Finite-element modelling of stress patterns along the Mid-Norwegian continental margin, 62° to 68°N

Abstract

The inferred nature of the regional intraplate stress field off the coast of Norway is based primarily on borehole breakout information and earthquake focal mechanisms. The stress indicators reveal a slightly clockwise rotation of the orientation of the principal horizontal stress S H to the northwest with respect to the dominant E-W orientation observed in the northern North Sea. The stress regime is predominantly strike-slip and thrust fault, although two normal faulting events have occurred on the shelf and shelf edge. Within the Mid-Norwegian margin segment (62° to 68°N), the Trøndelag Platform exhibits large variations of observed S H orientations. This may be due to an almost isotropic horizontal stress state ( S H = S h) resulting from the superposition of local and regional stress sources. This paper presents the predicted tectonic stresses along the Mid-Norweigan margin based on a 3-D finite-element analysis. The two principal modelling objectives are to elaborate the effects of: (1) changes in the regional stress magnitude S H; and (2) variations in rock properties (i.e., Young's modulus) on the horizontal stress anisotropy HSA of the Vøring Basin and the Trøndelag Platform. The model incorporates simplified mainland topography, the Trøndelag Platform, the Vøring Basin, the Vøring Plateau, and the Lofoten Basin. Our modelling results indicate that horizontal stress anisotropy (HSA) and the regional S H magnitude within the various structural units is dependent on their strike with respect to the orientation of the present-day regional stress field. Specifically, we find: (1) a near-isotropic horizontal stress magnitude configuration for the southern part of the Vøring Basin where the gradient of topography across the Mid-Norwegian margin is oriented oblique to the regional trend of S H; (2) a maximum horizontal stress anisotropy HSA in the southern part of the Trøndelag Platform, this local HSA maximum bordering the HSA minimum of the Vøring Basin; (3) a narrow band of high HSA at the eastern edge of the Trøndelag Platform imposed by compressive gravitational stresses related to the elevated topography of mainland Norway; (4) a low HSA at the western margin of the Trøndelag Platform, related to spreading stresses associated with the topographic lowering towards the Vøring Basin; and (5) in case of a far-field stress of S H = 1.1 S h, the modelling results predict exchanged principal horizontal stresses S H and S h at the western margin of the Trøndelag Platform.