The stress pattern of Iceland

Abstract

Iceland is located on the Mid-Atlantic Ridge which is the plate boundary between the Eurasian and the North American plates. It is one of the few places on earth where an active spreading centre is located onshore but the stress pattern has not been extensively investigated so far. In this paper we present a comprehensive compilation of the orientation of maximum horizontal stress (SHmax). In particular we interpret borehole breakouts and drilling induced fractures from borehole image logs in 57 geothermal wells onshore Iceland. The borehole results are combined with other stress indicators including earthquake focal mechanism solutions, geological information and overcoring measurements resulting in a dataset with 495 data records for the SHmax orientation. The reliability of each indicator is assessed according to the quality criteria of the World Stress Map project.The majority of SHmax orientation data records in Iceland is derived from earthquake focal mechanism solutions (35%) and geological fault slip inversions (26%). 20% of the data are borehole related stress indicators. In addition minor shares of SHmax orientations are compiled, amongst others, from focal mechanism inversions and the alignment of fissure eruptions. The results show that the SHmax orientations derived from different depths and stress indicators are consistent with each other.The resulting pattern of the present-day stress in Iceland has four distinct subsets of SHmax orientations. The SHmax orientation is parallel to the rift axes in the vicinity of the active spreading regions. It changes from NE–SW in the South to approximately N–S in central Iceland and NNW–SSE in the North. In the Westfjords which is located far away from the ridge the regional SHmax rotates and is parallel to the plate motion.