Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K–Ar illite fault gouge dating

Abstract

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. KAr 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 volcanism in 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.

Complex Bedrock Fracture Patterns: A Multipronged Approach to Resolve Their Evolution in Space and Time

“…The ages of the coarser grain size fractions likely still represent mixed ages affected by various contents of protolithic K‐bearing phases. The ages of the finest fractions remain maximum ages, but they are the best absolute chronological constraint of the last recorded slip event, as indicated by the steadily growing regional database, which is very consistent (Fossen et al, ; Fredin et al, ; Ksienzyk et al, ; Torgersen et al, ; Viola et al, ). The dated fault gouges yield therefore a Middle Triassic age (245 ± 6 Ma, TSC‐66), Early Jurassic ages (185 ± 4 Ma, TSC‐28 and 183 ± 8 Ma, TSC‐65), and a Late Jurassic age (162 ± 5 Ma, TSC‐8), with both the Early Jurassic ages being identical within error (Table and Figures d–i).…”

“…A ca. 116 Ma K‐Ar illite age lies within the age range of this alteration episode and has been interpreted as constraining fault reactivation in the Bergen area (Ksienzyk et al, ). Dilation along suitably oriented N‐S to NE‐SW striking preexisting fault‐ and fracture zones triggered enhanced fluid flow and associated calcite mineralization.…”

“…Similar extension directions have been calculated from subvertical, NNW‐SSE to N‐S striking doleritic dikes (Fossen, ; Valle et al, ), which intruded along the coastal area of western Norway during two pulses, in the Late Permian and the Late Triassic (Figures e and ) (Færseth et al, ; Fossen & Dunlap, ; Løvlie & Mitchell, ; Torsvik et al, ). Permian (K‐Ar) ages from fault rocks have also been reported from subvertical NNW‐SSE trending brittle faults in the Bergen area (Ksienzyk et al, ) and from N‐S trending normal faults affecting the Kongsberg area immediately west of the Oslo Rift (Torgersen et al, ). Middle to Late Permian faulting has also been constrained by paleomagnetic and/or 40 Ar/ 39 Ar dating of other major fault zones of western Norway (Dalsfjord Fault atop of the Nordfjord‐Sogn Detachment Zone (Eide et al, ), Lærdal‐Gjende Fault System, partly coinciding with the Hardangerfjord Shear Zone (Andersen et al, )), which indicates the localized structural reactivation of inherited Devonian detachment zones (Fossen et al, ).…”

“…The change in the extension direction from ENE‐WSW in the Permian to W(NW)‐E(SE) in the Jurassic agrees with the observation that strike‐slip movement along NW‐SE striking faults mostly overprints dip‐slip movement, and strike‐slip movement along N‐S striking faults is overprinted by dip‐slip movement (Table ). Predominantly Early Jurassic K‐Ar fault gouge ages constrain this extensional episode in the Bergen area (Ksienzyk et al, ) and in the Kongsberg area west of the Oslo Rift (Torgersen et al, ).…”

“…The brittle faulting record onshore SW Norway has mostly been related to pre‐dike deformation, that is, the late stages of Devonian NW‐SE extension and Permo‐Triassic rifting (Fossen, ; Larsen et al, ; Valle et al, ). More recent studies of onshore faults, including K‐Ar dating of fault rocks, however, indicate a more complex evolution, punctuated by multiple episodes of faulting and pervasive fault reactivation from the Permian down to the Cretaceous (Fossen et al, ; Ksienzyk et al, ; Viola et al, ).…”