Raman spectroscopy of carbonaceous material thermometry in the uppermost Austroalpine units, Eastern Alps – paleogeographic and tectonic implications

Raman spectroscopy of carbonaceous material (RSCM) estimates peak metamorphic temperatures in structural top units of the Eastern Alps (Steinach nappes, Gurktal nappes, Graz Paleozoic, Carboniferous of Nötsch, Kreuzeck-Goldeck-Gailtal Nappe, Diabase Zone of the North-Karawanken Range) and in their tectonic units below (Schneeberg Complex and Ötztal Nappe below the Steinach nappes, Heiligengeistklamm Nappe below the Gurktal nappes, Waxenegg Nappe below the Graz Paleozoic). The data are based on a standardized temperature calibration and describe the thermal structure of displaced tectonic units which were adjacent segments of the western Tethyan shelf during Permomesozoic times. In a west- to east profile through the Eastern Alps, RSCM data from the Koralpe-Wölz and Ötztal-Bundschuh nappe systems (Brenner area, Gurktal and Nock area, Remschnigg area, Grazer Bergland, Drau Range, North-Karawanken Range) show consistent peak metamorphic temperatures of 570-410 °C. Great similarities are found between the Steinach and Gurktal nappes. A thermal imprint of their post-Variscan (Pennsylvanian) molasse sediments is lower than 300 °C. Similar RSCM temperatures are found in Pennsylvanian sediments of the Graz Paleozoic and in the Saualpe, Remschnigg and Sausal sections of the Gurktal nappes, suggesting a similar burial depth during later times. The thermal structure of the Graz Paleozoic is characterized by an upright temperature trend within the nappe stack, by a thermal anomaly in the southeastern segment, and detachments disturbing the metamorphic pattern at the eastern margin. Permomesozoic burial heating of the Drau Range is constrained by upward decreasing RSCM temperatures. Temperatures <160 °C are estimated in the Carnian Raibl Formation. This pattern is overprinted in the central segment of the Gailtal Alps by a thermal anomaly with temperatures >200 °C in the Carnian stratigraphic level. It is suggested that the temperature gap at the base of the Drauzug-Gurktal Nappe System is related to low-angle detachments formed during post-orogenic collapse. Here, also RSCM temperatures increase locally.