Tectonometamorphic evolution of the western Austroalpine basement units: geometry and kinematics of the eoalpine collision (Texel Complex, South Tyrol)
The following work deals with the structural and metamorphic geology of the Austroalpine basement nappes west of the Tauern Window. The research area is situated at the westernmost end of the E-W striking Eoalpine High Pressure Belt (EHB) first described by Frank et al. (1987). These eclogite facies overprinted units are interpreted as resulting from the subduction of the Meliata ocean during the Late Jurassic and Cretaceous. In the eastern parts, both continental and oceanic crust were involved in this process, while the western part is characterized by an intracontinental shear zone. Based upon information about the well-known eastern parts it is generally assumed that the North Apulian plate was subducted underneath the South Apulian plate (Schuster 2003; Schmid et al. 2004). The setting in the western part is less investigated. Sölva et al. (2005) propose a somewhat contradicting model based on the recent geometrical situation in the Texel region. The investigation of this inconsistency between the models was the principal task of the following work. This problem stands close to the possible prolongation of an intrabasement shear zone, which starts in the upper Vinschgau as the brittle ,,Schlinig Line" and crosses the central Vinschgau as the ductile ,,Vinschgau Shear Zone", and becomes wider towards the E (Schmid & Haas 1989).
The mapped area is located 20 km NW of Meran in the National Park ,,Texelgruppe". It comprises approximately 40 km² with an elevation ranging from 1500 m to over 3300 m. Speckbacher (2009) worked in the ,,Pfossental" near the border to Austria, in this study the mapping area was the N-S striking ,,Zieltal". Because of the observed structures it is necessary to combine both parts as a single unit. Therefore this work presents the results of both investigations. The described problem was approached by a detailed field study in combination
with structural and petrological methods. The geological map was digitized with a GIS-application and connected to a database to display the field data with a geographic reference. The lithological and structural mapping allowed a sequence of deformation stages to be defined:
- D1 - mylonitic main foliation and contemporaneous isoclinal folding with W- to NW-dipping foliation
planes and well-defined NW-dipping stretching lineations. The shear sense criteria indicate top NW kinematics.
- D2 - large-scale refolding of the main foliation around steep NW-dipping fold axes. This deformation stage
produced a spectacular synformal (Zieltal) and antiformal (Pfossental) pair, and therefore some kinematics
are also refolded.
- D3 - brittle overprint with transtensive character. The results are normal faults with an offset between 10
m to 100 m. They are interpreted to be parallel structures to larger faults in the adjacent area.
Based on this sequence, structural-, microfabric- and petrological analyses were performed. The syn- and antiformal fold pair, as well as intersecting criteria in the mapping area were described. The analysis of microstructures made it possible to estimate temperature conditions of the single deformation stages. The petrological data correspond to the large existing dataset of the Ötztal area. Combined with literature data we were able to construct two ,,large-scale cross sections" to illustrate the recent geometry. Furthermore, we propose a geodynamic interpretation which describes the observed situation well. The results can be summarized as following:
- an originally SE-dipping subduction channel with ÖSC (South Apulia) in an upper plate, Campo (North Apulia) in a lower plate position
- the Schlinig Line - Vinschgau Shear Zone - Schneeberg Fault Zone represent a continuous shear zone from shallow levels to the deeper crustal levels. It can be interpreted as the subduction channel of the eoalpine orogeny.
- a SE vergent back-folding during the Eocene refolded the originally SE dipping subduction channel resulting in the recent NW-dip. A further constriction during the indentation of the Adriatic microplate (Southern Alpine units, Miocene) teared the fold along the Passeier Line and overturned the SE limb in the E.
- folding and indentation induced axial steepening of the structures, resulting in the map (plane view) showing the same view as a cross section.
Estimated temperatures from observed microstructures and thermal modelling by Fügenschuh et al. (2000) provide evidence that such large-scale folding is possible during Eocene in corresponding depths.