Gravity and magnetic modelling along seismic reflection profiles across the East Shetland Platform (Northern North Sea, UK)

Modelling of potential fields can significantly contribute to the understanding of the subsurface geology, particularly if constrained by field geology, well-data and seismic profiles. This approach becomes crucial to define the subsurface setting when some of such constraints are sparse like in underexplored marine settings. The East Shetland Platform and surrounding basins (i.e. the Dutch Bank Basin, DBB; the East Orkney basin, EOB) are examples of poorly explored areas in the UK Continental Shelf in the northern North Sea. In this area, a laterally discontinuous but locally thick Devonian-to-Tertiary sedimentary succession (up to 7-8 km in thickness) mainly consisting of sandstones, claystones and limestones with locally dolomites and anhydrites, unconformably overlies the Caledonian crystalline basement.

Starting from interpreted seismic profiles, we provide a first-order geophysical characterization through the combined forward modelling of the observed Bouguer gravity and reduced to the pole magnetic anomalies along five regional geological cross-sections. Furthermore, we return an overall tectono-stratigraphic framework of the Devonian-to-Recent sequences and tentatively define the crustal sources for the observed anomalies.

The modelling of the sedimentary sequence was supported by the available few exploration wells data and wireline logging (i.e. lithology, seismic velocity, bulk density) and their geometries were constrained by the time-to-depth conversion of five regional seismic reflection profiles recently acquired and processed.

The first-order contributors to the observed Bouguer gravity anomalies are related to the scattered distribution of the Mesozoic sedimentary sequences. In particular, two gravity lows result from the main Triassic-Jurassic sedimentary successions within the area (i.e. DBB, EOB). On the contrary, the gravity highs are mainly controlled by shallow exposures or structural highs of basement (i.e. Caithness Ridge, Fair Isle Platform) underneath the tertiary cover.

The Caledonian basement and high-susceptibility (up to 0.05 SI units) intrusive bodies are interpreted as the main sources of magnetic anomalies. Such intrusive bodies are modelled both inside the basement and the lower crust. These deeper sources are related with areas of high reflectivity observed in the seismic profiles and could be related to structural paleo-domains connected to the pre-Devonian evolutionary phases of the study area. If confirmed, this interpretation will provide important constraints to the reconstruction of the geodynamic evolution of the area, defining the off-shore extension of the first-order Caledonian and post-Caledonian tectonic lineaments exposed in the Scotland peninsula and surrounding islands.

This integrated forward modelling has proved valuable for the validation of the geometries retrieved after seismic profiles interpretation against the observed gravity and magnetic fields. Furthermore, we provide a more detailed and geologically-consistent reconstruction of the supra-basement sedimentary basins and retrieve location and geometries of the deeper intrusive bodies addressing their nature in the complex geodynamic evolution of the area. Some of such newly defined basins (i.e. the DBB and EOB) could be of interest in the topics of the energy transition and their need further detailed investigations.