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 1 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 Christina Schmidt 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 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 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, Christina Schmidt 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 3 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. 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 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