Post-Variscan thermal evolution of the flanks of the southern Upper Rhine Graben : fission-track analyses and thermal modelling

The Upper Rhine Graben (URG) is the most discernible part of the European Cenozoic rift system. Uplifted Variscan basement is exposed in the Black Forest and the Vosges and forms the flanks of the southern URG. The impact of the Jurassic hydrothermal activity on the interpretation of fission track (FT) data from the southern URG is elaborated by means of new zircon FT analysis on samples with known U/Pb crystallisation ages. Zircon FT central ages range from 162 Ma to 247 Ma. The analysed samples experienced substantial annealing prior to Cretaceous cooling that cannot be explained by burial alone. Instead, it is suggested that circulating hydrothermal fluids with temperatures in the order of 200-250 °C are responsible for the observed thermal anomaly, which is also evidenced by vein mineralizations. FT ages of 28 outcrop samples collected along two E-W trending transects from the Black Forest and Vosges vary from 136 Ma to 312 Ma (zircon samples) and from 20 Ma to 83 Ma (apatite samples). Broad and/or bimodal track lengths distributions indicate a complex thermal history, which was determined by inverse modelling of apatite FT parameters and tested against the observed dataset and independent geological constraints. Cooling below 120 °C in the Early Cretaceous to Palaeogene was followed by a discrete heating event during the late Eocene and subsequent cooling to surface temperature. The modelled time-temperature paths point to a total denudation of the flanks of the URG in the range of 1.0-1.7 km for a paleogeothermal gradient of 60 °C/km, and 1.3-2.2 km for a paleogeothermal gradient of 45 °C/km since the late Eocene. Zircon and apatite FT data from four boreholes, which penetrate the Mesozoic and pre-Mesozoic sediments and crystalline basement of northern Switzerland, are also presented. Inverse thermal modelling of the apatite FT parameters reveals the low-temperature thermal history of the crystalline basement of northern Switzerland. Moderate to rapid cooling of the samples through the apatite partial annealing zone (PAZ) at the end of the Mesozoic was followed by a distinct thermal event during the Eocene and subsequent slow cooling to present-day temperatures. The Eocene heating episode coincides with the initial rifting phases of the neighbouring URG and associated volcanic activity. Crustal-scale faults of the PermoCarboniferous Trough of northern Switzerland could have acted as major pathways for circulating hydrothermal fluids giving rise to the observed Eocene thermal event.