Post-Variscan thermal history of the Intra-Sudetic Basin (Sudetes, Bohemian Massif) based on apatite fission track analysis

Abstract

The Intra-Sudetic Basin, a ~ 12 km deep Variscan intramontane basin, has the best preserved post-orogenic sedimentary record available at the NE margin of the Bohemian Massif. Apatite fission track (AFT) analyses have been performed on 16 sedimentary and volcanic samples of Carboniferous to Cretaceous age from the Intra-Sudetic Basin to improve understanding of the post-Variscan thermal evolution. AFT central ages range from 50.1 ± 8.8 to 89.1 ± 7.1 Ma (Early Eocene to Coniacian), with 13 of them being Late Cretaceous. The mean track length values range from 12.5 ± 0.4 to 13.8 ± 0.5 (except for one sample 14.4 ± 0.2) µm. This relatively short mean track length together with the unimodal track length distributions and rather low standard deviation (0.8 to 1.7 µm) in most samples indicate a long stay in the partial annealing zone during slow cooling. However, in the northern part of the Intra-Sudetic Basin, samples show a wider track length distribution (standard deviation of 1.8 to 2.1 µm) that could indicate a more complex thermal evolution possibly related to Mesozoic reheating. Vitrinite reflectance data combined with thermal models based on the AFT results indicate that the Carboniferous strata reached maximum palaeotemperatures in the latest Carboniferous to Early Permian time, corresponding to a major coalification event. The second phase of temperature rise occurred due to Late Mesozoic sedimentary burial, but it had no influence on maturation of the Carboniferous organic matter. Final cooling phase in the Late Cretaceous–Paleogene was related to tectonic inversion of the Intra-Sudetic Basin, which occurred after deposition of a significant thickness of Cenomanian–Turonian sediments. Thermal modelling demonstrates that ~ 4 km thick cover of Upper Cretaceous sediments is required to obtain a good match between our AFT data and modelled time–temperature paths. This outcome supports a significant amount of Late Cretaceous–Paleogene inversion within the Variscan belt of Central Europe.