Geological setting and deformation history of a low-angle fracture zone at Finnsjön, Sweden

Abstract

In 1984 the Swedish Nuclear Fuel and Waste Management Company (SKB) initiated detailed investigations involving the geological, geophysical, hydrological and geochemical character of a fracture zone. For this purpose, a low-angle SSW-dipping fracture zone in a foliated granodiorite of Svecokarelian age (approximately 1.8 Ga), within the sub-Cambrian peneplain of northeastern Uppland, central eastern Sweden, was identified and investigated by extensive drilling. The initial formation of this zone, which is estimated to have taken place approximately 1.7 Ga ago as a result of ductile deformation, was followed by shearing during ductile-brittle transitional conditions, and subsequently by brittle deformation. It formed as a thrust in a 20–30 km WNW-ESE-trending strike-slip fault zone. The fracture zone is approximately 100m wide with an anastomosing shear pattern and is displaced by subvertical faults. Late reactivation of the zone has occurred preferentially in the upper part which is now open, permitting water transport. The palaeo-flow paths, indicated by bedrock alteration and fracture infillings, show that the hydraulic flow system in the rock mass has become increasingly restricted during geologic time. Flow now occurs predominantly along discrete zones which were initiated more than 1.7 Ga ago and since then have been repeatedly reactivated. The most intense penetrative fracturing, associated with hydrothermal activity, occurred more than 1.0 Ga ago. Fractures and fracture zones most likely to conduct water, i.e. forming transport pathways between a repository and the biosphere, tend to be those with a long and complex tectonic history.