, Variscan representing the coalescence of […] three Orogeny (after Carminati et al., 2004), subduction zones since Mesozoic times” which started in the Paleozoic, and during with a widespread extension development, the Alpine Orogeny, which started in the which resulted in a low topography in Mesozoic. the comparison to the Alps (after Carminati et Mediterranean area was dominated by al., 2004). All four subduction zones had three main subduction zones: “From east different to west the Cimmerian, the Dinarides, and characteristics, the Alps-Betics”, or Alpine-Betic (Carminati Mediterranean tectonics so special. The European plates In the after late the Mesozoic directions which and different made the 1 Alps-Betics or the Oligocene-Early Miocene” (Carminati et Dinarides-Hellenides-Taurides subduction al., 2012). In the eastern Mediterranean zones north- deformation is very active because of the “high involvement of five plates in this region: morphological and structural elevations, Africa, Greece, Anatolia, Eurasia, and double vergence, thick crust, involvement Arabia. The most prominent factor in this of shallow area is the “north-east-directed subduction foredeeps” (Carminati et al., 2004). In of Africa underneath Greece and the contrast the Apennines-Maghgrebides and Anatolian Plate (Eurasia)” (after Carminati Carpathian a et al., 2004). Another reason for the westward-direction and are characterized characteristic shape of the Mediterranean by trench retreat and “low morphological is the individual motion of and structural elevations, single vergence, microplates thin crust, involvement of shallow rocks, Alkapecia, and Tiszia, relative to the deep foredeeps, and a widely developed African and European plates since Late back-arc basin” (Carminati et al., 2004). Cretaceous (Handy, et al., 2010). This the Today’s shape of the Mediterranean is ‘westward’ drift of the lithosphere relative mainly a result of subduction zones to the mantle, at rates of about 49 mm per formed during the Cenozoic. This paper year” (Carminati et al., 2004). will now show in detail, but on a simplified The Mediterranean basin is divided into view, based on the main four subduction western, central and eastern. The central zones, how the Mediterranean area and and eastern Mediterranean basins are their basins developed and will mainly mainly relics of the Mesozoic-Cenozoic focus on the evolution during the last 45 Tethys Ocean (after Carminati et al., Ma in the Cenozoic. were Alpine-Betic eastwards- crustal rocks, subduction asymmetry or with eastwards-directed, deep and is and zones “ascribed had to Adria, Iberia, the five Alcapia, 2004). This assumption is based on the low heat flow (18-40 mWm-2) and the 4-8 km of sedimentary cover of the Ionian Sea (after Carminati et al., 2004). The western Tectonic Mediterranean basin is the youngest and Mediterranean developed during the last 40-30 Ma. The Cretaceous to Cenozoic lithosphere is here thinned to less than 60 During 170-131 Ma the subduction of km and the crust has a thickness of 8-15 remnant Tethyan basins was triggered by km (Carminati et al., 2012). This is a a “Neotethyan subduction slab along the consequence of the “coherent system of NE margin of the composite African- interrelated irregular troughs, mainly V- Adriatic slab” (Handy, et al., 2010). This shaped that began to develop in the Late subduction event “was linked by a sinistral features area of during the Late 2 transform system to E-W opening of the the Mediterranean at 45 Ma. Therefore Valais part of Alpine Tethys” (Handy, et al., figure 1 shows two different models: In a.) 2010). The effect of this subduction slab the Neotethys, or Ionian, Ocean “is was an intra-oceanic subduction of the assumed to be continuous from the Ionian Ligurian part of the Alpine Tethys during Basin to the Maghrebian Basin” (Carminati 131-84 Ma, which coincided with Eo-alpine et al., 2012) and b.) shows an alternative orogenesis in the Alcapia microplate (after with two distinct oceans: An Ionian and Handy, et al., 2010). During 84-45 Ma the Maghrebian Ocean (Carminati et subduction of the Piemont and Valais 2012). Figure 2 also shows the deep slab parts of the Alpine Tethys started and the structure of the subduction of the oceanic Ionian Sea slowly widened through NW lithosphere. translation of Adria with respect to Africa The Cenozoic development of the Central- (after Handy, et al., 2010). Western Mediterranean is characterized al., by a west-directed permanent subduction zone since the Late Cretaceous (Carminati et al., 2012). Figure 1 and 2 show the Tectonic features of the occurrence of a continuous Alpine belt Mediterranean area at 45 Ma before the Apennine subduction zone The central-western Mediterranean basin developed (Carminati et al., 2012). The moves from west to east and its evolution beginning of the Apennine-Maghrebides is connected to the three plates Africa, and Carpathian subduction zone ranges Adria and Europe, as seen in figure 2 from Late Cretaceous (~ 80 Ma) to Early (Carminati et al., 2012). Oligocene (~ 33 Ma) and is therefore “The Alps-Betics and Dinarides belts are neither shown in figure 1 nor in figure 2. collisional orogens that were preceded by the earlier […] subduction of several branches of the Neotethys and Alpine Tethys Ocean” (Carminati et al., 2012) and is a result of the collision of the African plate with Eastern and Western Europe which happened mostly during the Eocene (~ 55-35 Ma) (Carminati et al., 2012). This led to the formation of a Northeast to Southwest trending fold-and-thrust chain (Carminati et al., 2012). Scientists are not concurring about the existing oceans in 3 Figure 1: Geodynamic framework reconstruction of the Mediterranean at about 45 Ma. a.) Showing the reconstruction with a continuous Ionian Ocean and b.) showing the reconstruction with two oceanic basins (Carminati et al., 2012). 4 Figure 2: Geodynamic framework reconstruction of the Mediterranean at about 45 Ma. “The Alps were continuous with the Betics to Gibraltar, consuming an ocean located to the west”. The formation of the main subduction zones (Dinarides, Hellenides, Taurides) started. The Aegean extension is in progress (Carminati et al., 2004). Tectonic features of the Mediterranean area at 38 Ma 7 Ma later the eastern Alpine-Betic and Dinarides-Hellenides Belts grow further, but the Alpine-Betics slowed down. The western Apennines-Maghrebides and Carpathians Belts also start to develop while the Neotethys Ocean is consumed. During the development of these new subduction zones a slab breakoff of the former subducted oceanic lithosphere through the Alpine-Betics subduction has to occur. Figure 3 shows the geodynamic framework at 38 Ma. 5 Figure 3: Geodynamic framework reconstruction of the Mediterranean at about 38 Ma. Also showing the deep slab structure in the bottom picture (Carminati et al., 2012). 6 Tectonic features of the Mediterranean area at 31-30 Ma The westward-directed Apennines- Maghrebides and Carpathians subduction is going further “along the Alps-Betics retrobelt, where oceanic and thinned continental lithosphere occurred in the foreland to the east” (Carminati et al., 2004) and underneath the Adriatic and Mesomediterranean plate. The ApenninesMaghrebides and Carpathian subduction in the east is still growing further to the west consuming the Neothethys Ocean. Figure 4 and 5 show the geodynamic framework reconstruction during this time. Figure 5 also shows the deep slab structure on a defined section trace. Figure 4: Geodynamic framework reconstruction of the Mediterranean at about 30 Ma. “The AlpsBetics formed along the south-eastwards-dipping subduction of Europe and Iberia underneath the Adriatic and Mesomediterrenean plates. The Apennines developed along the Alps-Betics retrobelt to the east […] and the Carpathians started to develop along the Dinarides retrobelt” (Carminati et al., 2004). 7 Figure 5: Geodynamic framework reconstruction of the Mediterranean at about 31 Ma. Also showing the deep slab structure in the bottom picture (Carminati et al., 2012). 8 Tectonic features of the at about 20 Ma”, which is a result of “the Mediterranean area at 35 Ma Late During the last 35 Ma until today the counterclockwise Adriatic and African slabs are retreating (Carminati et al., 2004). The Apennines- while Africa and Europe have a slowed Maghrebides and Carpathians subduction down convergence. The cause for this zones are consuming further western parts behavior is the subduction rollback of the of the Alpine Tethys Ocean, which is Ligurian part of the Alpine Tethys, which visible in figure 6, which shows the coincided Alpine geodynamic framework reconstruction of orogeny. Also the very active Adriatic the Mediterranean at 21 Ma. Figure 6 microplate counter- shows also the deep slab structure on a clockwise rotation, which is caused by a defined section during this time. In this northward-directed push of the African picture it is discernible that the subduction plate, while slab pull has effected rapid below Sardinia in the depth of around 200 rollback subduction of the Ligurian part of km is now steeper than 10 Ma before. with the started Western with a Cretaceous to Early rotation of Tertiary Iberia” the Alpine Tethys and opening of the Western Mediterranean ocean basins of the (Handy, et al., 2010). Tectonic features Mediterranean area at 21 Ma “The maximum amount of north-south Africa/Europe relative motion […] was about 135 km in the last 23 Ma, more than five times shorter with respect to the eastward migration of the Apennines arc which moved eastwards more than 700 km during the last 23 Ma” (Carminati et al., 2012). Therefore the east-directed migration of the Apennine-Maghrebide arc is a consequence of Maghrebides the Apennine- subduction rollback (Carminati et al., 2012). The western Mediterranean started mainly forming after “the terminal convergence in the Pyrenees 9 Figure 6: Geodynamic framework reconstruction of the Mediterranean at about 21 Ma, also showing the deep slab structure in the bottom picture (Carminati et al., 2012). Tectonic features of the Belts are drifting further eastwards and Mediterranean area at 15 Ma consuming most of the eastern parts of the 6 Ma later, at 15 Ma, the Alps-Betics and Neotethys Ocean. Figure 7 and 8 both Dinarides-Hellenides subduction in the show east starts to retreat. In comparison the reconstruction of Mediterranean at 15 Ma. the geodynamic framework Apennines-Maghrebides and Carpathians 10 Figure 7: Geodynamic framework reconstruction of the Mediterranean at about 15 Ma. The Apennines-Maghrebides trench tend to drift eastwards. The Dinarides subduction slowed down, where the Hellenides subduction in the south got faster. The Carpathians in the east generate the Pannonian back-arc basin (Carminati et al., 2004). Figure 8: Geodynamic framework reconstruction of the Mediterranean at about 15 Ma (Carminati et al., 2012). 11 Tectonic features of the Mediterranean area at 5 Ma At 5 Ma the Neotethys Ocean is nearly consumed by the eastwards-drifting Apennines-Maghrebides and Carpathians subduction zones. In comparison the Alpine-Betics and Dinarides-HellenidesTaurides Belts in the east had a much lower drift. Figure 9 shows the geodynamic framework reconstruction of the Mediterranean at 5 Ma. Figure 9: Geodynamic framework reconstruction of the Mediterranean at about 5 Ma (Carminati et al., 2012). 12 Tectonic features of the “In the southern Apennines, the choking of Mediterranean area today (0 Ma) the subduction zone with the thicker Remnants of the Mesozoic Neotethys continental Ocean still exist and are at present-day platform slowed the eastwards migration of consumed and the subduction hinge, whereas in the Hellenides subduction zones (figure 10, central and northern Apennines and in Carminati et al., 2012) and until today the Calabria Apennines-Maghrebides subduction zone including rollback of the subduction hinge consumes old Tethyan domains by a due to the thin continental lithosphere speed of 25-30 mm per year (Carminati et (Carminati et al., 2004). al., 2004). Figure 10 also shows that Africa Figure 11 gives an overview of the is moving south-westwards in relation to present-day geodynamic framework of the Sicily. Mediterranean showing the topography “The recent stages of the evolution of the and bathymetry. Central in the Apennines Mediterranean region lithosphere subduction of is the still Apulia active”, are complicated” due to a tectonic inversion, which is “ascribed to the continuing AfricaEurope convergence (Carminati et al., 2012). Figure 10: Present geodynamic framework of the Mediterranean representing the main four subduction zones: The westwards-directed Apennines-Maghrebides and Carpathians, the northeastwards-directed Dinarides-Hellenides-Taurides, and the south-eastwards-directed Alps (Carminati et al., 2004). 13 Figure 11: Present geodynamic framework of the Mediterranean showing the topography and bathymetry (Carminati et al., 2012). Deep slab structure of the Today we have shallow slabs in the Alps Mediterranean (~ 40°), in the Betics (~ 45°) and in the During the last 45 Ma the deep slab Dinarides-Hellenides (~ 25°) subduction structure and the angle of the subduction zones (after Carminati et al., 2012). zones changed a lot. The Hellenides- Steeper slabs are below the Apennines (~ Dinarides-Taurides 70°) and the Carpathians (~ 75°) (after subduction zone consumed the Ionian Tethys Ocean from Carminati et al., 2012). east. The angle of this subduction zones Figure 12 shows the evolution of the deep didn’t and slab structure in the Mediterranean during Maghrebides subduction zones developed the last 45 Ma until today. At around 30 later out of the subduction of the Alps, at Ma a slab breakoff of the Alps-Betics belt 30 Ma, consuming the Ionian Tethys can be recognized due to the newly Ocean from the west. In this subduction formed zone the more the Ionian Tethys is Carpathians subduction zones. changed. The Apennines Apennines-Maghrebides and consumed the steeper is the subduction angle. 14 Figure 12: The evolution of the Mediterranean during the last 45 Ma as a result of the three main subduction zones: The early eastwards-directed Alpine subduction, the Apennine, and the DinaridesHellenides subduction. The Dinarides-Hellenides subduct the Tethyan Mesozoic oceanic lithosphere (Carminati et al., 2004). At 30 Ma a slab breakoff of the Alps-betics belt can be recognized, which is mainly caused by the new formed Apennines-Maghrebides and Carpathians subduction zones. 15 following a straight line. This gave the Conclusion The extension of the western Mediterranean its present-day shape. Mediterranean developed through relative The still active subduction zones are the convergence between Africa and Europe reason that figure 13 is not the final form at about 135 km in north-south-direction in of the Mediterranean: The Mediterranean the last 23 Ma, which is more than five tectonics are still in a process of change times slower than the migration of the and reorganization of plate-boundaries. Apennines arc, which was 700 km in eastward-direction in the same time (Carminati et al., 2004). Therefore the migration of the Apennines arc is “a consequence of the Apennines- Maghrebides subduction rollback, which was generated either by slab pull or by the ‘eastwards’ flow of the mantle relative to the lithosphere” (Carminati et al., 2004). Figure 13 gives a summary of the tectonic features during the last 45 Ma showing in detail how often the direction of the four subduction zones changed, never Figure 13: Summary of the main tectonic features of the Mediterranean during the last 45 Ma including the related subduction zones: ”The double-vergent Alps–Betics, the single eastwardsvergent Apennines-Maghrebides […], the double-vergent Dinarides-Hellenides-Taurides and related Aegean extension, the single eastwards-vergent Carpathians […], and the double-vergent Pyrenees”. (Carminati et al., 2004). 16 References Carminati, E., Doglioni, C., 2004. Mediterranean tectonics. GEOL: 00135. Carminati, E., Lustrino, M., Doglioni, C., 2012. Geodynamic evolution of the central and western Mediterranean: Tectonics vs. igneous petrology constraints. Tectonophysics 579, 173–192. Handy, M., R., Schmid, S., M., Bousquet, R., Kissling, E., Bernoulli, Reconciling D., 2010. plate-tectonic reconstructions of Alpine Tethys with the geological-geophysical record of spreading and subduction in the Alps. Earth-Science Review 102, 121-158. Şengör, A. M. C., 2009. Tectonic evolution of the Mediterranean: a dame with four husbands. Trabajos de Geología, Universidad de Oviedo, 29: 45-50. 17