Abstract

The Kozjak (Possruck) and Pohorje mountains form the southwestern basement rim of the Neogene Styrian Basin. This region was affected by two Tertiary magmatic events: the emplacement of the Oligocene Pohorje tonalite, and of Early/Middle Miocene dacites in the western Pohorje. Vitrinite reflectance and fission-track data are used to reconstruct the thermal history and to constrain the exhumation of the Pohorje/Kozjak area. Early Miocene sediments lacking a thermal overprint contain apatite grains of Eggenburgian (∼19 Ma) cooling age, only 1–2 Ma older than the time of deposition. The cooling rate of the mainly Austroalpine source units was very fast, denoting tectonic denudation. It means that in the Eastern Alps during Early/Middle Miocene time the Pohorje/Kozjak region in addition to the Tauern and Rechnitz windows was supplying sediment into the surrounding basins with nearly syn-sedimentary apatite cooling ages. Vitrinite reflectance anomalies in Early Miocene sediments in the Ribnica–Selnica trough, located between the Kozjak and Pohorje mountains, and at the eastern margin of the Kozjak mountains give evidence for a strong post-depositional thermal overprint. Thermal models based on nearby wells suggest that Miocene temperatures were as high as 220°C. Apatite fission-track ages indicate that the thermal overprint had terminated by middle Badenian (14.4±2.3 Ma) time. Vitrinite reflectance anomalies in the Ribnica–Selnica trough are a result of Early/Middle Miocene volcanic activity. Another vitrinite reflectance anomaly, situated at the eastern margin of the Kozjak mountains, extends eastward into the Somat–Radkersburg area. The heat source in this region is not obvious. Possible heat sources are: (1) a shallow pluton beneath this area, which easily can explain the reconstructed Early/Middle Miocene heat flow of more than 250 mW/m 2; and (2) advective heat transport due to rapid exhumation of hot metamorphic rocks in the Pohorje/Kozjak region. Rapid exhumation is proven by fission-track dating. It would explain the appearance of the nearly syn-sedimentary detrital apatite FT ages in the sediments and the post-depositional heating.

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