Subduction zone evolution and deep slab structure in the ... · Subduction zone evolution and deep...

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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-boundary-reorganization 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 Jurassic-Cretaceous 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

The Mediterranean is tectonically one of

the most complex and seismically most

active regions in the world (Sengör, 2009).

The main controlling factors are the

subduction of the Paleo-Tethys under

Pangaea and the opening of the Central,

South and North Atlantic Ocean. Therefore

the tectonic of the Mediterranean is a

result of rifting of the African and

European plates after the Variscan

Orogeny (after Carminati et al., 2004),

which started in the Paleozoic, and during

the Alpine Orogeny, which started in the

Mesozoic. In the late Mesozoic the

Mediterranean area was dominated by

three main subduction zones: “From east

to west the Cimmerian, the Dinarides, and

the Alps-Betics”, or Alpine-Betic (Carminati

et al., 2004). These subduction zones

consumed the previously formed Tethys

Ocean. During the Cenozoic the

Mediterranean was dominated by the

Alps-Betics or Alpine-Betic, the

Apennines-Maghrebides, the Carpathian

and the Dinarides-Hellenides-Taurides

subduction zones (after Carminati et al.,

2004). The Dinarides, Hellenides, and

Taurides are “a polyphase orogeny,

representing the coalescence of […] three

subduction zones since Mesozoic times”

with a widespread extension development,

which resulted in a low topography in

comparison to the Alps (after Carminati et

al., 2004). All four subduction zones had

different directions and different

characteristics, which made the

Mediterranean tectonics so special. The

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Alps-Betics or Alpine-Betic and the

Dinarides-Hellenides-Taurides subduction

zones were eastwards- or north-

eastwards-directed, with “high

morphological and structural elevations,

double vergence, thick crust, involvement

of deep crustal rocks, and shallow

foredeeps” (Carminati et al., 2004). In

contrast the Apennines-Maghgrebides and

Carpathian subduction zones had a

westward-direction and are characterized

by trench retreat and “low morphological

and structural elevations, single vergence,

thin crust, involvement of shallow rocks,

deep foredeeps, and a widely developed

back-arc basin” (Carminati et al., 2004).

This asymmetry is “ascribed to the

‘westward’ drift of the lithosphere relative

to the mantle, at rates of about 49 mm per

year” (Carminati et al., 2004).

The Mediterranean basin is divided into

western, central and eastern. The central

and eastern Mediterranean basins are

mainly relics of the Mesozoic-Cenozoic

Tethys Ocean (after Carminati et al.,

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

Mediterranean basin is the youngest and

developed during the last 40-30 Ma. The

lithosphere is here thinned to less than 60

km and the crust has a thickness of 8-15

km (Carminati et al., 2012). This is a

consequence of the “coherent system of

interrelated irregular troughs, mainly V-

shaped that began to develop in the Late

Oligocene-Early Miocene” (Carminati et

al., 2012). In the eastern Mediterranean

deformation is very active because of the

involvement of five plates in this region:

Africa, Greece, Anatolia, Eurasia, and

Arabia. The most prominent factor in this

area is the “north-east-directed subduction

of Africa underneath Greece and the

Anatolian Plate (Eurasia)” (after Carminati

et al., 2004). Another reason for the

characteristic shape of the Mediterranean

is the individual motion of the five

microplates Adria, Iberia, Alcapia,

Alkapecia, and Tiszia, relative to the

African and European plates since Late

Cretaceous (Handy, et al., 2010).

Today’s shape of the Mediterranean is

mainly a result of subduction zones

formed during the Cenozoic. This paper

will now show in detail, but on a simplified

view, based on the main four subduction

zones, how the Mediterranean area and

their basins developed and will mainly

focus on the evolution during the last 45

Ma in the Cenozoic.

Tectonic features of the

Mediterranean area during Late

Cretaceous to Cenozoic

During 170-131 Ma the subduction of

remnant Tethyan basins was triggered by

a “Neotethyan subduction slab along the

NE margin of the composite African-

Adriatic slab” (Handy, et al., 2010). This

subduction event “was linked by a sinistral

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transform system to E-W opening of the

Valais part of Alpine Tethys” (Handy, et al.,

2010). The effect of this subduction slab

was an intra-oceanic subduction of the

Ligurian part of the Alpine Tethys during

131-84 Ma, which coincided with Eo-alpine

orogenesis in the Alcapia microplate (after

Handy, et al., 2010). During 84-45 Ma the

subduction of the Piemont and Valais

parts of the Alpine Tethys started and the

Ionian Sea slowly widened through NW

translation of Adria with respect to Africa

(after Handy, et al., 2010).


Mediterranean area at 45 Ma

The central-western Mediterranean basin

moves from west to east and its evolution

is connected to the three plates Africa,

Adria and Europe, as seen in figure 2

(Carminati et al., 2012).

“The Alps-Betics and Dinarides belts are

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

the Mediterranean at 45 Ma. Therefore

figure 1 shows two different models: In a.)

the Neotethys, or Ionian, Ocean “is

assumed to be continuous from the Ionian

Basin to the Maghrebian Basin” (Carminati

et al., 2012) and b.) shows an alternative

with two distinct oceans: An Ionian and

Maghrebian Ocean (Carminati et al.,

2012). Figure 2 also shows the deep slab

structure of the subduction of the oceanic

lithosphere.

The Cenozoic development of the Central-

Western Mediterranean is characterized

by a west-directed permanent subduction

zone since the Late Cretaceous (Carminati

et al., 2012). Figure 1 and 2 show the

occurrence of a continuous Alpine belt

before the Apennine subduction zone

developed (Carminati et al., 2012). The

beginning of the Apennine-Maghrebides

and Carpathian subduction zone ranges

from Late Cretaceous (~ 80 Ma) to Early

Oligocene (~ 33 Ma) and is therefore

neither shown in figure 1 nor in figure 2.

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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).

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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).



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.

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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).

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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 Apennines-

Maghrebides 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 Alps-Betics 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).

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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).

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During the last 35 Ma until today the

Adriatic and African slabs are retreating

while Africa and Europe have a slowed

down convergence. The cause for this

behavior is the subduction rollback of the

Ligurian part of the Alpine Tethys, which

coincided with the Western Alpine

orogeny. Also the very active Adriatic

microplate started with a counter-

clockwise rotation, which is caused by a

northward-directed push of the African

plate, while slab pull has effected rapid

rollback subduction of the Ligurian part of

the Alpine Tethys and opening of the

Western Mediterranean ocean basins

(Handy, et al., 2010).



“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 the Apennine-

Maghrebides subduction rollback

(Carminati et al., 2012). The western

Mediterranean started mainly forming after

“the terminal convergence in the Pyrenees

at about 20 Ma”, which is a result of “the

Late Cretaceous to Early Tertiary

counterclockwise rotation of Iberia”

(Carminati et al., 2004). The Apennines-

Maghrebides and Carpathians subduction

zones are consuming further western parts

of the Alpine Tethys Ocean, which is

visible in figure 6, which shows the

geodynamic framework reconstruction of

the Mediterranean at 21 Ma. Figure 6

shows also the deep slab structure on a

defined section during this time. In this

picture it is discernible that the subduction

below Sardinia in the depth of around 200

km is now steeper than 10 Ma before.

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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).



6 Ma later, at 15 Ma, the Alps-Betics and

Dinarides-Hellenides subduction in the

east starts to retreat. In comparison the

Apennines-Maghrebides and Carpathians

Belts are drifting further eastwards and

consuming most of the eastern parts of the

Neotethys Ocean. Figure 7 and 8 both

show the geodynamic framework

reconstruction of Mediterranean at 15 Ma.

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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).

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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-Hellenides-

Taurides 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).

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Mediterranean area today (0 Ma)

Remnants of the Mesozoic Neotethys

Ocean still exist and are at present-day

consumed in the Apennines and

Hellenides subduction zones (figure 10,

Carminati et al., 2012) and until today the

Apennines-Maghrebides subduction zone

consumes old Tethyan domains by a

speed of 25-30 mm per year (Carminati et

al., 2004). Figure 10 also shows that Africa

is moving south-westwards in relation to

Sicily.

“The recent stages of the evolution of the

Central Mediterranean region are

complicated” due to a tectonic inversion,

which is “ascribed to the continuing Africa-

Europe convergence (Carminati et al.,

2012).

“In the southern Apennines, the choking of

the subduction zone with the thicker

continental lithosphere of the Apulia

platform slowed the eastwards migration of

the subduction hinge, whereas in the

central and northern Apennines and in

Calabria subduction is still active”,

including rollback of the subduction hinge

due to the thin continental lithosphere

(Carminati et al., 2004).

Figure 11 gives an overview of the

present-day geodynamic framework of the

Mediterranean showing the topography

and bathymetry.

Figure 10: Present geodynamic framework of the Mediterranean representing the main four subduction zones: The westwards-directed Apennines-Maghrebides and Carpathians, the north-eastwards-directed Dinarides-Hellenides-Taurides, and the south-eastwards-directed Alps (Carminati et al., 2004).

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Figure 11: Present geodynamic framework of the Mediterranean showing the topography and bathymetry (Carminati et al., 2012).

Deep slab structure of the

Mediterranean

During the last 45 Ma the deep slab

structure and the angle of the subduction

zones changed a lot. The Hellenides-

Dinarides-Taurides subduction zone

consumed the Ionian Tethys Ocean from

east. The angle of this subduction zones

didn’t changed. The Apennines and

Maghrebides subduction zones developed

later out of the subduction of the Alps, at

30 Ma, consuming the Ionian Tethys

Ocean from the west. In this subduction

zone the more the Ionian Tethys is

consumed the steeper is the subduction

angle.

Today we have shallow slabs in the Alps

(~ 40°), in the Betics (~ 45°) and in the

Dinarides-Hellenides (~ 25°) subduction

zones (after Carminati et al., 2012).

Steeper slabs are below the Apennines (~

70°) and the Carpathians (~ 75°) (after

Carminati et al., 2012).

Figure 12 shows the evolution of the deep

slab structure in the Mediterranean during

the last 45 Ma until today. At around 30

Ma a slab breakoff of the Alps-Betics belt

can be recognized due to the newly

formed Apennines-Maghrebides and

Carpathians subduction zones.

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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 Dinarides-Hellenides 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.

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Conclusion

The extension of the western

Mediterranean developed through relative

convergence between Africa and Europe

at about 135 km in north-south-direction in

the last 23 Ma, which is more than five

times slower than the migration of the

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

following a straight line. This gave the

Mediterranean its present-day shape.

The still active subduction zones are the

reason that figure 13 is not the final form

of the Mediterranean: The Mediterranean

tectonics are still in a process of change

and reorganization of plate-boundaries.

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 eastwards-vergent 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).

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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, D., 2010.

Reconciling 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.

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