Geological characterisation of geophysical lineaments as part of the expanded site descriptive model around the planned repository site for high-level nuclear waste, Forsmark, Sweden

Abstract: The Swedish Nuclear Fuel and Waste Management Co. (SKB) started a project in 2020 near Forsmark, Sweden with the aim to expand the deformation zone model to encompass the entire catchment area around the planned repository for spent nuclear fuel. This thesis builds upon a lineament model that was constructed for SKB which utilizes airborne magnetic data supported by airborne Very Low Frequency (VLF) and elevation data. A selection of the lineaments was investigated in the field to verify whether or not they correspond to deformation zones, with a subsequent structural analysis for each confirmed deformation zone. My results show that NW-SE trending ductile deformation zones are characterized by a high degree of localized strain and are locally brittlely reactivated. These NW-SE trending deformation zones are found in areas inferred to previously have been affected by high ductile strain. The ductile deformation zones are interpreted to be splays from a regional-scale deformation zone which makes up the northern tectonic boundary of the so-called Forsmark tectonic lens and have tectonic precursors that developed during the 2.0-1.8 Ga Svecokarelian orogeny. Tens to hundreds of meters wide strongly red-stained brittle deformation zones are found on the island of Gräsö, located outside the higher ductile strain area. Locally, mylonitic fabrics in the cores of the zones are visible, indicating also here a ductile precursor to brittle deformation. E-W striking red-stained brittle deformation zones hosting a fracture mineral assemblage of calcite, adularia and laumontite are extensional in nature and are most likely related to the 1.1-0.9 Ga Sveconorwegian orogeny. Some lineaments in the northern parts of Gräsö that are oriented E-W and NW-SE are parallel to the gneissic banding of the host rocks and do probably not represent distinct deformation zones.