Multiscalar 3D characterisation of the Mid-Norwegian passive margin evolution, Central Norway: A multi-technique approach to unravelling the structural characteristics and tectonic history of offshore basement highs

Smøla Island, situated within the Mid-Norwegian passive margin, contains crystalline basement-hosted intricate fracture and fault arrays formed during a polyphase brittle tectonic evolution. Understanding similar fracture patterns within basement structural highs offshore, which is crucial in that they are commonly associated with unconventional hydrocarbon reservoirs, remains challenging owing to the lack of rock exposure and geological and chronological constraints. As Smøla is an onshore analogue for basement highs offshore, any insights can therefore be applied to the offshore domain. In this study, we present a multiscalar multi-technique approach whereby various 2D and 3D field, drilling, geophysical, and K-Ar geochronology datasets are integrated to unravel the brittle evolution of Smøla and the surrounding margin. On the regional-scale, geophysical and DTM data-derived lineament analysis, along with local-scale field mapping, high resolution logging of four diamond drill holes, 3D modelling, and petrographic and microstructural analyses indicate that at least five deformation episodes affected Smøla through time. These deformation episodes are characterised by different geometric and kinematic patterns, and mineral assemblages: I) the earliest, ENE-WSW, WNW-ESE to NW-SE striking epidote-prehnite brittle structures are associated with syn to post-Devonian brittle sinistral transtension during post-Caledonian extension of the margin; II) NE-SW or NW-SE striking sericite-chlorite-calcite shear features formed from two separate brittle-ductile phases, which are related with both sinistral and later dextral strike-slip faulting during the Carboniferous to Late Triassic (~300 Ma to ~200 Ma); III) NE-SW, NNE-SSW striking chloritic-hematite breccias and gouges, linked to crustal extension, normal faulting, basin development (~200 Ma to ~100 Ma); IV) NE-SW to NNW-SSE and WNW-ESE striking hematite-zeolite-calcite veins and hydraulic breccias, likely associated to both late Mesozoic dextral strike-slip faulting, and regional ~E-W crustal extension (younger than ~150 Ma to ~75 Ma); and V) the last recorded episode characterised by NW-SE, WNW-ESE and NNE-SSW striking quartz-calcite veins cross-cutting all previous deformation features, associated with ~E-W to NE-SW crustal extension during the Cretaceous to Paleogene. Accordingly, 3D modelling of selected structures, by deformation episode, reproduce the complex brittle arrays on Smøla through time. The structural modelling also highlights the multiscalar nature of basement deformation features, with modelled down-dip and strike extents of the structures varying by D _1–D ₅. Overall, this ‘toolbox’ approach makes it possible to unravel the complex brittle deformation history of Smøla and the wider Mid-Norwegian margin and can be applied to other basement highs offshore Norway and elsewhere.