Fennoscandian Borderzone: thermal and tectonic history of a tuffaceous sandstone and granite from fission track analysis, Bornholm, Denmark

Abstract

Fission track ages for zircon and apatite together with track length distributions for apatite have been obtained from a tuffaceous sandstone of Wenlock age from Bornholm, Denmark. The zircon yields an age close to the depositional age of c. 420 Ma, which is similar to the age of bentonites from Gotland, Sweden. The apatite yields a much younger age, which is interpreted as a cooling/uplift age on the basis of the track length distribution. It is concluded that, due to burial since deposition, the sediment has been heated into the 130–190°C interval prior to 261 Ma. The complex apatite fission track age and length distribution obtained for a granite suggest reheating or prolonged heating in the 70–130°C interval probably associated with individual block movements in the area. Bornholm is located at the rim of the Fennoscandian Borderzone and thus fission track analysis of the thermal and tectonic history is important in understanding the evolution of this structure which is part of a major structural discontinuity stretching from Oslo to the Black Sea.