Variscan geodynamic evolution of the Carnic Alps (Austria/Italy)

Abstract

The South-Alpine Carnic Alps are part of the southern flank of the European Variscides and display a continuous sedimentary record from Late Ordovician to Devonian times followed by Carboniferous S-directed nappe stacking and Late Carboniferous to Early Permian post-collisional collapse. The tectonometamorphic and sedimentary evolution of the Carnic Alps resembles a continuous process where pre-and syn-orogenic volcanism, syn-orogenic flysch sedimentation, deformation including nappe stacking, metamorphism and tectonic collapse shift in age from internal zones in the N towards external zones in the S. New structural, petrological and sedimentological data are presented concerning the tectonometamorphic history of the Carnic Alps. We distinguish three thrust sheets or tectonic nappes differing in their stratigraphic, sedimentological, deformational and metamorphic histories which were thrust over each other in Carboniferous times. Our data lead to a new geodynamic model showing an evolution from rifting or backarc spreading in the Late Ordovician to the establishment of a mature passive continental margin in the Late Devonian/Early Carboniferous, flysch sedimentation in an active continental margin setting during the Visean/Namurian and finally collision during the Late Carboniferous between the northern margin of Gondwana and a microcontinent to the N.

Polyphase metamorphism in the eastern Carnic Alps (N Italy–S Austria): clay minerals and conodont Colour Alteration Index evidence

“…the Palaeocarnic Chain) can be split into Venturini et al 2001Venturini et al -2002 showing sampled localities: small squares conodontproductive limestones providing Colour Alteration Indices (CAI); small triangles pelitic samples providing Kübler Indices two zones (W and E) marked by different deformative characters and history. The W Zone (Passo di Monte Croce Comelico-Forni Avoltri) consisting of Early Ordovician-Early Carboniferous succession is known as epimetamorphic (Hubich et al 2000;Läufer et al 2001;Sassi et al 2004). The E Zone (Forni Avoltri-Tarvisio) represented by Late Ordovician-Late Carboniferous successions is stated as non-to anchimetamorphic with minor epizonal areas (Läufer 1996;Rantitsch 1997;Sassi et al 1995).…”

“…Then the sedimentary style changed from carbonate sedimentation to siliciclastic turbidites (Hochwipfel Formation) and, subsequently, accumulation of volcanics interpreted as intraplate alkali-basalts (Spalletta et al 1980;Rossi and Vai 1986;Läufer et al, 2001) and volcaniclastics (Dimon Formation). This cycle continued until the late Bashkirian (Westfalian A).…”

“…The Carnic Alps E zone region has undergone fold and thrusts tectonics during the Variscan compressive phase (late Bashkirian-Moscovian) resulting in structures trending N120°E (Venturini 1990;Läufer et al 2001). …”

“…The basins closed at the Guadalupian-Lopingian boundary either due to transpressional reactivation of synsedimentary faults (Venturini 1991), or to continuing Variscan orogeny (Läufer et al 2001). Unconformably overlying that sequence is the famed Late Permian-Late Triassic carbonate-dominated Southern Alpine succession whose accumulation has been suggested to have been controlled by Late Permian crustal extension and by break-up unconformities during the Triassic (Cassinis et al 2000).…”

“…Previous workers (Schramm 1991(Schramm , 1992Sassi et al 1995Sassi et al , 2004Á rkai et al 1995;Läufer et al 1997Läufer et al , 2001Rantitsch 1997;Venturini and Spalletta 1998;Rantitsch and Russegger 2000;Pondrelli 2002) hypothesize at least the presence of two metamorphic episodes, Variscan as well as Alpine, although with some reservation regarding allocation of phenomena to one cycle rather than the other.…”