Deep structure of the Lofoten‐Vesterålen segment of the Mid‐Norwegian continental margin and adjacent areas derived from 3‐D density modeling

Abstract

To understand the major structural features of the sedimentary cover and crystalline crust of the Lofoten‐Vesterålen margin and the northern part of the Vøring segment of the Mid‐Norwegian continental margin, a lithosphere‐scale 3‐D structural model has been constructed. This model extends from the exposed crystalline rocks of the Fennoscandian Shield in the east to the Cenozoic oceanic domain of the Norwegian‐Greenland Sea in the west, covering the Vestfjorden, Ribban, and Røst Basins and the northern parts of the Vøring Basin and Trøndelag Platform. All available published and/or released data have been used to set the initial 3‐D model which has been validated by means of 3‐D density forward modeling to obtain a gravity‐consistent 3‐D structural/density model. Results from the 3‐D density modeling reveal that relatively thick sedimentary rocks are present in the distal Røst Basin below the lava flows. The presence of a low‐density more than 20 km thick granitic body has been modeled within the middle‐upper crystalline crust beneath the eastern part of the Vestfjorden Basin and the adjacent mainland. Moreover, the results of the 3‐D density modeling indicate the presence of an atypical low‐density lithospheric mantle beneath a large part of the Lofoten‐Vesterålen margin which is required to fit the regional component of the modeled gravity with the observed one. The pronounced crustal feature within the model area is the Bivrost Lineament that appears to be the deeply seated lithosphere‐scale boundary that delineates clearly the Lofoten‐Vesterålen segment from the Vøring margin showing contrasting densities and crustal thicknesses.

Relation between seismicity and tectonic structures offshore and onshore Nordland, northern Norway

“…At the shelf edge the rapid changes in both the Moho depth and the density of the rocks have earlier been associated with earthquake occurrence (Fejerskov & Lindholm, 2000), while the deeper structures, mapped by Maystrenko et al (2017), coincide quite well with the earthquake distribution observed at the northern end of the Nordland Ridge/Utrøst High (Fig. 1).…”

“…Although the BTZ is a geological transition zone, as shown by Maystrenko et al (2017), it did not appear to be an active earthquake source zone.…”

“…As Maystrenko et al (2017) noted, there are two low-velocity zones in Norway: one under the Lofotens and another beneath SW Norway (e.g., Kolstrup et al, 2015), and these zones coincide spatially with the areas of increased seismicity. Therefore, these authors suggested that the velocity Bugge et al (2002), and Bøe et al (2008); Precambian granitoids - Gustavson & Gjelle (1991), and Gustavson & Blystad (1995); Detachments mainland - Eide et al (2002); Detachments offshore -Olesen et al (2002);Faults -IKU (1995), Bugge et al (2002), Olesen et al (2002), and Bøe et al (2008); Basement depth - Olesen et al (2002) …”

“…Moreover, it was reported that the territory of Nordland exhibits features of neotectonic movements (Dehls & Olesen, 2000), which together with the seismicity have been related to stress generation from the ridge push of the Atlantic Ocean, the post-glacial rebound, and the sediment redistribution and topography (e.g., Olesen et al, 2013a). In addition, the geological settings in Nordland are very complex: the Caledonian nappes are folded down to 5 km deep (Midtun, 1988;Olesen et al, 2002), intrusive igneous bodies, such as the Transscandinavian Igneous Belt (e.g., Gaal & Gorbatschev, 1987;Henkel & Eriksson, 1987;Larson & Berglund, 1992;Hogdahl et al, 2004), are located onshore, while the Moho depth varies from less than 20 km offshore to the west to more than 45 km onshore to the east with a convex Moho bulge under the Lofoten-Vesterålen region (Olesen et al, 2002;Maystrenko et al, 2017). As earthquakes generally occur along pre-existing zones of weakness and result from a buildup of stress and/or reduced effective shear strength along favourably oriented faults (e.g., Davis & Reynolds, 1996), all the factors mentioned above could influence the stress field and may have an effect on the tectonic structures, which are abundant in the Nordland area.…”

Deep structure of the northern North Sea and southwestern Norway based on 3D density and magnetic modelling

“…An inherent non-uniqueness of the used approach has been described in detail by Maystrenko et al (2017). Here, we would only like to point out that numerous density/magnetic and structural models can be responsible for the same modelled gravity/magnetic field.…”