Download Evolution of the Eastern Alps

Christina Schmidt
Matriculation no.: 295386
Evolution of the Eastern Alps
C. SCHMIDT
Field school “Alps” (26/08/2013 – 05/09/2013)
CONTENTS:
 Introduction
 Present state of the Eastern Alps
 Development toward the status quo
 Outlook
 References
I. INTRODUCTION
Depending on which author is consulted the Alps are subdivided into Eastern, Western,
Central and Southern Alps. This is a purely geographic distinction of alpine regions, not to be
mistaken for the geologic classification. Some authors propose bisection into Western and
Eastern parts because of a barely visible border between Western and Central Alps (Pfiffner,
2010). The Western boundary of the Eastern Alps towards the central part of the orogen is
easily recognizable between the Swiss town St. Margrethen, situated south of the Lake
Constance, the city Chur, east of the stream bifurcation of Vorder- and Hinterrhein and the
Italian town Sondrio, 50 km north of Bergamo. (For Froitzheim1 though, the line from Lake
Geneva through the Rhone Valley to the Swiss town Martigny, along the Great St. Bernard
Pass through the Aosta Valley to the Italian town Ivrea represents a clear distinction
between Western and Central Alps). Additionally Froitzheim2 as well as Pfiffner (2010)
denominate a series of stretched valleys that form the border between the three northerly
regions and the Southern Alps: Valtellina/Valtelline Valley, Pustertal/Puster Valley and
Gailtal/Gail Valley.
The geologic distinction between Helvetic, Penninic, Austroalpine and Southalpine nappes
relies on the paleogeographic domain in which the corresponding lithologies were formed
(Pfiffner, 2010). In the Eastern Alps there are outcrops of Helvetic nappes; their sediments
originate from the former European continental margin. The Penninic nappes represent
pelagic deposits of the Piedmont Ocean basin separating the European and the Adriatic
continental margins; the Austro- and Southalpine nappes are the former continental margin
of the Adriatic plate.
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http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-thealps-part-1-general-remarks-austroalpine-nappes, 15.08.13
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http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-thealps-part-1-general-remarks-austroalpine-nappes, 15.08.13
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Matriculation no.: 295386
II.
PRESENT STATE OF THE EASTERN ALPS
Today the surface of the Eastern Alps is primarily constituted of Austroalpine nappe stacks.
Two windows that enable a look onto Penninic and Helvetic rocks and the most northern
regions of the orogen are the only exceptions. In the northernmost area, the Northern
Calcareous Alps, three different stacks can be determined: The Bajuvarian, Tirolian and
Juvavian nappes. Those nappes are constituted of Mesozoic, primarily calcareous sediments.
The Bajuvarian Lechtal nappe has been folded in the course of several deformation episodes.
The profile derived by Pfiffner, 2010, from manifold interpretations of the TRANSALP seismic
profile offers information about the subsurface (Fig. 1 at the end):
A complex of southward dipping Subalpine Molasse, Helvetic rocks and Penninic flysch
sediments underlies the thrust plane at the base of the Austroalpine nappes. The crystalline
basement and the Mesozoic authochthonous cover in the footwall of this complex also dip
slightly towards the Periadriatic line and have experienced normal faulting with planes
dipping towards the fault lineament. Further southwards the geology of the Eastern Alps’
surface differs from West to East. In the West the Northern Calcareous sediments overlie the
corresponding crystalline basement composed of the Silvretta and Öztal nappes (the latter is
present at the Schneeberg complex as gneiss, amphibolite and mica schist, Konzett et al.,
1996). To the East instead Palaeozoic Austroalpine greywackes or quartzphyllites are present
(Pfiffner, 2010). Outcrops of Austroalpine Palaeozoic sediments are rare; sites are situated
along the Periadriatic Line. The lineament zone and the Austroalpine basement around it dip
to the North which will be a point of discussion later in the paper (see III. Development
toward the status quo). The huge antiform of the Tauern Window in the profile’s centre is an
accumulation of European crystalline basement. In turn the crystalline basement is missing
beneath the Northern Calcareous Alps; this phenomenon is argued to have occurred either
due to thrust faulting or a pop-up structure (Pfiffner, 2010, Schmid, 2004). It is overlain by a
thin authochthonous Mesozoic sediment cover of the Helvetic domain. The dominating
structures inside the basement rocks are isoclinal and plunging folds which indicate ductile
deformation at high temperatures. The Penninic nappe has overthrusted the continental
rocks; the basal décollement is equally deformed as the whole massif; therefore the
deformation process must have followed onto the nappe emplacement. The antiform is
locally still covered by Austroalpine units, in the northern areas either by quartzphyllites or
greywackes, in the South by crystalline basement rocks.
All of the Austroalpine units are crossed by various strike slip faults (Fig. 2, at the end); those
are structures formed by continuous N-S directed shortening and periodical E-W directed
extensions. The Periadriatic Line strikes W-E and is displaced to the North by the Giudicarie
Fault that strikes SSW-NNE around Trento. This strike slip zone hosts a range of plutons (see
the paper Alpine Granites by Jacqueline Engmann).
A narrow strip of Helvetic and Penninic nappes is aligned along the northern boundary of the
Northern Calcareous Alps: the Helvetic nappes predominate around Vorarlberg in the West,
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Matriculation no.: 295386
towards Vienna instead the Penninic nappes which are characterized by Rhenodanubic
Flysch sediments (Pfiffner, 2010).
Noticeable features in the Eastern Alps are the Tauern and Engadin Window. The central
Tauern Window offers a view onto Penninic and even Helvetic rocks. Faults with extensive
components to the West and East enabled the deeper nappes to be exhumed. The Engadin
Window only exhibits Penninic nappes from the former Piedmont Ocean.
Pfiffner observed high pressure conditions (HP) during metamorphosis at various sites in the
Eastern Alps. Eclogite facies rocks can be found in the far SE Austroalpine crystalline
basement between Graz and Klagenfurt in Austria.
More eclogitic rocks are present SE and SW of the Tauern Window. The eclogite facies in
both cases is surrounded by amphibolite facies rocks. All of those outcrops were dated to
the Middle Cretaceous orogeny (110-90 Ma) – an age that is not present in any other
metamorphic rock in all of the Alps. Accordingly, orogenesis or rather HP conditions of the
Middle Cretaceous only affected the Austroalpine nappes. A much broader zone of
greenschist alteration surrounds the amphibolite facies and spreads all over the Eastern
alpine orogeny. Metamorphism of a very low grade can be found in the Northern Calcareous
Alps (Pfiffner, 2010).
As shown in the metamorphosis map produced by Pfiffner (2010) (Fig. 3, at the end)
pressure dominated blueschist facies is an exception in the Austroalpine rocks and is only
present as a narrow zone encircling the eclogites SW of the Tauern Window. It is more
common in Penninic nappes, visible in the Tauern and Engadine Window. There the
metamorphism dates back to the Cenozoic erathem and altered Cretaceous pelagic
sediments. The pressure dominated metamorphic rocks of the Penninic zone overlie the
Helvetic nappes that have been primarily altered by high temperatures. Conclusively the
Penninic nappes underwent deeper subduction before having been thrusted onto the
Helvetic nappes.
III.
DEVELOPMENT TOWARD THE STATUS QUO
The Alpine orogenesis is mainly constituted of the Cretaceous orogeny and the Cenozoic
orogeny. The complexity of the orogen is owed to irregular plate boundaries and the varying
directions of plate movements. This led to continent-continent collisions that occurred in
different regions of the Alps at different geologic stages. This is also visible from the map
displaying grades of metamorphism in the Alps (Fig 3. after Pfiffner, 2010): For example, the
Eastern Alps are the only region where metamorphosed Austroalpine nappes are cropping
out whose alteration was dated to the Middle Cretaceous. The exposed eclogite facies can
only form under high pressure conditions that are typically ascribed to subduction zones.
Consequently, a separate discussion of the different evolution paths over time for the
Eastern, Central, Western and also Southern Alps and Dolomites is reasonable.
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Matriculation no.: 295386
The Cretaceous orogeny is determined by the convergent movement of Adria, a subplate of
Africa, and the European tectonic plate. But initially the E-W directed convergence
subducted the Piedmont Ocean beneath the Adriatic microplate (Pfiffner, 2010). At the same
time the continental margin of Adria was compressed and the first Austroalpine nappes
were stacked. Indicators from structural geology show a WNW directed movement of Adria
and a subduction of the Piedmont Ocean to the ESE, respectively (Pfiffner, 2010). Crystalline
Austroalpine nappes show HP alteration which implies transportation of the continental
material to high depths (>30 km) at the border of Early and Late Cretaceous (AlbianTuronian). Stöckhert and Gerya (2005) explain this phenomenon with the accumulation of an
accretionary wedge that is not only formed out of oceanic sediments scraped off the lower
plate but foremost out of continental material derived by subduction erosion. Radiometric
estimates revealed age differences in the metamorphosed oceanic crust that were probably
caused by slab break-offs. Those could have led to interruptions of the metamorphic
alteration during a continuous convergent movement.
As the overthrusting moved towards external regions nappe stacks were built
synsedimentarily; this is visible in the Upper Austroalpine nappes that crop out in the
Northern Calcareous Alps. In the Cretaceous recently deposited sediments were eroded on
the foot wall of the décollement while on the hanging wall identical sedimentation
continued. The erosive contacts of the Mesozoic sediments allow for a dating of the
overthrusts with regard to the ages of deposited sediments. According to Pfiffner (2010) the
stacking occurred successively from the end of the Barremian to the end of the
Maastrichtian stage:
Table 1: Overview of Cretaceous overthrusting events, edited by author (nomenclature of
nappes from Horninger and Weiss, 1980, and Oberhauser and Bauer, 1980)
Time span
Precise age [Ma] Overthrusting event
End of Barremian
Aptian – Albian
120
110-100
End of Cenomanian
94
End of Turonian
End of Maastrichtian
89
65
Juvavian nappes Tirolian nappes
Lechtal nappe  Allgäu nappe
(Bajuvarian nappes)
Inntal (Tirolian nappe)  Lechtal (Bajuvarian
nappe)
Reactivation of Lechtal-overthrust
Krabachjoch nappe  Inntal nappe
(Tirolian nappes)
While Pfiffner (2010) marks tectonic evolution in the Cretaceous by identifying events of
overthrusting, Froitzheim3 defines phases of orogenic movement. For the Mesozoic
deformation processes the following phases have been determined:
The Vinschgau shear zone limiting the Öztal- and the Campo nappe (Upper Austroalpine
crystalline basement) represents a deformation phase of the Cenomanian, 100 Ma (Fig. 2, at
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http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-thealps-part-1-general-remarks-austroalpine-nappes, 15.08.2013
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Matriculation no.: 295386
the end). The shearing only affected the Upper Austroalpine crystalline basement. It
precedes the Trupchun Phase which is characterized by a compressive regime. The
compression induced the westward stacking of nappes as well as local strike slip faults (e.g.
caused by sinistral transpression in Graubünden, westernmost Eastern Alps) and tight or
even isoclinal folding (opening angles from 0-30°). It lasted from the Cenomanian to the
Santonian and was replaced by a period of extension, the Ducan-Ela Phase (also Gosau
event, Schmid et al., 2004). Exhumation of the Austroalpine nappes and successive cooling
took place at that time (Schmid et al., 1996). During that extensive period the predominant
normal faults are directed east or SE and show an inversion of the tectonic movements in
the Campanian. The fault planes are often the reactivated thrusts of the Trupchun Phase.
Normal faulting mainly affected the Upper Austroalpine nappes; the lower ones exhibit
recumbent folding as a result of gravitational collapse (vertical shortening). Such folds (Fig.
4) developed from strata that reached an almost vertical dip during the former compression.
Figure 4: Recumbent folds in Graubünden
(Switzerland), Lower Austroalpine Ela nappe
(http://www.steinmann.unibonn.de/arbeitsgruppen/strukturgeologie/lehre/wiss
en-gratis/Abb.57.jpg/image_preview, 19.08.2013)
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In the Late Cretaceous, at 70 Ma, the Eastern Alps were paleogeographically a shallow
mountain range composed of the Austroalpine nappe stack. At that stage the Brianҫonnais
microcontinent was still located NE of the subduction zone and marine sedimentation took
place. The remnants of the Piedmont Ocean and the Valais Trough merged. The movement
of the Eastern Alps independent from the Southalpine nappes and the Piedmont oceanic
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Matriculation no.: 295386
crust was possible because of strike slip faults to the north and south (Fig. 5). The southern
strike slip is the precursor of the Insubric Line.
Figure 5: Situation of Eastern Alps at 70 Ma
(http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-thealps-part-1-general-remarks-austroalpine-nappes, 15.08.2013)
During this initial form of orogeny the European continent was not yet involved; it moved
south towards the subduction and sedimentation still prevailed on its continental margin
(Helvetic nappes) and in the Piedmont Ocean (for example the Rhenodanubic Flyschzone).
The plate movements shifted from E-W towards a N-S convergence during the Cenozoic or
Tertiary orogenesis (Pfiffner, 2010). The orogenesis included large overthrusts and folding
that influenced the inversion and relief of the mountain range.
In this period the Piedmont Ocean was closed completely by subduction. After the final
closure the Brianҫonnais microcontinent entered the subduction zone and the resulting
compressive forces caused the European continental margin to bulge, too. Consequently
nappes were formed. After the migration of sedimentation processes, the deformation
successively extended towards more external parts of the continental margin. This is an
indicator for the two directional growth of an orogen like the bivergent nappe stacks of the
Alps. In the Eocene the whole microcontinent was subducted and experienced peak
pressures 1 - 1.3 MPa (50 Ma, Schmid et al., 1996). But the delayed heating overprinted the
HP metamorphism. For the Eastern Alps the actual continent plate of 30 km thickness
arrived at the eastern part of the subduction zone in the Paleocene (for Central Alps 50 Ma,
Schmid et al., 1996, Schmid et al., 2004). Then the frontal part, the Adula nappe, collided
with the land mass of the Adriatic plate.
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Matriculation no.: 295386
The continuous compression promotes the formation of more nappe stacks. The only
indicators for the deformation time spans are the end of sedimentation of the Rhenodanubic
Flysch. It represents the possible starting point for compression of the Piedmont Ocean’s
crust (Late Cretaceous). The deformation must have ended at the latest in the Early Miocene
when the thrust of the infrahelvetic sediments stopped at the Subalpine Molasse (Pfiffner,
2010), the most external site of those Helvetic nappes. As a consequence of the continent
collision all Austroalpine nappes were pushed northwards onto the Penninic nappes. This
compressive regime is described as the Blaisun Phase (Pfiffner, 2010). Internal folding of the
Penninic nappes resulted from the thrusting. During this overthrust the Penninic sediment
cover doubled in thickness because the southern, ophiolite-bearing Bündner schists (from
Piedmont oceanic crust) was pushed onto the northern schists that do not comprise
ophiolitc material (crust in the Valais trough). In the second half of the Eocene the nappe
complex of Penninic and Austroalpine rocks was moved further to the North on top of the
Helvetic nappes (like the Tauern massif). By the same process infrahelvetic sediments from
the European shelf were sheared off and transported until they reached their final position
on the Subalpine Molasse. Their transportation lasted until the Oligocene (it is classified as a
separate deformation phase of the infrahelvetic complex, the Pizol Phase).
Pfiffner (2010) refers to a short period of extension in the Eastern Alps in between the
overthrusts, the Turba Phase. An extensional stress regime acted parallel to the orogeny,
associated with a structural inversion of the Central Lepontine Alps. Structures derived from
that phase are normal faults situated in Austroalpine and Penninic nappes. At the end of the
Eocene magmatic intrusions evolved north of the Periadriatic line like the Riesenferner,
Karawanken and Pohorje Plutons (Schmid et al., 2004).
Pfiffner (2010) identified the Domleschg Phase as a third deformation period in the
Oligocene. But it only produced a minor shortening in NNW-SSE direction.
Those three deformations are all displayed today in the Northern Calcareous Alps.
Furthermore minor Eocene deformations of the Rhenodanubic Flysch are known as well as
the Pizol Phase influencing the infrahelvetic sediments during the Oligocene.
The Tauern massif (part of the Helvetic nappes) experienced horizontal shortening and
contemporarily thickened from the end of the Oligocene to the Miocene. It is considered
another major deformation period. According to Pfiffner (2010) the large amount of uplift
induced normal faults to the East and to the West of the Tauern Window in the Miocene;
those are the Brenner fault and the Katschberg fault. The driving force of the shortening was
the Tauern ramp: A large thrust plane at the base of the Helvetic nappes transported the
crystalline basement from beneath the Northern Calcareous Alps towards the Periadriatic
Lineament zone (Pfiffner, 2010). Schmid et al. (2004) proposed alternatively that the
structure of the Tauern massif developed from a pop-up structure. The necessary
transpressive regime is supposed to have resulted from the juxtaposition of the dextral
strike slips of the Periadriatic Fault zone and the sinistral SEMP (Salzach-Ennstal-MariazellPuchberg) strike slip fault (Fig. 2, at the end).
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Matriculation no.: 295386
The Miocene is also the starting point of lateral extrusion in between those faults
(Ratschbacher et al., 1991). Lateral extrusion is based on two processes: tectonic escape
(from the indentation of e.g. Adria) and extensional collapse (from the weight of the
orogen). In the Eastern Alps presumably both mechanisms were active and induced an
eastward movement of the regions between SEMP and Periadriatic Lineament. The
extrusion resulted also in the Brenner normal fault and the doming of the Tauern Window.
The main cause for those processes was the slab break-off of the European plate advancing
to the West. The slab retreated towards the Carpathians and provided space for extension in
the Pannonian basin as well as the easternmost alpine orogen (Schmid et al., 2004,
Ratschbacher et al., 1991). This extension in turn led to a sinking movement of the Adriatic
plate east of the Tauern Window and therefore to the polarity change of the subduction
zone (Schmid et al., 2004, Regard et al., 2008).
Pfiffner (2010, Fig. 1, at the end) as well as Nagel et al. (2013) show the Tauern Window with
bulging thrusts on top of the Helvetic Tauern massif (Fig. 6). This proves that the inversion
was preceded by the loading of the massif with Penninic and Austroalpine nappes from the
South.
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Figure 6: a) top view of the Eastern Alps, b) detail: the Tauern Window with profile line A-A’, c) profile A-A’
through Tauern Window displaying bulged nappes and overthrusts, d) corresponding lithological column
showing thrust nappes (Nagel et al., 2013)
IV. OUTLOOK
In the 1990s the responsible national institutions produced precise levellings for France,
Austria and Switzerland (Institut Géographique National, Bundesamt für Eich- und
Vermessungswesen Österreich, Bundesamt für Landestopografie swisstopo). Pfiffner (2010)
combined the results of the different projects in one map (Fig. 7). The recent uplift and
subsidence tendencies show that the collision of the Adriatic and European plate has not
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Matriculation no.: 295386
ceased. North of the Eastern Alps the Molasse basin subsides while uplift is concentrated at
the border to the Central Alps. Uplift maxima are located at the eastern and western margin
of the Tauern window. Fission track dating revealed that the recent vertical movement
tendencies have been present for a few million years.
Figure 7: Detail of map displaying regions of relative uplift or subsidence, dotted areas = subsidence, areas with
crosses and dots = uplift > 1 mm/a, red plusses = uplift maxima, red minuses = subsidence maxima, black stars
= point of reference for national levelling (Pfiffner, 2010)
The plate movements in general are directed to the NNW in the Eastern Alps (Fig. 8) but with
a discrepancy between the northern and southern boundary of the whole orogen. While the
northern outline is pushed to the North by 0.7 mm/a, the southern border has much greater
velocity of 1.2 mm/a. The consequence is a horizontal shortening of the whole mountain
range by 0.5 mm/a along a NNW-SSE axis.
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Matriculation no.: 295386
Ostalpen
Figure 8: Map of recent plate movements with thrusts, normal faults and strike slip faults, white rectangle
being the Eastern Alps (Pfiffner, 2010)
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V.
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REFERENCES
Oberhauser, R., Bauer, F. K., 1980. Der geologische Aufbau Österreichs. Springer Verlag. Wien, New York.
Pfiffner, O. A., 2010. Geologie der Alpen. Haupt Verlag. Bern, Stuttgart, Wien.
Horninger, G., Weiss, E. H.: Engineering geology in mountainous regions. Abh. Geol. Bundesanst 34 (1980). p. 257-286.
Konzett, J., Hoinkes, G.: Paragonite-hornblende assemblages and their petrological significance: an example from the
Austroalpine Schneeberg Complex, Southern Tyrol, Italy. In: Journal of Metamorphic Geology 14. (1996). p. 85-101.

Ratschbacher, L., Frisch, W., Linzer, H. G.: Lateral extrusion in the Eastern Alps: Part II: Structural analysis. In: Tectonics 10.
(1991). p. 257-271.

Regard, V., Faccenna, C., Bellier, O., Martinod, J.: Laboratory Experiments of Slab Break-off and Slab Dip Reversal: Insight into
the Alpine Oligocene Reorganization. In: Terra Nova 20. (2008). p. 267-273.

Stöckhert, B., Gerya, T. V.: Pre-collisional high pressure metamorphism and nappe tectonics at active continental margins: a
numerical simulation. In: Terra Nova 17. (2005). p. 102-110.

Schmid, S. M., Pfiffner, O. A., Froitzheim, N., Schönborn, G., Kissling, E.: Geophysical-geological transect and tectonic evolution
of the Swiss-Italian Alps. In: Tectonics 15. (1996). p. 1036-1064.

Schmid, S. M., Fügenschuh, B., Kissling, E., Schuster, R.: TRANSMED transects IV, V and VI: Three lithospheric transects across
the Alps and their forelands. Cavazza W, Roure F, Spakman W, Stampfli GM, and Ziegler PA (eds) The TRANSMED Atlas: The
Mediterranean Region from Crust to Mantle. (2004). Springer Verlag.
Figures:

Nagel, T. J., Herwartz, D., Rexroth, S., Münker, C., Froitzheim, N., Kurz, W.: Lu-Hf dating, petrography and tectonic implications
of the youngest alpine eclogites (Tauern Window, Austria). In: Lithos 170-171. (2013). p. 179-190.
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http://www.sciencedirect.com/science/article/pii/S0024493713000406, 19.08.13
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http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-the-alps-part-1-generalremarks-austroalpine-nappes, 15.08.13
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http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/Abb.57.jpg/image_preview, 19.08.13
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http://earth.unibas.ch/tecto/research/Alps_tecto.png, 20.08.13
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Matriculation no.: 295386
Nappe stack, Northern Calcareous Alps
Tauern massif
Lower and Upper European crust
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Lower and Upper
Adriatic crust
Lithosperic mantle
Figure 1: Profile of the Eastern Alps striking N-S (after Pfiffner, 2010)
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Matriculation no.: 295386
SEMP strike slip fault
Austroalpine Mesozoic sediments
Austroalpine Paleozoic sediments
Penninic nappes
Austroalpine crystalline
basement
Vinschgau shear zone
Figure 2: Tectonic map of the Eastern Alps, modified by the author (http://earth.unibas.ch/tecto/research/Alps_tecto.png, 20.08.2013)
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Matriculation no.: 295386
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Figure 3: Metamorphosis map with numbers displaying metamorphic ages in Ma; small dots: eclogite facies, large dots: blueschist facies, narrow diagonals: amphibolite facies,
intermediate diagonals: greenschist facies, large diagonals: anchizone; eastwards dipping diagonals = Cenozoic, westwards dipping diagonals = Cretaceous (modified by author,
after Pfiffner, 2010