2010:43 Displacement along extensive deformation zones at the two SKB sites: Forsmark and Laxemar

The Fennoscandian shield is distinguished by that the exposed bedrock is mainly composed of Precambrian metamorphic and igneous rocks older than a billion or one and a half billion years with few easily distinguished testimonies for the younger history. Large parts of the present ground surface closely coincides with a late Precambrian denudation surface; the sub-Cambrian peneplain. Palaeozoic and younger sediments were deposited on the peneplain, but these sediments have been removed from most areas that now form the mainland of Sweden and Finland and where there area just some few remnants. However, Palaeozoic sediments are abundant in the Baltic Sea. The Palaeozoic sedimentary rocks may form a memory of the late Palaeozoic and younger tectonic events in the underlying basement rocks. Such data are used in this report to complement the structural observations made at sites located on the mainland, giving information on displacement along faults. For construction of a geological repository for disposal of nuclear waste and for its long term safety it is important to understand the late history of geological and seismic events to be able to estimate its influence and consequences for the repository.

The purpose of the current project is to describe displacement along tectonic structures forming the boundary of the sites Forsmark and Laxemar where the Swedish Nuclear Waste Management Co (SKB) recently has finished site investigations for a repository for spent nuclear fuel. The description of displacement will be based on information gained from marine geological investigations performed in the Baltic Sea. General observations of late displacement along faults in the Baltic-Sea Basin are also made in order to compare these with structures of similar orientation in the site areas. Of special interest are structures with evident indications of late bedrock movements where future movements cannot be excluded.