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Home , Austroalpine nappes, Eastern Alps, Engadin window, Geology of the Alps, Helvetic (geology), Helvetic nappes

Christina Schmidt Matriculation no.: 295386

Evolution of the Eastern

C. SCHMIDT

Field school “Alps” (26/08/2013 – 05/09/2013)

CONTENTS:  Introduction  Present state of the  Development toward the status quo  Outlook  References

I. INTRODUCTION Depending on which author is consulted the Alps are subdivided into Eastern, Western, Central and . 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 1 easily recognizable between the Swiss town St. Margrethen, situated south of the , the city , 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 through the Rhone Valley to the Swiss town Martigny, along the Great St. Bernard Pass through the Aosta Valley to the Italian town 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: /Valtelline Valley, Pustertal/ and Gailtal/Gail Valley. The geologic distinction between , , Austroalpine and Southalpine relies on the paleogeographic domain in which the corresponding were formed (Pfiffner, 2010). In the Eastern Alps there are of ; their 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 .

1 http://www.steinmann.uni-bonn.de/arbeitsgruppen/strukturgeologie/lehre/wissen-gratis/-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 Christina Schmidt Matriculation no.: 295386

II. PRESENT STATE OF THE EASTERN ALPS Today the surface of the Eastern Alps is primarily constituted of Austroalpine 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 , 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 , Helvetic rocks and Penninic sediments underlies the thrust plane at the base of the . The crystalline 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 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, and mica , 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 2 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 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 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 . This strike slip zone hosts a range of plutons (see the paper Alpine 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 in the West, Christina Schmidt Matriculation no.: 295386 towards instead the Penninic nappes which are characterized by Rhenodanubic Flysch sediments (Pfiffner, 2010). Noticeable features in the Eastern Alps are the Tauern and 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 only exhibits Penninic nappes from the former Piedmont Ocean.

Pfiffner observed high pressure conditions (HP) during metamorphosis at various sites in the Eastern Alps. rocks can be found in the far SE Austroalpine crystalline basement between Graz and Klagenfurt in . 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 (110-90 Ma) – an age that is not present in any other metamorphic 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 alteration surrounds the amphibolite facies and spreads all over the Eastern . 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 facies is an exception in the Austroalpine rocks and is only present as a narrow zone encircling the SW of the Tauern Window. It is more 3 common in Penninic nappes, visible in the Tauern and Engadine Window. There the metamorphism dates back to the 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 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 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 is reasonable.

Christina Schmidt 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 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 (Albian- Turonian). Stöckhert and Gerya (2005) explain this phenomenon with the accumulation of an that is not only formed out of oceanic sediments scraped off the lower plate but foremost out of continental material derived by subduction . Radiometric estimates revealed age differences in the metamorphosed oceanic that were probably caused by 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 4 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 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 Christina Schmidt 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 (), Lower Austroalpine Ela nappe (http://www.steinmann.uni- bonn.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 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 Trough merged. The movement of the Eastern Alps independent from the Southalpine nappes and the Piedmont oceanic Christina Schmidt 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-the- alps-part-1-general-remarks-austroalpine-nappes, 15.08.2013) 6

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 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 (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. Christina Schmidt 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 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 (from Piedmont ) 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 (it is classified as a separate deformation phase of the , 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 7 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 (-Ennstal-Mariazell- Puchberg) strike slip fault (Fig. 2, at the end). Christina Schmidt 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 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 , 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 tendencies show that the collision of the Adriatic and European plate has not Christina Schmidt Matriculation no.: 295386 ceased. North of the Eastern Alps the 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 9 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. Christina Schmidt 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) 10

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 , . 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: 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

Christina Schmidt Matriculation no.: 295386

Nappe stack, Northern Calcareous Alps

Tauern massif

Lower and Upper European crust

11 Lower and Upper Adriatic crust

Lithosperic mantle

Figure 1: Profile of the Eastern Alps striking N-S (after Pfiffner, 2010) Christina Schmidt Matriculation no.: 295386

SEMP strike slip fault

Austroalpine Mesozoic sediments

Austroalpine Paleozoic sediments

Penninic nappes 12

Austroalpine crystalline basement

Vinschgau

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

Christina Schmidt 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