THE EXHUMATION HISTORY OF THE CENTRAL ALPS: A COMBINED APPROACH OF ZIRCON FISSION TRACK DATING, ND ISOTOPE GEOCHEMISTRY, AND THE SEDIMENT BUDGET OF THE FORELAND BASINS

The concept of dating detrital zircons from foreland basins is widely used to unravel the cooling and denudation history of the orogenic hinterland. This approach, however, only monitors the erosion of zircon-bearing lithologies while Zr-poor or -free lithologies as, e.g., ophiolitic rocks are neglected. Recognizing the latter is important to detect former suture zones, which may have large bearing on the reconstruction of the geodynamic evolution of the orogen.

In this study we try to reconstruct the exhumation history of the Central Alps by using a combination of fission track (FT) dating on detrital zircons, Nd isotopic ratios on detrital epidote, and the sediment budget of the foreland basins.

For the Oligocene molasse strata geochronology and isotope data yield concurrent results, pointing to an exposure of sedimentary cover nappes and some basement rocks of the Austroalpine upper plate on the northern flank of the Central Alps. For Miocene times, Zr FT data imply that metamorphosed units of the Penninic lower plate became first exposed in the western Central Alps at 21 Ma and subsequently in the eastern part of the Central Alps at around 13 Ma. In contrast, the Nd isotope data show that ophiolitic rocks of the lower plate became already exposed at 21 Ma in the eastern Central Alps. Therefore, the Penninic lower plate became exposed simultaneously over large areas of the Central Alps. At the same time a drastic decrease of the sediment discharge to the foreland basins points to the collapse of the relief in the hinterland. The combination of enhanced exhumation and relief collapse suggests tectonic denudation rather than erosion is driving the exhumation of the Central Alps. This fits well to the scenario of large-scale extension processes affecting the Central Alps in post-collisional times.