From widespread faulting to localised rifting: Evidence from K‐Ar fault gouge dates from the Norwegian North Sea rift shoulder

[Abstract Although seismic and stratigraphic well information put tight constraints on rift basin evolution, eroded shoulders commonly expose polydeformed prerift basement whose deformation history may be difficult to constrain. In this work, we apply K‐Ar dating of fault gouge samples from 18 faults explore the brittle well‐exposed eastern margin northern North Sea rift. We find evidence clay formation since Late Devonian, with distinct Permian Jurassic activity peaks that closely match early stages two well‐established phases. A marked decay in density away confirms a close relationship between rifting onshore faulting. The results show initial rift‐related extension affected much wider area than resulting offshore Hence our data support model where strain is initially distributed over several 100 km wide region, as prelude development ~150–200 Permo‐Triassic defined by large marginal faults. Towards end second phase, localises even more strongly 25–50 Viking Graben. Interestingly, period widespread seen for both phases general characteristic continental rifting. documented faulting fracturing Devonian weakened probably facilitated subsequently localised form rift, further localisation its relatively narrow central part (Viking Graben)., Onshore illite dates indicate evolved an phase widely reached far into margins, followed large‐displacement This pattern was repeated during rifting, suspect similar exist other rifts. ]