Dieter Hubich^* 1, Andreas L. Läufer ², Jörg F.A. Loeschke ¹

Extensive field mapping, structural-geological, sedimentological, as well as illite "crystallinity" data reveal a new interpretation of the geodynamic evolution of the Southalpine Variscan basement of the Carnic Alps. Based on these results and on comparisons with other parts of the European Variscides, we present a plate tectonic model for the southern margin of the Variscan orogen.

The Variscan basement of the Carnic Alps is divided into the anchizonal metamorphic central and eastern Carnic Alps and the epizonal metamorphic western Carnic Alps. The boundary between these units is approximately located in the area of the Bordaglia fault zone.

In contrast to previous nappe concepts we distinguish three Variscan nappes (see figure 1) which differ in their stratigraphic, sedimentological, deformational and metamorphic evolution: (1) Fleons nappe, (2) Cellon-Kellerwand nappe, and (3) Hochwipfel nappe:

(1) The westernly located Fleons nappe generally correlates with the western Carnic Alps. It shows a continuous stratigraphic sequence of Ordovician to Devonian or presumably early Carboniferous sedimentary rocks of a passive continental margin. The decrease of grain size and quartz content of the Ordovician Val Visdende schists from south to north represent the transition from coastal deposits in the south to deposits at a position close to the shelf edge in the north. The early Carboniferous intraplate basalts of the Raabtal indicate the climax of the Variscan rifting. These strongly altered basalts are overlain and reworked by a newly discovered flysch unit, the Streifwald flysch. According to field mapping south of Maria Luggau, the intraplate basalts, Streifwald flysch, and pelagic carbonate rocks are imbricated at an accretionary wedge over a north-directed subduction zone.

(2) The Cellon-Kellerwand nappe comprises Plenge and Gamskofel massiv, Biegengebirge, Seewarte-Hohe Warte massiv and Cellon-Kellerwand group, the area of Plöckenpass and Pizzo di Timau, and Monte Zermula-Rosskofel massiv. As the Fleons nappe, the Cellon-Kellerwand nappe reveals a continuous late Ordovician to Lower Carboniferous stratigraphic sequence, ranging from Ordovician shelf deposits, Devonian reef carbonates to early Carboniferous flysch sediments (Cellon-Kellerwand flysch). The flysch covers and reworks the carbonate pre-flysch sequence, which was subsided probably below the CCD and dissolved by CO₂-rich currents in deep sea environment. The subsidence was caused by intensified rifting that started in Frasnian time indicated by the break-up of the carbonate platform and deposition of pelagic limestone. The nappe contact between Fleons nappe and Cellon-Kellerwand nappe is marked by a klippen zone, where remnants of the higher-metamorphic Fleons nappe rest on top of the lower-metamorphic Cellon-Kellerwand nappe. Duplex structures lead to a tectonic mixing of the lower and the upper nappe. The already weakened zones in the crust were repeatedly reactivated in late-Variscan and Alpine times by extensional and contractional tectonics along the Bordaglia fault.

(3) The Hochwipfel nappe is characterized by wildflysch deposits (Hochwipfel flysch), which contain all Ordovician to early Carboniferous pre-flysch units as olistolites and hence does not show a continuous stratigraphic sequence in contrast to the other two nappes. Due to resedimentation processes at an active continental margin the Hochwipfel flysch appears to be more mature and rich in quartz than the feldspar-rich Cellon-Kellerwand flysch of the Cellon-Kellerwand nappe. The Hochwipfel nappe can be subdivided into a northern proximal unit with several huge olistolites, which is thrust over a southern distal unit overlying the Lower Carboniferous intraplate basalts of the Dimon Formation. The volcanics of the Dimon Formation are presumably younger than the basalts of the Raabtal in the western Carnic Alps, but formed in a similar geotectonic position. They represent the peak of extension of the passive continental margin prior to contraction. The nappe contact between the Cellon-Kellerwand nappe and the Hochwipfel nappe is marked by broad mylonitic thrust planes and duplex structures.

A first ductile deformation event (D1, presumably Visean/Namurian) and epizonal metamorphism (Tmax 300-450°C) affected only the Fleons nappe. A roughly N-S oriented stretching lineation of quartz and calcite lies within around E-W striking penetrative foliation planes. Shear sense indicators in the XZ plane of the finite strain ellipsoid indicate top-to-south thrusting. A second deformation event (D2, presumably Namurian/Westfalian) affected the Fleons nappe, Cellon-Kellerwand nappe and finally the external Hochwipfel nappe under anchizonal metamorphic conditions (Tmax 260-300°C). Corresponding structures are south-verging thrusts with thick quartz mobilisates and tight folds associated with an axial planar cleavage. In the Fleons nappe, incompetent layers additionally reveal small-scale folds in the order of some cm. A stretching lineation, as during D1, did not develop, neither in the Fleons nappe, nor in the other two nappes. Both deformations are of Variscan age, because their styles and metamorphic grades differ from the ones in the less intensely deformed post-Variscan cover rocks.

The Eder unit, Luggau unit as well as the area of the Mauthner Klamm at Kötschach Mauthen do not represent separate Variscan nappes, as previously suggested, but represent late-Alpine tectonic blocks, which are genetically linked to activities along the Periadriatic lineament. The rocks of these units show a strong E-W oriented, subhorizontal stretching lineation within steeply dipping foliation planes oriented subparallel to the Periadriatic lineament. Shear sense indicators in the XZ plane of the finite strain ellipsoid indicate dextral shear. The Variscan structures are overprinted by the Alpine deformation. However, the deformational and metamorphic influence of the Periadriatic lineament decreases gradually towards the south.

We interpret the stratigraphic sequences and the arrangements of the three nappes in a plate tectonic model which shows first rifting processes during the Upper Ordovician, Silurian, Devonian and the Lower Carboniferous (prior to the Middle Visean), the formation of a passive continental margin, after that the onset of wild flysch sedimentation along an active continental margin during subduction and then the collision between the northern margin of Gondwana and a microcontinent to the north.

The beginning of rifting can best be demonstrated by bimodal volcanism in the Fleons nappe where Upper Ordovician acidic pyroclastic rocks (Comelico porphyroid) and both clastic pyroxene grains and basic volcanic fragments in greywackes of the Fleons formation indicate the occurrence of acidic and basic volcanism. This is a characteristic Ordovician rifting period which can also be observed in other places along the northern margin of Gondwana, for example in the Cantabrian Mountains (NW Spain) where alkali-basaltic dykes and tuffaceous layers occur in Cambro-Ordovician sedimentary rocks. During the Silurian and Devonian, a widespread facies differentiation in the Carnic Alps is typical for extensional tectonism and horst-and-graben structures on continental crust. These extensional processes are also known from places in the Eastern Alps (Gurktal nappe, Graz Palaeozoic, Northern Greywacke Zone). There is no doubt that the rifting processes increased in intensity until the Upper Devonian when the well-known nodular limestones of the Cellon-Kellerwand nappe und equivalent deep water limestones in the Hochwipfel flysch (now as olistolites) developed. The subsidence continued until the Lower Carboniferous when alkali-basaltic submarine basalt flows and subaerial pyroclastics erupted in a plate tectonic setting which could have been similar to an oceanic island setting (Raabtal intraplate basalts, Dimon formation, alkali intraplate basalts of Eisenkappel / Karawanken Mountains). Whether true oceanic crust ever developed in the Carnic Alps and Karawanken Mountains during the Lower Carboniferous is not quite clear. An argument for the formation of oceanic crust is that there is not a single outcrop known in the whole area where the proven stratigraphic basement of the alkali-basalts can be seen. It might have been the oceanic mantle. However, the climax of the extensional period was reached during the Lower Carboniferous. This is a time when the last pre-flysch basalts of the Variscan mountain chain erupted also in other areas, i e. the so-called "Deck-Diabas" in the Rhenish Schiefergebirge at the northern margin of the Variscides. We therefore see a common plate tectonic development at the southern and northern margin of the Variscides during the Devonian and Lower Carboniferous inside and outside the Alpine domain with rifting and crustal thinning before the beginning of flysch sedimentation.

The sedimentation of turbidites in the three flysch basins of the Carnic Alps (Streifwald flysch, Cellon-Kellerwand flysch, Hochwipfel flysch) indicate the beginning of subduction of oceanic or thinned continental crust to the north under an active continental margin which is now represented by the metamorphic domains of the Eastern Alps north of the Periadriatic lineament. In comparison with northern parts of the Variscides (Rhenish Schiefergebirge) where the flysch deposition migrated from south to north - and having the bilateral symmetry of the Variscan orogen in mind – it could be concluded that flysch deposition in the Carnic Alps migrated from north to south so that the Streifwald flysch is the oldest and the Hochwipfel flysch the youngest flysch deposit. The turbidites on top of the Eisenkappel pillow basalts (Karawanken Mountains) are interpreted as an equivalent of the Streifwald flysch. They are therefore considered to be of Lower Carboniferous age and older than the Hochwipfel flysch in the Seeberg area. A similar development of an active continental margin with the formation of huge olistolites within a flysch sequence at the southern margin of the Variscides can be seen in the Montagne Noire.

The youngest flysch deposits (Hochwipfel flysch) contain the biggest olistolites indicating the increasing uplift of the hinterland due to the transition from subduction to collision. The suggestion of the migration of flysch basins from north to south within the southern half of the Variscides is supported by the formation of Upper Devonian to pre-Upper Visean flysch clastics in the southern Black Forest which contain the erosional fragments of a volcanic arc.

The three nappes of the Carnic Alps containing the deposits of a rift and passive continental margin sequence of Ordovician to Lower Carboniferous age and an active continental margin flysch sequence of Middle Visean to Namurian age were finally thrust upon each other forming a south verging nappe complex or fold-and-thrust belt during the collisional stage in the Upper Namurian to Lower Westfalian whereby the previous depositional basins were closed. Clear unconformities between Permian red clastics and greenschist facies metamorphic basement rocks of the Fleons nappe prove this collision. However, in the eastern parts of the Carnic Alps and in the Karawanken Mountains this unconformity is less clear. The basin of the Upper Carboniferous Auernig formation is therefore interpreted as a successor basin of the Hochwipfel flysch which has continuously been filled up with clastics and some marine limestones. In this area, a clear-cut unconformity between the two series can only be seen in some places.

Figure 1 Schematic geological map of the Carnic Alps, showing the three Variscan nappes.