We have utilised potential field data in combination with seismic interpretation and bedrock mapping to improve the understanding of basement structures and rifting processes on the continental shelf offshore Nordland, northern Norway. Comparing the Bouguer gravity field to gravity responses from Airy roots at different depths for the northern Scandinavia mountains it is shown that the compensating masses are situated at a relatively shallow depth in the upper crust. The Tysfjord granite within the Transscandinavian Igneous Belt extends to a substantial depth (minimum 22 km) in the crust. These voluminous granites may be related to Proterozoic plate subduction along the western edge of the Baltic shield, analogous to the Sierra Nevada Batholith in California. The NW-SE trending late Devonian extensional structures (Kollstraumen detachment, and Nesna and Sagfjord shear zones) extend from the mainland north-westwards below the Helgeland, Vestfjorden and Ribban basins. The Bivrost Lineament most likely represents a detachment dipping 5-15° to the southwest and may constitute the offshore extension of the Nesna shear zone. The entire Nordland mainland and offshore area is to a great extent affected by the NE-SW trending late Caledonian gravity collapse. Later (Late-Palaeozoic – Mesozoic) offshore rifting events have resulted in a shallow Moho and rotated fault-blocks separated by transfer zones giving rise to intermediate wavelength gravity and aeromagnetic anomalies. Changes in Moho depths occur across the Surt, Bivrost and Vesterålen transfer zones indicating that these structures are continuous to great depths within the crust. The more local transfer zones separate structural domains characterised by different fault-polarities within the Ribban and Vestfjorden basins. The Myken intrusive complex (Palaeocene age?) to the northeast of the Utgard High gives rise to large gravity and magnetic anomalies comparable to the anomalies originating from the basement structuring.
Bridging the gap between the onshore and offshore geology in Nordland, northern Norway