Marie-Andrée Dumais

The Geological Survey of Norway has collected frequency-domain electromagnetic and gamma-ray spectrometric data through airborne mapping since 1972 on the mainland of Norway. These data were acquired and processed using the technologies available at the time of the campaigns. Consequently, the resolution and quality of individual surveys vary across the country.Over the years, helicopter-borne frequency-domain electromagnetic data were acquired using various instruments with up to five different frequencies. While today, these data are inverted to determine apparent resistivity using a half-space earth model, inversion has not been consistently carried in the past. To build a homogeneous compilation, we are re-processing and inverting all existing data using modern levelling and noise-reduction tools. By limiting instrumental and environmental noise, we create a country-scale map of conductors. For each frequency, apparent resistivity data from all surveys are merged into a single, seamless compilation.The primary objective of reprocessing and compilation is to recover the maximum amount of legacy airborne data and produce a uniform coverage map. This unique compilation serves as a crucial tool for identifying conductors in evaluating mineral resources and for general bedrock mapping. The location and continuity of conductive structures are interpreted, across survey boundaries, providing critical insights into the deeper sources of mineral systems at a regional scale.Similarly, gamma-ray spectrometric data were collected using different instruments with varying detector volumes, leading to discrepancies in large-scale resolution. Presently, these data are corrected for live time, cosmic and aircraft background removal, radon removal, Compton stripping, and height attenuation following the International Atomic Energy Agency (IAEA) recommendations. Since 2002, the final products are the ground concentration for potassium, uranium and thorium. Prior to this, window counts of gamma rays for each respective radioelement were reported. A homogeneous compilation is obtained after a careful data re-processing including noise reduction, levelling and calibration. For surveys where original calibration parameters are missing, data from neighbouring overlapping surveys allow to derive the ground concentration. The final compiled ground concentration maps provide geochemical insight about the top half meter of the ground. Combined with electromagnetic data, links between surface lithology and deeper sources can be studied.Airborne geophysical operations are capital-intensive. A standardized and homogeneous re-processing of frequency-domain electromagnetic and gamma-ray spectrometric data maximizes the value of Norway’s existing geophysical assets. These new regional datasets will contribute to mineral exploration, effective bedrock mapping, and societal safety by identifying natural and anthropogenic radioactivity, quick clay and rock instabilities, and other environmental hazards.

The Svalbard archipelago is well‐known for its outcropping geology, but its subsurface geometry and physical properties are less well constrained. To address this knowledge gap, a multidisciplinary approach has been applied to integrate potential field data with seismic reflection profiles, exploration boreholes and geology. We constructed five 2D gravity and magnetic forward models along pre‐existing seismic lines in central Spitsbergen, utilizing newly acquired ground‐borne gravity, helicopter‐borne magnetic, and regional gridded potential field data. Specifically, we focused on characterizing the geophysical signature of the pre‐Devonian basement composition and topography, position of the Billefjorden Fault Zone (BFZ), and the magmatic bodies related to the High Arctic Large Igneous Province (HALIP). Regional trends show a 15 km wide NNW‐SSE trending magnetic and gravity high east of the BFZ and a long‐wavelength elliptical magnetic anomaly of ca. 50 km diameter below central Isfjorden with no anomalous gravity response. We observe outcropping igneous rocks and interpret HALIP‐related igneous sills on 2D seismic data in this region matching the short‐wavelength signal. Our models show that most of the gravity signal is related to the arrangements of the sedimentary basins and the influence of the BFZ on the basement topography. Large lateral intra‐basement magnetic susceptibility contrasts and a deep magnetic source were necessary to fit the magnetic signal, indicating basement heterogeneity and locally the influence of magmatic bodies.

Magnetic data, along with their associated chrons, have played a crucial role in deepening our understanding of oceanic crust formation and seafloor spreading dynamics. Over the past 25 years, the Geological Survey of Norway has conducted extensive magnetic surveys, acquiring more than 172,846 km of new aeromagnetic profiles in the Norwegian-Greenland Sea (NGS). This contribution presents our latest regional compilation of the NGS, shedding light on the complex tectonic evolution of the region since the onset of continental breakup. The NGS witnesses diverse tectonic regimes and structural features, including sub-oceanic basins of different ages, microcontinents, and conjugate volcanic passive margins, between the Greenland-Iceland-Faroe Ridge in the south and the Arctic Ocean in the north. The new aeromagnetic compilation suggests that the highly magmatic breakup in the NGS was diachronous and initiated as isolated and segmented seafloor spreading centres. The early seafloor spreading system, initiating in the Early Eocene, gradually developed into atypical propagating systems, with subsequent breakup(s) following a step-by-step thinning and rupture of the lithosphere. Newly formed spreading axes initially propagated towards local Euler poles, died out, migrated or jumped laterally, changed their propagation orientation, or eventually bifurcated. The final line of lithospheric breakup may have been controlled by highly oblique extension, associated plate shearing, and/or melt intrusions before and during the formation of the Seaward Dipping Reflectors (SDRs). The Inner SDRs and accompanying volcanics formed preferentially either on thick continental ribbons or moderately thinned continental crust. The segmented and diachronous evolution of the NGS spreading activity is also reflected by a time delay of 1–2 Myrs expected between the emplacement of the SDRs imaged at the Møre and Vøring margins. Further north, the initiation of spreading that led to the formation of the Knipovitch Ridge began around C6 (~20 million years ago) within a distinct and oblique oceanic segment in the Fram Strait region. Magnetic observations indicate a broader continent–ocean transition, interpreted as exhumed lower continental material adjacent to the Barents Sea margin, which significantly reduces the mapped extent of the oceanic domain expected in the Fram Strait. This configuration also suggests the presence of a failed oceanic basin east of the Boreas Basin, which helps explain the resulting asymmetry in the spreading system. Meanwhile, several significant changes in spreading kinematics were recognised in the Norway Basin, with the first occurring in the Middle Eocene around 47 Ma (magnetic chron C21r), initiating rifting in the southern part of the Jan Mayen Microplate Complex. Inheritance and magmatism likely influenced the complex reorganisation of rifting, ultimately leading to the final dislocation of the Jan Mayen Microplate Complex from Greenland during the Late Oligocene/Early Miocene. The mechanism behind this final dislocation, likely triggered by overlapping rift systems, also raises questions about the controversial nature of crust and lithosphere stretching between the Faroes and Iceland.

Gravity data provide constraints on lateral subsurface density variations and thus provide crucial insights into the geological evolution of the region. Previously, gravity data from the Norwegian Arctic archipelago of Svalbard comprised an onshore regional gravity database with coarse station spacing of 2–20 km, offshore gravity profiles acquired in some fjords, airborne gravity, and satellite altimetry. The sparse regional point‐based onshore coverage hampered the direct integration of gravity data with seismic profiles acquired onshore Svalbard in the late 1980s and early 1990s. In April 2022, we acquired gravity data at 260 new stations along seven profiles from western to eastern Spitsbergen, with a cumulative length of 329 km. The profiles were acquired directly along selected seismic profiles and provide much closer station spacing (0.5–2 km) compared to the regional inland grid (2–20 km) acquired in the late 1980s (total number of onshore stations: 1,037). Having processed the data, we compared the first‐order density trends of our new data with the legacy regional grid. The new gravity data are consistent with the regional data, imaging a gravity low in the western part of the area underlying a foreland basin and a gravity high in the northwestern part of the area likely associated with a basement high or denser basement. We compare the new and vintage gravity using maps and profiles, linked to the known major tectonic features such as major basinal axes and fault zones, as well as other geophysical data sets including seismics and magnetics.

A recent aeromagnetic survey was carried out to refine our understanding of the structure and nature of the basement rocks beneath the Finnmark Platform (Norwegian Barents Sea). Magnetic depth source estimation and potential field models were used to explore the platform's basement architecture and evaluate the existence of Paleozoic basins. In the western part, magnetic fabrics suggest a continuation of Precambrian rocks into the adjacent platform. Further east, magnetic data reveal NW–SE-trending basement lineaments related to the Sørøya–Ingøya Shear Zone, formed during the Caledonian orogeny. Additionally, lower-frequency and discrete NNW–SSE lineaments may reflect the influence of older Precambrian structures. The Gjesvær Low, previously considered a post-orogenic basin, is now seen as a Caledonian backthrust system tied to the Sørøya–Ingøya Shear Zone. Both Precambrian and Caledonian inherited structures controlled and accommodated the development and segmentation of subsequent rift basins during the Paleozoic and Mesozoic. The NW–SE to WNW–ESE Precambrian trends guided early rift segmentation from the Devonian to Early Carboniferous, while Mesozoic rift border faults align with NE–SW to ENE–WSW-trending magnetic lineaments, largely interpreted as Caledonian in origin.

The interest for the polar regions and complex continental margins and ocean has increased during the last few decades. New technologies allow to conduct research in this hostile environment, permitting to investigate the tectonic and geodynamic history of the North Atlantic and Arctic oceans. In particular, the crustal and lithospheric structure of the Fram Strait and the transition from the Knipovich Ridge to the Barents Sea shelf and Svalbard are still poorly understood. Several multi-geophysical investigations from various campaigns since the 90s along the Western Barents Sea margin and the Northeast Greenland margin resulted in limited and contradicting interpretations of the crust and upper mantle. In this work, we study the spreading of the Knipovich Ridge and the regional tectonic of the Fram Strait and the Svalbard Margin.Our new KRAS-16 aeromagnetic data survey the complexity of the seafloor spreading history of the Fram Strait region. The high-resolution data identified the magnetic isochrons around the Knipovich Ridge and suggest the presence of several oceanic fracture zones and lineaments in the Fram Strait. The Knipovich ridge spreading initiated at C6 (20 Ma) and a ridge jump occurred at C5E. The oceanic crustal domain was consequently delineated. This new survey suggests that the continent-ocean boundary on the east Barents margin should be relocated up to 150 km farther west compared to previous studies. A 3-D magnetic inversion modelling identified zone with weak magnetization along the rift valley correlated with the absence of volcanic or bathymetric rise evidence. Combined with seismicity data available along the Knipovich Ridge, amagmatic and magmatic accretions show a segmentation of the seafloor spreading that correlates with the variation in magnetization along the rift valley. Furthermore, the new location of the continent-ocean boundary prompted to revise the existing 2-D seismic interpretations in terms of crustal interpretation and tectonic. This is tested further using joint 2-D gravity and magnetic field modelling and electromagnetic/magneto-telluric (CSEM/MT) data. A wide transition lithospheric domain likely comprising an exhumed lower crust or mantle is delineated from our interpretation. These results provide insights of the regional and structural nature of the Knipovich Ridge and its intricate development.

We integrated high-resolution aeromagnetic data and 2D/3D seismic data from the Norwegian Southwestern Barents Sea. The main objective is to address the long-standing question on the role of pre-existing basement structures in controlling strain accommodation and extension in the Finnmark Platform and adjacent rift basins. The thorough qualitative analysis of the high-resolution magnetic data reveals fault geometries, regional kinematics, magmatism and inheritance of older Precambrian/Caledonian structures. Through the application of second order derivative filters and depth-to-magnetic-source modelling, the trends of the Caledonian metamorphic fabrics are identified and correlated with the structure of buried basement faults and shear zones also imaged at the same level of resolution on 2D/3D seismic data. The magnetic data reveal an unprecedented detail of the basement fabrics dominated by high-frequency NW-SE trending magnetic lineaments associated with the semi-regional Sørøya-Ingøya Shear Zone. The high-frequency magnetic lineaments are superimposed by lower frequency NNW-SSE trending magnetic lineaments that reflect the inheritance of older Precambrian structures. At the edge of the Tromsø Basin, the new magnetic data highlight sill intrusions also visible on seismic data. Fault geometries, regional kinematics, and spatial distribution of the magnetic sources suggest that old detachments and younger Mesozoic faults reactivated the basement fabrics found along the graben borders. Focusing of strain accommodation at the edge of the Hammerfest Basin is helped as well as modulated by the presence of back-thrusted Caledonian nappes interpreted on the Finnmark Platform. Offshore, surface ruptures associated with graben formation align with the dominant NNW-SSE trending magnetic lineaments defining steeper normal faults that are characterised by right-stepping segments along the southern flank of the Hammerfest Basin. Based on potential field models, we finally quantify the crustal architecture of the rift and platform system. At upper crustal level, we test the presence and significance of potential Palaeozoic basin preserved at the edge of the basement hinge-zones. Potential field modelling also highlights and quantifies several rift domains defined by moderate to extreme thinning of the crust (low-β stretched domain, necking, and high-β hyperextended regions). The development of the necking zone is clearly influenced by the existence of former first-order and multi-scale inherited basement features preserved in the Finnmark Platform.

The lithospheric structure of the Fram Strait and the extent from the Knipovich Ridge to the Barents Sea shelf and Svalbard are poorly understood. Several multi‐geophysical investigations from various campaigns since the 90s along the Western Barents Sea margin and the Northeast Greenland margin resulted in insufficient and contradicting interpretations of the crustal and upper mantle settings in the oceanic and continental domains. New airborne magnetic data across the Knipovich Ridge and west of Svalbard provide new insights, reveal the complexity of the seafloor spreading history of the Arctic Atlantic Ocean, and indicate a European‐Eurasian continent‐ocean boundary located ∼150 km farther west than previously suggested. This new location of the continent‐ocean boundary prompted to revise the existing 2‐D seismic interpretations in terms of crustal domains and tectono‐stratigraphic setting. This is tested using joint 2‐D gravity and magnetic field modeling to derive an improved crust‐mantle model of the study. One recently acquired combined 2‐D controlled source electromagnetic/magneto‐telluric (CSEM/MT) profile across the Mohns Ridge was also modeled with potential field data and provided new insights into the tectonic settings of the crust and the mantle thermal anomalies. This study proposes to unify the various seismic and CSEM/MT interpretations using the new aeromagnetic compilation.

SUMMARY: Insights into the spreading evolution of the Knipovich Ridge and development of the Fram Strait are revealed from a recent aeromagnetic survey. As an ultraslow spreading ridge in an oblique system located between the Svalbard–Barents Sea and the Northeast Greenland rifted margins, the dynamics of the Knipovich Ridge opening has long been debated. Its 90° bend with the Mohns Ridge, rare in plate tectonics, affects the evolution of the Fram Strait and motivates the study of crustal deformation with this distinctive configuration. We identified magnetic isochrons on either side of the present-day Knipovich Ridge. These magnetic observations considerably reduce the mapped extent of the oceanic domain and question the present understanding of the conjugate rifted margins. Our analysis reveals a failed spreading system before a major spreading reorganization of the Fram Strait gateway around magnetic chron C6 (circa 20 Ma).