The south-western black forest and the Upper Rhine Graben main border fault: Thermal history and hydrothermal fluid flow

The thermal history of the south-westernmost Black Forest (Germany) and the adjacent Upper Rhine Graben were constrained by a combination of apatite and zircon fission-track (FT) and microstructural analyses. After intrusion of Palaeozoic granitic plutons in the Black Forest, the thermal regime of the studied area re-equilibrated during the Late Permian and the Mesozoic, interrupted by enhanced hydrothermal activity during the Jurassic. At the eastern flank of the Upper Rhine Graben along the Main Border Fault the analysed samples show microstructural characteristics related to repeated tectonic and hydrothermal activities. The integration of microstructural observations of the cataclastic fault gouge with the FT data identifies the existence of repeated tectonic-related fluid flow events characterised by different thermal conditions. The older took place during the Variscan and/or Mesozoic time at temperatures lower than 280°C, whereas the younger was probably contemporary with the Cenozoic rifting of the Upper Rhine Graben at temperatures not higher than 150°C.