The post-Caledonian development in the western Bergen Arc System and the nearby offshore area is characterized by a gradual transition from ductile through semi-ductile to brittle deformation. Interpretation of offshore seismic data shows a dominance of pre-late Jurassic faulting, with relatively minor late or post-Jurassic reactivation. This history is also seen in the Bjorøy tunnel in the Hjeltefjord fault zone, where most, but not all of the (semi)brittle faulting history is pre-Oxfordian (pre-late Jurassic). Pre-Oxfordian faults are typically cohesive fault rocks with striations on slip surfaces, whereas post-Oxfordian faults are identified as non-cohesive gouge zones without clear kinematic indicators. Slip analysis of the fault population with cohesive fault rocks in the Øygarden, Sotra, Løvstakken and Hjeltefjord area gives a relatively consistent NW -SE extension direction. Detailed investigation of Permian dikes in western Sotra and Permo-Jurassic dikes in Sunnhordland indicate a very precise E-W extension direction at about 260 Ma. These dikes intruded along pre-existing faults and fractures, and are mostly undeformed or weakly jointed. Much of the brittle or semi-brittle deformation in the region is therefore thought to be older than the Permian dikes. This suggests that a change in the orientation of the minimum principal stress axis happened at some time between the formation of the fault systems and intrusion of the Permian dikes. Since the NW extension direction is similar to the early to mid-Devonian stretching direction detected in the Caledonian nappe region, it is suggested that the main population of faults with well-developed slip indicators west of Bergen reflects the (late) Devonian stress field.
Haakon Fossen, Geologisk institutt, Universitetet i Bergen, Allégt. 41, N-5007 Bergen, Norway