Tectonic mélanges formed of continental rocks in the Tauern Window

Tectonic mélanges formed of oceanic rocks provide key observations on the chemical, thermal and mechanical processes active at the plate interface of oceanic subduction zones. Mélanges formed of continental rocks are less documented, though they potentially provide similar observations pertaining to continental subduction and collision. Here, we present a geometric and lithological characterization of tectonic mélanges formed of continental rocks based on comprehensive mapping in the Tauern Window (Rauris and Zirknitz valleys, Austria).

The investigated melanges comprise mostly well identifiable cover and basement units formed prior to the opening of the Alpine Tethys, occurring juxtaposed without lithostratigraphic coherence. They are strongly deformed and show systematic top-to-the-N shearing consistent with the subduction and collision kinematics. Field observations allow to distinguish four mélange types based on their thickness, the degree of continuity, the tectonic position as well as the occurrence of a matrix and its nature.

Type 1 mélanges are continuous, several hundreds of meters thick and located directly above thick and coherent continental nappes. The matrix is composed of Carboniferous and Jurassic-Cretaceous schists. Within it are found large lenses of metasediments from all known Permo-Triassic lithostratigraphic units, together with rare orthogneiss. A Type 2 mélange occurs as a single 100-200 m thick continuous unit between oceanic nappes. The matrix is composed of Jurassic-Cretaceous schists containing smaller lenses of the same lithologies as described for type 1 mélanges. Additionally, a discontinuous and heterogeneous slab of orthogneiss occurs. In contrast, type 3 mélanges are characterized by the absence of a matrix and a thickness of less than 20 m. They are built of slivers composed of a single lithostratigraphic unit that alternate within the mélange at meter to kilometer scale. Metasediments of possible oceanic origin are locally observed. Finally, type 4 mélanges are strictly speaking not mélanges. They correspond to strings of 1-5 m thick boudins of Triassic marble found within oceanic nappes.

We interpret the four identified mélange types as a mixture of cover and basement units scraped off the down going plate during continental subduction. The evolution from mélange type 1 to type 4 corresponds to an increase of strain. The occurrence of up to three mélanges sandwiched between oceanic nappes on each investigated profile, indicate out-of-sequence thrusts within the hanging wall oceanic unit active during collision subsequent to continental subduction.

The units described here are not an exception in the Tauern Window. A mélange of type 3 can be followed over more than 100 km all around the Eastern Dome. Additionally, the Eclogite Zone shares many similarities with the type 2 mélange. As such, our observations raise several questions: Are mélanges formed of continental rocks widespread in the Alpine-Mediterranean orogenic system? Why are continental elements scraped off whereas deformation is expected to localize in weak oceanic metasediments or serpentinite? Why are oceanic rocks barely found in the mélanges? Why did thickening compartmentalize independently within continental and oceanic units during collision?