Fission-track dating of apatite from the Gory Sowie Massif, Polish Sudetes, NE Bohemian Massif: implications for post-Variscan denudation and uplift

Abstract

Six samples from the Gory Sowie Massif gneisses in the West Sudetes, analyzed for the purpose of apatite fission- track dating (AFT) yielded ages ranging from 43 to 57 Ma. No regional variation in the results was observed and the samples form a rather uniform population. From confined-track length measurements, short values between 8.8 and 9.6 µm were obtained. Track length distribution in four of the six samples is bimodal. This suggests residence in temperatures corresponding to the par- tial annealing zone until relatively recent times and subsequent rapid cooling to ambient conditions. We propose a mid-Tertiary rise in the geothermal gradient as a possible factor responsible for reheating the Gory Sowie Massif after it had cooled to temper- atures around 60 - 40 uC in the Oligocene. According to reverse-modelling results, the cooling began in the Cretaceous and there- fore correlates with the event reported from elsewhere in the northern part of the Bohemian Massif. As Lower Carboniferous sed- imentary rocks overlie the gneisses, the overburden which was responsible for keeping temperatures above 100 uC until early Ter- tiary was probably the Carboniferous-Permian Variscan molasse. The amount of the Cretaceous/early Tertiary denudation can be estimated to be 4 - 8 km. The recent, rapid cooling phase began 7- 5 Ma ago and is interpreted as the combined result of decrease of geothermal gradient and/or increased tectonic activity in the Sudetes as evidenced by the presence of the Pliocene, coarse-clas- tic Gozdnica formation.