Search for Glacio-Isostatic Faults in the Vicinity of Olkiluoto

Glacio-isostatic land uplift continues in Finland at a moderate rate of 2 – 9 mm/year and is at its greatest in the Quarken area. In the vicinity of Olkiluoto Island the rate is 5 mm/year. This land uplift is thought to happen along the ancient fracture zones of the earth’s crust and continues without creating new faults. Possible glacio-isostatic faults near Olkiluoto could give new information about the movement of bedrock. In this work the aim was to find out if there exist significant postglacial faults. Research included the mapping of all long fractures and faults at the shore outcrops of islands surrounding Olkiluoto. During the mapping all such marks were observed, which could be exposed if the fault has been active after glaciation. Most of the observations were made on the south and west side of Olkiluoto, because on the north side there are only a few islands. In addition, the work included some targets from Kustavi and Korsholm, where (by personal communication) some possible postglacial faults have been observed. Observations were done on 136 outcrops and 619 fractures and faults. Only 30 were true faults. The shortest measured fractures were 4 m long and the longest 58 m. The average length was 11 m. From the observed 30 faults 13 were located in a rapakivi area in Kustavi and 17 around Olkiluoto. In the rapakivi area some mylonites and one clear fracture were deep and old features, except the fault, which possibly was reactivated also after glaciation. Other young faults in the rapakivi do not reach deeper than the first horizontal fracture. In the vicinity of Olkiluoto Island there were no post-glacially active faults, which has undisputedly been active after glaciation. Marine forces and frost have moved some small blocks on outcrops.