Structure and nature of the Palaeozoic basement based on magnetic, gravimetric and seismic investigations in the central Upper Rhinegraben

Abstract

Abstract New and old magnetic and gravimetric data acquired in the frame of a geothermal project have been processed in N. Vosges and the adjacent Rhinegraben in order to identify the nature and tectonic framework of the graben basement and to select areas with a geothermal potential. In maps, the ENE–WSW striking units of N. Vosges can be followed beneath the sedimentary cover as far as the N35°-striking Upper Rhenish Shear Zone, which shifts these units towards NE. The magnetic basement S and SW of Strasbourg is interpreted as akin to the southern Champ du Feu Massif, consisting of I-type granitoids. Combined with new and old reflection seismic sections, the data show that the petrologic and tectonic contexts in the Illkirch-Grafenstaden area, where a geothermal project is planned, which is similar to the deep geothermal wells of Soultz and Rittershoffen 70 km to the north. The target zone is a N–S fault, partly normal and partly transpressive, which offsets the magnetic, I-type granitoids with high contents of uranium and thorium by about 800 m. Further exploitation of oil along the N–S footwall block attests to the geothermal potential of the area. The NW–SE transpressional regime favours the formation of cracks and increases the porosity and consequently fluid convection. The close borehole ESC-7 and modelling of the temperature distribution lead to expectations of a temperature of at least 150 °C on top of the granite and show that the temperature anomaly results from the heat-productive granitoids and from the blanket effect of the low thermal conductivity tertiary sediments.

Keywords

• Rhinegraben
• Palaeozoic basement
• Gravity
• Magnetism
• Seismics
• Geothermal project