Crustal inhomogeneities in the Northern North Sea from potential field modeling: Inherited structure and serpentinites?

A new crustal model for the northern North Sea was developed by gravity and magnetic modeling along the deep seismic line NSDP84-1. Utilizing vertical gradients allowed distinguishing between shallow and deep crustal sources. The upper crust is characterized by low magnetic susceptibilities and low densities, which is typical for felsic rocks. A new finding was that the deep crust below the western Viking Graben and the East Shetland Basin is the source of high magnetic anomalies combined with low gravity anomalies, which was interpreted to represent rocks with very high magnetic susceptibilities and low to intermediate densities. Such rock parameters may indicate serpentinites, but intermediate intrusives or a combination of both is also possible. Honoring the string of three near equidistant magnetic maxima, which follow the trend of the NNE–SSW striking East Shetland Basin in the map plane, it is suggested that this area is part of an island arc of the Iapetus Ocean which has been assembled during the collision between Laurentia and Baltica in late Silurian times. Partly serpentinized peridotites and intermediate intrusives will relate in such a model to slab dehydration of the subducting oceanic plate below the island arc. These inherited or synorogenic serpentinites are expected to persist in the geothermal regime of the Caledonian orogeny to a depth of at least 50 km. Increased heat flow by later rift phases will have caused metamorphism of the remaining serpentinites to meta-peridotites at depth below the present day Moho. Fluid release related to dehydration of the serpentinites may have triggered further serpentinization of the inherited, partly serpentinized rocks at shallower depth. An alternative origin for the suggested serpentinites, valid only for the area under the western part of the Viking Graben, may be synrift serpentinization due to the heavy faulting during the Jurassic rift phase.

► North Sea crustal model by gravity and magnetic modelling of deep seismics (NSDP84-1).
► Lower crustal rocks with high susceptibilities – low densities under the East Shetland Basin.
► Interpreted as serpentinites/intrusives from subduction at an Iapetus Ocean island arc.
► P-T conditions in the Caledonian orogeny save serpentinites shallower than 50 km.
► Secondary serpentinization by dehydration of deep serpentinites during later rift phases.