Density contrasts in the upper mantle and lower crust across the continent-ocean transition: constraints from 3-D gravity modelling at the Norwegian margin

Abstract

SUMMARY 3-D gravity models that consider the highly complex density structure of the Norwegian passive volcanic continental margin require a significantly lower density in the mantle below the ocean than below the continent. This is especially pronounced on the crust-free residual gravity anomalies obtained by 3-D gravity stripping. This density contrast in the mantle indicates different physical properties either related to thermal or compositional conditions beneath the two domains. We assess the gravity response of thermal differences and find that a hotter oceanic mantle compared to a colder continental mantle could cause the density contrast. Accordingly, 3-D lithospheric-scale gravity models considering temperature-dependent mantle densities indicate that the transition between continental and oceanic lithosphere is rather sharp with a steep gradient in thickness beneath the continent–ocean transition zone. Furthermore, a mantle wedge of reduced density extends from the oceanic Jan Mayen Fracture Zone to the continental Jan Mayen Lineament and points to increased mantle temperatures and/or compositional changes beneath the lineament. The models further indicate the presence of high-density bodies within the lower crust at the continent–ocean transition that are continuous with the lowermost oceanic layer 3B in terms of geometry and density. This supports the concept of underplated lower crust beneath the western Norwegian margin. Finally, NE–SW trending linear zones of increased density are required in the lower crust beneath the eastern Voring and More Basins to fit the short-wavelength gravity signal.