Reverse-slip offsets and axial fractures in road-cut boreholes from the Caledonides in Finnmark, northern Norway: neotectonic stress orientation indicators

Abstract

Reverse-slip offsets of road-cut drillholes, and axial fractures developed in the walls of many such holes, have been recorded from areas in the Porsangerfjord and Laksefjord districts of the Caledonides of Finnmark, northern Norway. Displacement vectors of the offset boreholes indicate movement towards 120–136° in phyllites of the Laksefjord Nappe. At the base of the Kalak Nappe and in the Gaissa Nappe in western Porsanger, reverse-slip offset is directed almost due east. Observations of the extensional axial fractures, which are considered to have developed normal to the least principal in situ stress, σ 3, at the time of blasting, support a c. ESE–WNW orientation of S Hmax at or close to the surface in these parts of Finnmark, possibly with a slightly more E–W trend in western areas. The consistency of orientation of these structural features is considered to reflect a regional stress pattern and is not merely a manifestation of the local geology or topography.