ORIGIN AND SIGNIFICANCE OF RELAXATION FRA CTURES IN EPS1 GRANITE CORESECTIONS (SOULTZ-SOUS-FORETS, RHINEGRABEN)

Detailed analysis of fracture patterns was carried out in the granite core sections of a deep borehole, which reached the hot and fractured granitic basement of the Rhinegraben at Soultz-sous-Forets (European G eothermal Project). Two main types of fractures were identified, respe ctively older than and subsequent to drilling. Most pre-existing fract ures show a conjugate, normal fault type pattern which reflects the co nditions of the Oligocene graben opening. The other types of fractures are revealed by core relaxation. We demonstrate that they correspond to a latent fracturing due to the same Oligocene extension. Such devel opment of relaxation fractures during core extraction may thus be stro ngly influenced by the tectonic history of the rock massif, and not by the present-day stress and drilling conditions exclusively.