Geophysical modelling of the Bjerkreim–Lobe, southern Norway

Abstract

SUMMARY The Bjerkreim–Sokndal (BKS) intrusion in southern Norway has been studied for decades due to the presence of magnetic remanence creating anomalies 12 000 nT below background as measured by airborne magnetic surveys. The strong magnetic remanence also makes the BKS intrusion a good Earth analogue for remote studies of planets that have prominent magnetic signatures, such as Martian geological environments. Although numerous geophysical surveys and samples have been collected in the area, there are limited 3-D geological interpretations of the subsurface. Here, we used existing geophysical data to conduct forward and inversion modelling of the Bjerkreim lobe to investigate the subsurface geometry of the BKS intrusion. An extensive petrophysical property compilation was used as input data for the models, in combination with airborne magnetics and digital elevation models. This petrophysical compilation was initially analysed using principal component analysis to understand which variables would have the greatest impact on the models. Forward and inversion modelling show that cross-cutting jotunite bodies, and small anorthosite blocks within the Bjerkreim lobe have a limited depth extent of 1 km. Massive and foliated anorthosites to the west of the Bjerkreim lobe extend to depths greater than 4 km indicating that the BKS intruded into these anorthosites. Complications in magnetic field fitting during the forward modelling of megacyclic units with strong magnetic remanence and the results from a new ground magnetic survey support the need to revisit mapped contacts of the cyclical units.