Late Neogene extension in the vicinity of the Simplon fault zone (central Alps, Switzerland)

Brittle faults and kinematic indicators have been measured systematically below and above the major late alpine extensional Simplon fault in the vicinity of Simplonpass. Paleo-stress inversion has been conducted for a total of over 1200 fault measurements, from 62 sites. Minor faults in both hanging- and footwall of the Simplon fault define a coherent paleo-stress field with SW-NE oriented maximum horizontal extension and near vertical compression. The kinematics of these latest brittle deformations are in very good agreement with the ductile, slightly older deformations recorded within the Simplon fault mylonites (Mancktelow 1990). Paleo-stress orientations obtained from sets of the latest brittle faults are clearly different from the present day stress field its derived from earthquake focal plane mechanisms. The latter define a nearly N-S oriented maximum horizontal extension direction (Kastrup 2002), at a high angle to the downdip direction of the Simplon fault lineation. The present day stress orientation is ill orientatated for a reactivation of the Simplon fault and its associated minor fractures.