Miocene thermal history of the southwestern margin of the Styrian basin: vitrinite reflectance and fission-track data from the Pohorje/Kozjak area (Slovenia)

The Kozjak (Possruck) and Pohoje mountains form the southwestern basement r im of the Neogene Styrian Basin. This region was affected by two Tertiary m agmatic events: the emplacement of the Oligocene Pohorje tonalite, and of E arly/Middle Miocene dacites in the western Pohorje. Vitrinite reflectance and fission-track data an used to reconstruct the the rmal history and to constrain the exhumation of the Pohorje/Kozjak area. Ea rly Miocene sediments lacking a thermal overprint contain apatite grains of Eggenburgian (similar to 19 Ma) cooling age, only 1-2 Ma older than the ti me of deposition. The cooling rate of the mainly Austroalpine source units was very fast, denoting tectonic denudation. It means that in the Eastern A lps during Early/Middle Miocene time the Pohorje/Kozjak region in addition to the Tauern and Rechnitz windows was supplying sediment into the surround ing basins with nearly syn-sedimentary apatite cooling ages. Vitrinite reflectance anomalies in Early Miocene sediments in the Ribnica-S elnica trough, located between the Kozjak and Pohorje mountains, and at the eastern margin of the Kozjak mountains give evidence for a strong post-dep ositional thermal overprint, Thermal models based on nearby wells suggest t hat Miocene temperatures were as high as 220 degrees C, Apatite fission-tra ck ages indicate that the thermal overprint had terminated by middle Badeni an (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 an omaly, situated at the eastern margin of the Kozjak mountains, extends east ward 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 ex humation of hot metamorphic rocks in the Pohorje/Kozjak region. Rapid exhum ation is proven by fission-track dating. It would explain the appearance of the nearly syn-sedimentary detrital apatite FT ages in the sediments and t he post-depositional heating. (C) 1998 Elsevier Science B.V. All rights res erved.