Paleozoic basin reactivation and inversion of the underexplored Northern North Sea platforms: a cross-border approach

Well penetrations on the UK East Shetland Platform (ESP) prove 1-8 km thick Devonian post-orogenic extensional collapse-related successions. Conversely, extremely thick (1-6 km) Permo-Triassic basin fills without Devono-Carboniferous units were in the past interpreted west of the Utsira High, on the Norwegian Horda Platform and Stord Basin, albeit Pre-Triassic well penetrations are here very rare. In this work, the nature and age of Paleozoic-Triassic strata and structures in these underexplored platform regions are tentatively constrained by performing cross-border regional seismic interpretation east and west of the Viking Graben. We highlight cross-border analogies in structural style and seismic facies, with a similar evolution dominated by polyphase inversion tectonics and structural grain preservation. In the Norwegian study areas, much of the half-graben sedimentary fills may be interpreted as Devonian-?Carboniferous in age as in the ESP, rather than overly thick Permo-Triassic successions. Major graben-bounding extensional faults are low-angle (~25-33°), approximately northerly-striking and likely rooting downwards into reactivated Caledonian shear zones. Rifting development occurred in multiple episodes, possibly creating different traps. Prior to Permian-Jurassic rifting, many low-angle Caledonian thrusts were subject to extensional inversion in the Devonian and then to Variscan compressional reactivation, causing vertical extrusion and deformation of Devonian syn-rift wedges.