Evolution of the Eastern Alps - RWTH Aachen University · Evolution of the Eastern Alps C. SCHMIDT Field school “Alps” (26/08/2013 – 05/09/2013) CONTENTS: Introduction Present

Christina Schmidt Matriculation no.: 295386

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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.

1 http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-the-

alps-part-1-general-remarks-austroalpine-nappes, 15.08.13 2 http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-the-

alps-part-1-general-remarks-austroalpine-nappes, 15.08.13


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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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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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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 (Albian-

Turonian). 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 120 Juvavian nappes Tirolian nappes

Aptian – Albian 110-100 Lechtal nappe Allgäu nappe (Bajuvarian nappes)

End of Cenomanian 94 Inntal (Tirolian nappe) Lechtal (Bajuvarian nappe)

End of Turonian 89 Reactivation of Lechtal-overthrust

End of Maastrichtian 65 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

3 http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-the-

alps-part-1-general-remarks-austroalpine-nappes, 15.08.2013


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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.uni-

bonn.de/arbeitsgruppen/strukturgeologie/lehre/wiss

en-gratis/Abb.57.jpg/image_preview, 19.08.2013)

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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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-the-

alps-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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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-Mariazell-

Puchberg) strike slip fault (Fig. 2, at the end).


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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.

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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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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Figure 8: Map of recent plate movements with thrusts, normal faults and strike slip faults, white rectangle

being the Eastern Alps (Pfiffner, 2010)

V. 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. http://www.sciencedirect.com/science/article/pii/S0024493713000406, 19.08.13

http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/geology-of-the-alps-part-1-general-

remarks-austroalpine-nappes, 15.08.13

http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/Abb.57.jpg/image_preview, 19.08.13

http://earth.unibas.ch/tecto/research/Alps_tecto.png, 20.08.13

Ostalpen


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Figure 1: Profile of the Eastern Alps striking N-S (after Pfiffner, 2010)

Tauern massif

Nappe stack, Northern Calcareous Alps

Lower and Upper European crust

Lithosperic mantle

Lower and Upper

Adriatic crust


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Figure 2: Tectonic map of the Eastern Alps, modified by the author (http://earth.unibas.ch/tecto/research/Alps_tecto.png, 20.08.2013)

Austroalpine Mesozoic sediments

Penninic nappes

Austroalpine Paleozoic sediments

Vinschgau shear zone

SEMP strike slip fault

Austroalpine crystalline

basement


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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

Documents

Evolution of the Eastern Alps - RWTH Aachen University · Evolution of the Eastern Alps C. SCHMIDT Field school “Alps” (26/08/2013 – 05/09/2013) CONTENTS: Introduction Present