Geochronological investigation of eroding areas is able to clear only some interval of the uplift history. The dating of surface samples from mountains is rather meaningless if the thickness of the removed layer is much less than the depth of the closure temperature. On the other hand, if the removal is very deep (only 'freshly' formed cooling ages can be found at the surface), the evaluation of its size is strongly limited. This is especially true for tectonic denudation. Thus, for the reconstruction of the exhumation-uplift-denudation history the geochronology of the clastic sediment deposited around the mountain chain is indispensable (see e.g. Cerveny et al., 1988).
We have compared two well-controlled geological settings from the Eastern Alps and from the Pannonian basin. Both can be characterized by tectonic unroofing of a deeper structural level and both were formed during the same Middle Miocene 'escape period' (Ratschbacher et al., 1989). In case of the Tauern Window the footwall is the Penninic unit, (the cooling ages can be found in Frank et al., 1987 and Staufenberg, 1987). The existence of metamorphic core complexes of the SE Pannonian basin and their formation at around 18 Ma was proved by zircon fission track dating (internal reports of the Hungarian Oil and Gas Company). Although the time and rate of cooling of the two structures are the same, the mechanisms are different. The exhumation of the Penninic level in the Tauern Window was the results of a continent-continent collision and crustal thickening, but the formation of the core complexes in the SE Pannonian basin was related only to extension.
Siliciclastic post-rift Molasse sediments were dated by single grain fission track technique. Apatite and zircon grain-age-spectra refer to the composition of the eroding areas at the time of deposition. Prior to the exhumation and erosion of the lower structural unit only detritus of 'old' ages from the hangingwall was transported to the basins. In higher members of the stratigraphic sequence appear the freshly cooled particles from the lower unit which reveal FT ages close to the depositional age. When the young group of ages becomes distinguishable and the cluster can be characterised by its own age we can calculate the cooling rate for the thermal range below the apatite closure temperature considering the depositional age of the sediment.
The East Alpine sediments at the age of deposition contained ~4 million years old grains, while the sediments from the surroundings of the Pannonian metamorphic core complexes show more contrast (~12 Ma). It means that in the zone of active Alpine thrusting the fast cooling rate (due to uplift) had continued after the 'check point' of the closure thresholds of apatite, while in the Pannonian basin it had slowed down. The cooling rates of the final uplift (from the closure T to the surface) were 25-30 °C/Ma and ~9 °C/Ma respectively. On this base it is possible to conclude on the paleo-relief. In the Middle Miocene the western part of the Eastern Alps were already a high mountain chain while the Pannonian core complexes were forming a relative flat, humpy landscape.
Cerveny et al., In New perspectives in basin analysis (ed. Kleinspech & Paola) (1988).
Frank et al., In Geodynamics of the Eastern Alps, (ed. Flügel & Faupl) 270-281 (1987).
Ratschbacher et al., Geology 17, 404-407 (1989).
Staufenberg, Jb. Geol. B.-A. (Wien) 130, 571-586 (1987).