Tectonic interpretation of the metamorphic field gradient south of the Koralpe in the Eastern Alps

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In order to constrain tectonic models for the nature of the Eoalpine high pressure belt at the eastern end of the Alps, we investigate the formation pressure of metamorphic rocks along a profile between the Koralpe and the well-known UHP rocks of the southern Pohorje mountains. Rocks from three different regions are considered: (i) the rocks of the southernmost Koralpe to the north, (ii) the rocks of the Plankogel Unit between the Plankogel detachment and the Drava valley and (iii) the rocks between the Possruck range and the southern Pohorje mountains. In the Koralpe, pelitic rocks record a formation pressure around 15 – 18 kbar, as reported in the literature. For the Plankogel Unit, we derive pressures between 7.1 ± 1.95 kbar and 11.5 ± 3.42 kbar at 650 °C and recognize only a single Eoalpine metamorphic event. For the high grade rocks of the Pohorje mountains, we derive peak metamorphic pressures (explored with the garnet-muscovite-kyanite-quartz assemblage) that rise from 16.2 ± 3.45 kbar (at 700°C) in the north, to 23.9 ± 2.49 kbar (at 700 °C) in the south. There, we also recognize a later lower pressure event that is derived from pressure calculations with the full equilibrium assemblage. This lower pressure event yields similar conditions around 10 ± 2 kbar at 650 °C for the entire north-south transect within the Pohorje mountains. Peak metamorphic conditions in the Koralpe and Pohorje regions are matched by a continuous field gradient of about 1.3 kbar per 10 kilometers distance corresponding to a depth increase of about 0.5 km per kilometers distance assuming lithostatic conditions. We suggest that this supports that the two units may be interpreted together in terms of a 45° dipping subducting plate. Above this subducting plate, it is inferred that a slab was extracted that was located between the Plankogel Unit and the high pressure rocks, causing a first exhumation stage that is associated with buoyant upwards tilting of the subducted slab to mid crustal levels. Within this model, the Plankogel Unit was located in the hanging wall of the extracted slab and the Plankogel detachment forms the suture of the extracted slab. Exhumation from mid crustal levels to the surface during a 2nd stage occurred due to erosion and normal faulting. This normal faulting is responsible for some 10 km of upward displacement of the Pohorje mountains relative to the Koralpe and ultimately for the current distribution of lithologies on a map scale.

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