Quantification of neotectonic stress orientations and magnitudes from field observations in Finnmark, northern Norway

Fieldwork was conducted in Finnmark, northern Norway, with the purpose of detecting and measuring stress-relief features, induced by quarrying and road works, and to derive from them valuable information on the shallow-crustal stress orientations and magnitudes. Two kinds of stress-relief features were considered in this study. The first consists of drillhole offsets that were found along blasted road-cuts and which were triggered by the sudden rock unloading following the actual blasting. Vertical axial fractures found in the concave remains of boreholes represent the second kind of stress-relief feature. The axial fractures are tension fractures produced by gas overpressure inside the drillhole when the blast occurs. As such, their strike reflects the orientation of the ambient maximum horizontal stress axis. The borehole offsets show mostly reverse-slip displacements to the E-SE and the axial fractures trend NW-SE on average, in agreement with NW-SE compression induced by North Atlantic ridge-push forces. Mechanical considerations of the slip planes offsetting some of the drillholes lead to the conclusion that the magnitude of the maximum horizontal stress at the surface is in the range ∼0.1-∼1 MPa. This range of magnitudes is 1-2 orders less than the horizontal stress magnitudes measured at the surface in other post-glacial environments (e.g. Canada). It is suggested that this difference is related to the marked decline in stress that followed the tremendous post-glacial burst of earthquake activity that affected Fennoscandia but apparently not the Canadian Shield.