FISSION-TRACK DATING OF THE WESTERN BORDER OF THE BOHEMIAN MASSIF - THERMOCHRONOLOGY AND TECTONIC IMPLICATIONS

Since 1985, apatite fission-track analysis was applied to more than 70 samples from surface outcrops and shallow boreholes at the western ma rgin of the Bohemian massif. Apatite ages were determined by the grain -population method. Additional information from the frequency distribu tions of fully confined spontaneous tracks was used for modelling of t -T paths in the low-temperature range (<120 degrees C). Seven zircon s amples were dated by the external detector method. Zircon ages between 283 and 215 Ma indicate unroofing during the Permian molasse stage an d the Triassic. Tectonic quiescence and slow subsidence prevailed from the Jurassic until the middle Cretaceous. In the basement area south of Weiden, a Mesozoic partial annealing zone (for apatite fission trac ks) is now exposed at the surface. Farther north, the basement was aff ected by stronger Cretaceous and Palaeogene erosion, which yielded coo ling ages between 110 and 49 Ma. This second period of post-Variscan d enudation was correlated to reverse faulting along the Franconian Line .