Gravity and magnetic data integration for forward modeling in Svalbard

Abstract

The Svalbard archipelago is located in Arctic Norway and forms the northwestern corner of the Barents Shelf. It has a complex geological history with multiple phases of deformation. NNW-SSE orientated long-lived tectonic lineaments separate the Pre-Caledonian basement rock of Svalbard into three distinct provinces. Svalbard is recognized for its high-quality geological outcrops, but less knowledge is available on density structure and geometry of the subsurface. To put further constraints on the subsurface, we performed a multi-physical data integration and used these as constraints for forward modeling. The specific focus was on the geophysical signal of the emplacement of magmatic bodies associated with the High Arctic Large Igneous Provinces (HALIP) and one of the main fault zones, the Billefjorden Fault Zone (BFZ). Input data from various data sets sensitive to different physical parameters in the subsurface were considered. Among them were regional magnetic and gravity data, 2D seismic profiles, borehole logs and recently acquired inland gravity data. This new high resolution gravity data was collected in April 2022 along seismic lines in central Spitsbergen. We completed a satisfactory quality check of the data, after which they were used as input data for forward modeling where available. Potential field data, magnetic and gravity data, are sensitive to changes in magnetic susceptibility and density, respectively. We enhanced the preliminary understanding of the regional potential field data by applying filter techniques to the data grids, amplifying long wavelength trends, and allowing depth and edge detection. The subsurface magnetic susceptibility and density structure was then quantified by magnetic and gravity forward modeling on five roughly E-W orientated profiles on Spitsbergen located along seismic lines. We made seismic interpretations to establish the subsurface geometry of the models, after which suitable physical parameters were assigned based on the available data. An iterative procedure followed to obtain the best fit of the modeled subsurface to the observed potential field data. The regional data showed that the BFZ coincided with a high in magnetic and gravity anomalies, coming from a relatively shallow source. A long wavelength, deep magnetic anomaly was positioned below central Isfjorden but had no gravitational high. HALIP-related outcropping igneous rocks and sills were previously interpreted in this region. However, the depth and signal of the magnetic anomaly indicated a deeper situated source was necessary. Forward modeling showed that the gravity anomaly trends were mostly explained by the basement topography, with an elevated basement along the BFZ and the influence of a thick sedimentary basin. To match the magnetic data on the three profiles intersecting the Isfjorden anomaly, large basement susceptibility contrasts were needed. As the other two profiles beyond the extent of the Isfjorden anomaly did not require heterogeneities in basement susceptibility, we favor the presence of a local magmatic body in the deep subsurface. Further geological implications of the forward modeling results are discussed, including scenarios for the emplacement of this magmatic body. The regional potential field analysis and the forward models have enabled us to put more constraints on the geometry and physical properties of the subsurface of Svalbard.