Jurassic Austroalpine .doc

Only in the westernmost part of the Austroalpine extensional tectonics led to the formation of the newly formed eastern passive continental margin of the Penninic Ocean. In Middle Jurassic times the situation generally changed due to the partial closure of the Neotethys Ocean. The Austroalpine became in that time a lower plate position.

Westdirected nappe thrusting and the formation of deep-water trenches in front of the prograding nappes crosscut the former facies belts. Tectonic shortening decreased in Late Jurassic times. New shallow-water carbonate ramps and platforms established, they seal the main tectonic shortening and prevail untill the Early Cretaceous.

Northern Calcareous Alps

At the Triassic/Jurassic boundary the carbonate production rate was significantly reduced in connection with the environmental crisis leading to mass extinction. A lack of sufficient sediment supply led to the drowning of the Hauptdolomite/Dachstein Carbonate Platform.

Later on a horst and graben morphology developed (Bernoulli & Jenkyns, 1974; Eberli, 1988; Krainer et al., 1994) and triggered breccia formation along submarine slopes and escarpments, in the Northern Calcareous Alps mainly in Pliensbachian to Early Toarcian times (Böhm et al., 1995) (Fig. 1).

Breccia formation in late Early Jurassic times was mostly interpreted due to the opening of the Penninic Ocean (e.g., Bernoulli & Jenkyns, 1974; Eberli, 1988; Krainer et al., 1994).

Whereas the early Early Jurassic sequences of the Lower Austroalpine show the typical features of a rifted margin (e.g., Eberli, 1988), the Bavaric and Tirolic units were only slightly influenced by these rifting processes.

In contrast, late Early Jurassic tectonic movements affected mainly the Tirolic unit and resulted in a completely new palaeogeographic setting, whereas the Bavaric to Lower Austroalpine units show only mild or no influence of these tectonic processes (Fig. 1). This change in the late Early Jurassic was interpreted by many authors also as a result of the opening of the Penninic Ocean (e.g., Eberli, 1988, 1991; Krainer et al., 1994).

In contrast, Frisch & Gawlick (2003) and Missoni & Gawlick (in press) attributed this “event” to the onset of subduction in the Neotethys Ocean.

Due to the subduction processes in the Neotethys realm and contemporaneous out-of-sequence thrusting in the more continentward parts (Juvavic and Tirolic nappes), the sedimentation pattern in the Northern Calcareous Alps changed dramatically in the Middle Jurassic (Gawlick & Frisch, 2003), and not in the Oxfordian as formerly assumed (Tollmann, 1985).

The main difference of Hallstatt and Tauglboden Mélanges was the earlier onset and the different composition of huge mass-flows in the Hallstatt Mélange basins. The mélanges are interpreted as carbonate-clastic-radiolaritic trench fills formed in sequence due to the closure of parts of the Neotethys Ocean.

The Plassen Carbonate Platform (Late Oxfordian to Early Berriasian) developed above the trenches in a shallowing-upward cycle during tectonically active periods in a convergent regime (Gawlick & Schlagintweit, 2006).

In the Late Kimmeridgian to Early Berriasian huge masses of shallow-water carbonates were formed. The platform carbonates are covered by calpionellid-radiolaria wackestones to packstones, of Late Berriasian age. A siliciclastic influenced drowning sequence (Schrambach Formation) sealed the highly differentiated Plassen Carbonate Platform.

The onset, evolution and drowning of the Plassen Carbonate Platform took place in a tectonic active regime. The tectonic evolution of the Northern Calcareous Alps during Kimmeridgian to Berriasian times and the final drowning of the Plassen Carbonate Platform can be interpreted as a result of further tectonic shortening and uplift of the accretionary prism after the closure of parts of the Neotethys Ocean.

This led to erosion of siliciclastic material, which reached at this time the inner parts of the Northern Calcareous Alps.

Bavaric nappe group