THE PLANKOGEL DETACHMENT OF THE EASTERN ALPS: PETROLOGICAL EVIDENCE FOR AN OROGEN-SCALE EXTRACTION FAULT

The Plankogel detachment is an east-west trending, south-dipping low-angle normal fault that juxtaposes the high pressure rocks of the eclogite type locality of the eastern European Alps towards amphibolite facies rocks to the south. It occurs in both the Saualpe- and Koralpe Complex in eastern Austria. During Cretaceous intracontinental subduction, the footwall and the hangingwall units of the Plankogel detachment were buried to different crustal levels as inferred by pseudosection modelling and conventional thermobarometry: ~23–24 kbar and 660–690 °C for the eclogite facies units in the footwall of the detachment and ~12–14 kbar and 550–580 °C for the amphibolite facies metapelites in the hangingwall. Despite the different peak metamorphic conditions, both sides of the detachment display a common retrograde overprint at conditions around 10 kbar and 580–650 °C. From this, we infer a two-stage exhumation process as a reflection of slab extraction during early Eoalpine subduction. The first stage of exhumation occurred due to the downward extraction of a crustal boudin that was localized in the trace of the Plankogel detachment, the later stage however, is attributed to more regional erosion or extension driven processes. Since the Plankogel detachment is geometrically related to a crustal-scale shear zone further north (the Plattengneiss shear zone), we further suggest that our model may solve the long standing discussion on the enigmatic shear sense of the Plattengneiss shear zone.