Simultaneous normal faulting and extensional flexuring during rifting: an example from the southernmost Upper Rhine Graben

The southern end of the Upper Rhine Graben (URG) is formed by a major continental transfer zone, which was localised by the reactivation of ENE-oriented basement faults of Late Palaeozoic origin. A combination of subcrop data (derived from exploration wells and reflection seismic lines) and palaeostress analysis provided new constraints on the timing and kinematics of interacting basement faults. Rifting in the southern URG began in the Upper Priabonian under regional WNW–ESE-directed extension, oriented roughly perpendicular to the graben axis. In the study area, this led to the formation of NNE-trending half-grabens. Simultaneously, ENE-trending basement faults, situated in the area of the future Rhine-Bresse Transfer Zone (RBTZ), were reactivated in a sinistrally transtensive mode. In the sedimentary cover the strike-slip component was accommodated by the development of en-échelon aligned extensional flexures. Flexuring and interference between the differently oriented basement faults imposed additional, but locally confined extension in the sedimentary cover, which deviated by as much as 90° from the regional WNW–ESE extension. The interference of regional and local stresses led to a regime approaching radial extension at the intersection between the URG and RBTZ.