The history of alpine tectonics and a visit to the Tauern window

Soon after the systematic geological mapping of the Alps and with the establishment of a stratigraphic column between 1830 to 1880 geologists get interested how mountain ranges form. The first geological studies of sedimentary successions in France and United Kingdom required only vertical movements to be explained, movements thought to result by igneous intrusions in the underground (idea proposed by Hutton) or by the contraction of a cooling earth. The German geologist von Buch related the growth of mountains and folding to igneous bodies, intruding and displacing older sediments. In contrasts the French geologist Elie de Beaumont developed a theory involving a periodic contraction of earth, resulting in volcanoes and earthquakes and tectonic movements. The periodicity of these movements were based on his observations of different tilted sediment layers, formations, deposition and presumed erosion phases.

Fig.1. De Beaumont’s two classes of sedimentary deposits, constraining the age of sudden upheaval pulses of mountain ranges: (1) previously horizontal beds (b), tilted up and contorted on flanks of rising core (a), and younger flat beds (c) extending up to the foot of the chain;(2) in this case, also beds (c) are disturbed and flanked by new horizontal deposits (d) (from DalPIAZ 2001).

The hypothesis of Beaumont becomes quickly accepted by the majority of European and American geologist, besides being in contradiction of the slow processes invoked by Lyells gradualism. With the introduction of the concepts of nappes and mountain fold-belts in geology by the observations of various geologists during the late 19th century, like the Austrian geologists Eduard Suess and Otto Ampferer, the picture become more complicated, tangential movements of earth crust seemed necessarily to explain these features.

Until them contorted relationships of different formations were interpreted as results of folding, Marcel Bertrand, a young French geologist that never worked in the Alps, reinterpreted in 1884 completely one of the classic profiles of the Glarus double fold drawn by the Swiss geologist Heim.

Fig.2a. Section with the "double fold" through part of the Glarus Alps, by Albert Heim, from Livret- Guide Géologique, 1894, figure from FRANKS & TRÜMPY 2005.

Fig.2b. The Glarus overthrust as depicted in a watercolour by the geologist H.C. Escher in 1812 (figure from PFIFFNER 2009). The thrust forms the contact between older (Helvetic) Permo-Triassic rock layers of the dark Verrucano (Permian - Triassic sandstones and conglomerates) group and younger (external) Jurassic and Cretaceous white limestones and Paleogene flysch and molasse.

Bertrand replaced the double fold by a single nappe, laterally displaced over 40km - but his paper was widely ignored. Suess after an excursion to the Glarus Alps in 1892 was inclined to Bertrand idea, but even he failed to persuade the dominant geological establishment. With the works of the Swiss Geologist Hans Schardt in 1893 and 1898, where he demonstrated that some prominent mountains of the external Swiss Alsp represents the eroded remains of much larger cover nappes, the existence of nappes, not after great aversions, become accepted. Pierre Termier in 1904 extended the nappe structure to the Eastern Alps and developed the concept of tectonic windows.

Fig.3. The geologist Argand adopted between 1909 and 1934 the idea of nappes in the geology of the Alps, here a generalized view of the Europe-vergent Alpine belt. Note that the Eastern Alps (4) override the western Alpine nappe stack (2-3), and its root zone is indented and back-folded by the Southalpine hinterland, in turn deformed by south-vergent thrust. The Western Alps consist of ophiolite-bearing cover sequences (3) and Penninic nappes (2), squeezed out from the contraction of Alpine geosyncline (I-III: Simplon-Ticino nappes;IV-V-VI: Gran St. Bernard-Monte Rosa-Dent Blanche nappes), and overthrown onto the sliced (a-b: Helvetic basement) and undeformed (c) European foreland (1) (from DalPIAZ 2001).

To the east of the Brenner Pass the south-western branches of the Zillertal Alps reach the South Tyrolean realm. Here a sudden and exceptional lithological change can be observed, crystalline schist's lay in contact with a succession of marbles (Fig.5. and 6.) and metamorphic sandstones and breccias, followed by green calc-schists and serpentinites (Fig.4.) and amphibolites, and finally a vast area of gneiss, a rock that is more resistant to erosion and forms a characteristic landscape with prominent peaks.

Fig.4. Prasinite and calc-schist boulder (geologic map) of the "Bündner Schiefer".

Fig.5. Dolomitic marbles (Jurassic) of the Tristenspitze (geologic map) "sitting" on thrusted gneiss.

Fig.6. Dolomitic marbles (Jurassic) of the Tristenspitze.

This varied succession contrasts strongly with the surrounding monotone schist's, and aroused the interest of generations of geologists.
The schist represents former Ordovician to Devonian marine sediments.
The metamorphic sediments (marbles and metamorph clastic formations) were interpreted as lithologies of an early Permomesozoic and a later Mesozoic ocean (Penninic Ocean), becoming in the Cainozoic sandwiched between the two collision fronts of the European continent and various plate fragments of the Adriatic realm.
The central gneiss, denominated appropriately "Zentralgneis" or Tauern gneiss, comprises a large number of metamorphic rocks, mainly of granitic origin intruded 250 million years ago in the Permomesozoic sediments, and is part of the European basement.

Fig.7. The Tauern gneiss (Central gneiss) in the background surrounded by tectonized and eroded gneiss in the foreground. From the central gneiss it is possible to traverse the ocean floor magmatites, then sediments, to arrive in the tectonic overthrusted older metamorphic and monotone schists of the Austroalpine units.

During the Alpidic orogenesis the sediments were overthrusted by the Austroalpine units, which constitute the frame around the window today.
Thus, the window allows insight into the Penninic units, the deepest tectonic units of the Eastern Alps. This extraordinary situation designated the Tauern window as a key of understanding the Alpine nappe stack since early times of research.
The exhumation of the Tauern window occurred mainly in the Oligocene and Miocene, tectonic units east of the window were pushed further to the east, a phenomenon designated as lateral extrusion.

A sign of ongoing tectonic activity are earthquakes, which occur predominantly on the western boarder (Brenner Pass and Wipp- and Eisacktal, following the fault zone of the Periadriatic lineament), the northern boarder (Inntal thrust zone), and in minor entities and magnitude on the southern limit (Defreggen-Antholz-Pustertal thrust zone).
A new published research (PLAN et al. 2010), carried out in a cave in Styria, dated earthquake-damaged speleothems and scratched flowstone to as recent as the last glacial maximum, between 118ka an 9ka, suggesting that the Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) fault, a major strike-slip system in the European Alps developing from the Inntal fault north of the Tauern Window, is still active, and lateral extrusion of Alpine units on a large scale in direction of the Pannonian Basin are still ongoing.

References:

DalPIAZ, G.V. (2001): History of tectonic interpretations of the Alps. Journal of Geodynamics 32: 99-114
FRANKS, S. & TRÜMPY, R. (2005): The Sixth International Geological Congress: Zürich, 1894. Episodes, Vol. 28(3): 187 - 192
PLAN, L.; GRASEMANN, B.; SPÖTL, C.; DECKER, K.; BOCH, R. & KRAMERS, J. (2010): Neotectonic extrusion of the Eastern Alps: Constraints from U/Th dating of tectonically damaged speleothems. Geology v.38(6): 483-486.
PFIFFNER, O.A. (2009).Geologie der Alpen.Haupt Verlag Bern-Stuttgart-Wien: 359