| Title | Relative chronology of deep circulations within the fractured basement of the Upper Rhine Graben |
|---|---|
| Authors | Dezayes, Lerouge, Ramboz, Wille |
| Year | 2013 |
| Conference | European Geothermal Conference |
| Keywords | Rhine Graben, granite, fracture, mineralogical filling, palæo-circulation. |
| Abstract | In the Upper Rhine Graben, the deep geothermal reservoirs constitute fractured dominated systems. However, the hydraulic behaviour of the fracture network is poorly known but its knowledge constitutes an important way to better target the exploration works. In this paper, we propose a combined structural and mineralogical analysis of the fractures and their fillings based on samples collected on the EPS1 Soultz cores and outcrops on the both flanks of the Upper Rhine Graben. These mineralogical fillings have been sampled within fractures identified by measuring their orientation and defining their relative age. Petrological study (Optical microscope, Scanning Electron Microscope, cathodoluminescence) completed by chemical analyses (Electron Microprobe Analyses,) have been performed in order to determine the filling minerals and their relationships. Several stages of fracturing following by several pulses of fluid have been occurred in the Rhine Graben. This study shows that the fracture sets are characterized by different polyphased mineralogical filling. In the Hercynian and Permian fracture sets two successive stages are highlighted: 1- shear/cataclasis phase with illite and quartz; 2- precipitation of dolomite. Those are the consequences of fluid circulation prior to the Oligocene graben opening. The N-S fractures linked to the Eocene-Oligocene graben formation, with reactivation of Hercynian structures, have no macroscopic mineralogical filling on outcrops because of surficial weathering alteration. However, under the microscope, tension fractures are filled with radial illite, which probably recorded fluid circulation associated to the early graben opening stage. The reactivation of Hercynian structures in relation with the Tertiary tectonic develops deep fluid/basement interaction. In the contrary, the N-S large structures are more recent circulation system and rather constitute recharge drai |