Norwegian Journal of Geology Volume 91-
Post-Caledonian extension during orogenic collapse and Mesozoic rifting in the West Norway–northern North Sea region was accommodated by the formation and repeated reactivation of ductile shear zones and brittle faults. Offshore, the Late Palaeozoic–Mesozoic rift history is relatively well known; extension occurred mainly during two rift phases in the Permo–Triassic (Phase 1) and Mid–Late Jurassic (Phase 2). Normal faults in the northern North Sea, e.g., on the Horda Platform, in the East Shetland Basin and in the Viking Graben, were initiated or reactivated during both rift phases. Onshore, on the other hand, information on periods of tectonic activity is sparse as faults in crystalline basement rocks are difficult to date. K–Ar dating of illite that grows synkinematically in fine-grained fault rocks (gouge) can greatly help to determine the time of fault activity, and we apply the method to nine faults from the Bergen area. The K–Ar ages are complemented with X-ray diffraction analyses to determine the mineralogy, illite crystallinity and polytype composition of the samples. Based on these new data, four periods of onshore fault activity could be defined: (1) the earliest growth of fault-related illite in the Late Devonian–Early Carboniferous (>340 Ma) marks the waning stages of orogenic collapse; (2) widespread latest Carboniferous–Mid Permian (305–270 Ma) fault activity is interpreted as the onset of Phase 1 rifting, contemporaneous with rift-related volcanismin the central North Sea and Oslo Rift; (3) a Late Triassic–Early Jurassic (215–180 Ma) period of onshore fault activity postdates Phase 1 rifting and predates Phase 2 rifting and is currently poorly documented in offshore areas; and (4) Early Cretaceous (120–110 Ma) fault reactivation can be linked either to late Phase 2 North Sea rifting or to North Atlantic rifting.