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1
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
3
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).
Tectonic features of the
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.
4
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).
5
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.
6
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).
7
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 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).
8
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).
9
Tectonic features of the
Mediterranean area at 35 Ma
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).
Tectonic features of the
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 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.
10
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
Mediterranean area at 15 Ma
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.
11
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).
12
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-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).
13
Tectonic features of the
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).
14
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.
15
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.
16
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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Geodynamic evolution of the central and
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