Basement inhomogeneities and crustal setting in the Barents Sea from a combined 3D gravity and magnetic model

Abstract

This thesis provided a new understanding of the lithospheric structural setting and anomalies observed in the greater Barents Sea.The Barents Sea has been focus of attention for geological and economical reason since the 70ies-80ies. The crustal and lithospheric evolution of this area has been poorly understood and relatively challenging because it involved a large number of different processes. During its evolution, plate aggregation and orogenesis have been alternated with episodic rifting and magmatic events. The final result is the present day continental shelf area formed by a heterogeneous crust and continental mantle, dominated by structural highs, platforms and structural lows (grabens, half graben and large sag basins). Although the general structures have been well outlined, numerous problems have been encountered in understanding the structure and evolution of the basins and the crust and their relation with the geodynamic evolution. Moreover the offshore propagation of Timanian, Caledonian and Uralian structures are still disputed.In this thesis, I have developed a new understanding of the lithospheric setting and properties of the Barents Sea using an integrated geophysical approach. The results have been obtained by combining gravity, magnetic and geoid potential fields with seismic, petrophysics and offshore observations. 3D magnetic anomalies inverse models have been developed first to define the distribution of magnetic crustal properties. The inversion results have been further integrated with 2D forward modelling and potential field analysis. The magnetic crustal units have been further refined by a 3D forward model which beside defining magnetic and density properties provided, major sedimentary interfaces, top basement, upperlower crust boundary and Moho depth. Finally, thermal and compositional models have been constructed to constrain the nature of the lithospheric mantle.The adopted integrated approach provides an enhanced regional understanding of the structure and devolution of the Barents Sea Four distinct Barents Sea basement types have been distinguished: Archaean-Paleoproterozoic, Timanian, Caledonian, and Uralian terranes. A distinction of the different allochthon nappes forming the Caledonian terranes is also proposed and a correlation with the onshore structures made. The crustal setting resulting from the models support the bifurcated extension model of the Caledonian into the western Barents Sea. The existence of a Svalbard micro-plate (Barentsia) has been identified as a thick and distinct crustal unit located in the northwest Barents Sea.The southeastern Barents Sea results to be the extension of the Timan-Pechora basement terranes, that are traced further north in the North Barents Basin. The lithospheric mantle velocity anomaly under the East Barents Sea has been interpreted as a relict Paleo-Proterozoic craton accreted toward Paleo-Baltica during the Timanian event.Thermal and compositional models of the lithospheric mantle provide also evidences of lithospheric-sublithospheric mantle interaction due to rifting and due to large magmatic events, as for example the magmatism on Svalbard and the opening of the North Atlantic.