The 3D stress field of Nordland, northern Norway - insights from numerical modelling

The Nordland area in northern Norway is the seismically most active area on mainland Fennoscandia. It exhibits patterns of coastal extension, which contrasts with the first-order regional stress pattern that reflects compression aligned with the North Atlantic ridge-push. The regional stress field has been considered to emanate from the interaction of ridge push and glacial isostatic adjustment; while the local stress pattern can be additionally influenced by gravitational, topographic stresses, as well as the flexural effects of erosion and sediment deposition. We employ finite element numerical models at a crustal scale to study the 3D stress field, using existing geometric constraints from previous geophysical studies. Internal body forces, induced by variations in density, topography or Moho depth, already yield significant deviatoric stresses. In the models tested, these can strongly influence the near-surface stress regime, in particular for the continental margin setting we are investigating. In addition, redistribution of rock mass, which occurred mainly under Pleistocene glaciation, can modify the stress field significantly on a semi-regional scale. We consider this process to be the main driver for the coastal extension, in particular in areas where erosion has been high. The northern Atlantic coast of Norway is an area of neotectonic activity and anomalous patterns in seismicity and land uplift. This study aims to improve our understanding of the present-day stress regime in this area. In addition, our study sheds light on the importance of stresses due to redistribution of sediments, a stress driver that is often neglected in stress studies. An earlier version of this manuscript was included in a non-peer reviewed project report (Gradmann et al., 2018).