KONTINENTALSOKKEL

A high sensitivity aeromagnetic survey was carried out over an area of c. 9164km2 in Vestfjorden. Data acquisition was carried out under magnetic conditions, thus giving high quality data. After second order polynomial statistical levelling of the tie lines and lines, residual levelling errors were reduced using floating differential median filtering. Six maps were produced for the investigated region. One of these showes magnetic total field anomalies after correction for the IGRF of 1995. Two maps show residual magnetic values after application of different highpass filters.ø In addition, maps portraying the magnitude of the vertical and total magnetic gradients are presented. The residual maps after highpass filering delineate anomalies with amplitudes as low as 0.1-0.2 nT. Therefore, the high sensitivity aeromagnetic data include information on shallow and weak magnetic sources in the sedimentary column in addition to the more familiar crystalline basement sources.

A high sensitivity aeromagnetic survey (RAS-03) was carried out in an area of c. 41400 km2 over the Røst Basin offshore Lofoten, Northern Norway. Data processing comprised spike removal and data editing, IGRF correction, loop closure levelling and floating differential median micro-levelling. Four maps were produced for the survey area. One of these shows magnetic total field anomalies after correction for the IGRF of 2000. Two maps show residual magnetic values after application of two different high pass filters. A map showing magnetic analytic signal is also presented. The residual maps after high pass filtering delineate anomalies with amplitudes as low as 0.1-0.2 nT, representing shallow and weak magnetic sources in sedimentary basins.

Aeromagnetic data from the Røst Basin (RAS-03) were acquired in 2003 and compiled with neighbouring data-sets. The new Ra 3 compilation (Røst Aeromagnetics 2003 consisting of LAS-89, NAS-94, VAS-98 and RAS-3) has been reprocessed and subsequently merged with the SPT-93, NGU-69, NGU-73 and NRL-73 surveys in addition to the mainland of Norway aeromagnetic grid data. A compilation of the existing gravity data in the area has also been carried out. Forward 3D modelling along 14 profiles is constrained by available data on density, magnetic properties and reflection and refraction (OBS) seismics. The new data show that the previously interpreted oceanic fracture zones (Bivrost, Jenegga and Vesterålen) do not exist. Apparent offsets in the oceanic spreading anomalies were caused by poor navigation and wide line spacing of the vintage datasets. Consequently, opening of the Norwegian-Greenland Sea occurred along a stable axis without offsets of the oceanic spreading anomalies. A basement structure map has been compiled, and contains depth to magnetic basement, regional basement faults, and transfer zones. The Sandflesa and Flakstad basement highs at depths of approximately 6 km occur to the southwest and west of the Utrøst Ridge. The Bivrost, Vesterålen and Lenvik transfer zones define the regional segmentation of the Lofoten margin. The local Mosken and Melbu transfer zones are also located where shifts in polarity occur along faults and separate individual rotated fault blocks. The transfer zones are not spatially connected to younger oceanic fracture zones as previously interpreted. Depths to magnetic basement are locally larger than the depths obtained from gravity and seismic data (e.g. in the Helgeland and Vestfjorden basins and the Nordland Ridge). We interpret these to represent down-faulted Caledonian nappes, Devonian sediments or low-magnetic (amphibolite facies) Precambrian rocks. Voluminous mafic intrusions occur in both the basement and the overlying sediments in the Sandflesa high area and cause large gravity and magnetic anomalies. Basement depth estimates from gravity and magnetic data to the east of the Utrøst Ridge and north of the Marmæle Spur show that there may exist an approximately 2 km thick sequence of low-density sediments below the base-Cretaceous unconformity in this area.

The crustal structrue in the outer Vøring Basin is investigated by forward modelling of gravity data and interpretation of the magnetic field and the isostatic state. In addition all available magentic and gravity data for the Vøring Basin have been reprocessed. BP delivered depth-converted interpretation along six seismic profiles. These, together with the available results of OBS arrays, constrain the geometry of the structural models. The modelling has been done in 2D and where needed in 3D, Especially in the central and northern Vøring Basin, lateral changes in the density structures perpendicular to the seismic profiles had to be regarded to adjust the models.
The 2D and 3D models show that the gravity fields is mainly influenced by the thickness of the sediments, the depth of the Moho and the occurrence of magmatic underplating. The high density magmatic underplated material is probably related to the intrusion of sills from the Vøring Escarpment into the Vøring Basin and stops westwards of the Fles Fault Complex. To the east of the Fles Fault Complex normal lower crustal material is present instead.
Seismic results show in the whole area deep crustal reflectors, which have been previously interpreted to be connected to magmatic underplating or top basement. The modelling results indicate that these reflections are connected to magmatic underplated structures and intracrustal reflectors. The modelling results are subsequently used to produce new maps of the depth to Basement and Moho.
The shape of the magnetic field in the outer Vøring area, suggests, in contrast to the gravity field, shallow sources, which are probably connected to igneous rocks. But in general the magnetic signature in the outer Vøring Basin shows no prominent anomalies. The main magnetic anomalies are caused by the transition to oceanic domains.

Aeromagnetic data from the Røst Basin (RAS-03) were acquired in 2003 and merged with 12 neighbouring datasets. A compilation of the existing gravity data in the area has also been carried out. Forward 3D modelling along 17 profiles is constrained by available data on density, magnetic properties and reflection and refraction (OBS) seismics. The new data show that the previously interpreted oceanic fracture zones (Gleipne, Surt, Bivrost, Jenegga and Vesterålen) do not exist. Apparent offsets in the oceanic spreading anomalies were caused by poor navigation and wide line spacing of the vintage datasets. Consequently, opening of the Norwegian-Greenland Sea between the Jan Mayen and Senja-Greenland fracature zones occurred along a stable axis without offsets of the oceanic spreading anomalies of jumps in spreading axis. Reconstruction of the East Greenland aeromagnetic data to Chron 22 (c. 47.7 Ma) reveals that a c. 50 km wide igneous complex cut across the spreading anomalies 24A, 24B and 23 from the Vøring Marginal High on the Norwegian margin to the Traill Ø on the East Greenland coast. The complex (refereed to as Traill Ø-Vøring igneous complex in the present report) links up with the NE-SW trending initial magmatic lineament (IML) between the Traill Ø and the opening history and excludes the need to invoke the abandoned spreading ridge and the Gleipne Fracture Zone along the Vøring margin.
(Forkortet)

Aeromagnetic data from the Jan Mayen Fracture Zone (JAS-05) were acquired and processed during the autumn of 2005 and compiled with neighbouring data-sets. Data processing comprised spike removal and data editing, IGRF correction, statistical levelling of tie lines and profiles. The new JAS-05 survey has been merged with more than 40 other surveys including the mainland of Norway, Svalbard and East-Greenland. A compilation of the existing gravity data in the area has also been carried out. Potential field modelling has been constrained by available data on density, magnetic properties and reflection and refraction (OBS) seismics. New features have been revealed by the interpretation of the Jan Mayen Aeromagnetic Survey 2005 (JAS-05). The Aegir Marginal High (AMH) represents an inverted structure, uplifted during late Oligocene-early Miocene time. The thickening of the oceanic crust is syn-accretion and is not necessarily related to a Late Miocene underplaying (post-accretion), as previously assumed. The magnetic anomaly map shows also evidences of earlier deformation and block rotation linked with a propagator system along the Aegir through. A late Eocene displacement of the magnetic chrons suggests a progressive plate reorganisation leading to a major pulse in Oligocene. The major pulse coincides with two main unconformities seismically observed along the Jan Mayen Corridor (PM and MU) and could explain the main uplift phase of the AMH as well. We propose also a new challenging model for the Mid-Norwegian breakup system. Our tectonic analysis suggests that a triple junction Ridge-Ridge-Fracture Zone was initiated during the Late Paleocene-Early Eocene breakup between Greenland-Jan Mayen and Norway. Our interpretation infers also that the north Atlantic breakup probably started earlier at C24R or C25 time south of the EJMFZ. Spreading rates between the Aegir and Mohns ridges appear on the new magnetic compilation to differ by ~2 mm/yr for much of the period ranging from continental break up to extinction of the Aegir Ridge. The contrasting spreading rates of either side of the Jan Mayen Fracture Zone (JMFZ) resulted in dextral strike-slip along the JMFZ. Preliminary modelling suggests that such a minor difference in spreading rates could have led to significant shortening in the Vøring Basin. This model can explain why most of the mid-Norwegian domes are located to the north of the Jan Mayen Lineament.

Confidential to 31.12.2008
The report presents a 3D model of the entire mid-Norwegian margin, with special emphasis on the Trøndelag Platform. Based on the interpretation of OBS profiles, long-offset seismic reflection and petrophysical data the crustal structure was modelled. Seismic horizons provided by Statoil were used to refine the model in the area of the Trøndelag Platform.

The high-sensitivity Southern Nordkapp Basin Aeromagnetic Survey (SNAS-06) was carried out in an area of app. 4000 km2 in the Barents Sea, offshore Finnmark, northern Norway. Data processing comprised spike removal and data editing, IGRF correction, statistical levelling, floating differential median micro-levelling and directional cosine filtering (decorrugation). Three maps were produced for the survey area. One map shows the magnetic total field anomalies after correction for the IGRF of 2006. Another map shows residual magnetic values after application of a 20 km high-pass filter. A map showing magnetic analytic signal is also presented. The residual map after high-pass filtering delineates anomalies with amplitudes as low as 0.1-0.2 nT, representing shallow and weak magnetic sources in the sedimentary basin. Salt diapirs coincide clearly with negative magnetic anomalies. Small positive magnetic anomalies within the salt-related negative anomalies may reflect slivers of sedimentary rocks (e.g. claystones) within the diapirs.