[en] The rocks of Olkiluoto can be divided into two major classes: (1) supracrustal high-grade metamorphic rocks including various migmatitic gneisses, tonalitic-granodioriticgranitic gneisses, mica gneisses, quartz gneisses and mafic gneisses, and (2) igneous rocks including pegmatitic granites and diabase dykes. The migmatitic gneisses can further be divided into three subgroups in terms of the type of migmatite structure: veined gneisses, stromatic gneisses and diatexitic gneisses. On the basis of refolding and crosscutting relationships, the metamorphic supracrustal rocks have been subjected to polyphased ductile deformation, consisting of five stages, the D2 being locally the most intensive phase, producing thrust-related folding, strong migmatisation and pervasive foliation. In 3D modelling of the lithological units, an assumption has been made, on the basis of measurements in the outcrops, investigation trenches and drill cores, that the pervasive, composite foliation produced as a result of polyphase ductile deformation has a rather constant attitude in the ONKALO area. Consequently, the strike and dip of the foliation has been used as a tool, through which the lithologies have been correlated between the drillholes and from the surface to the drillholes. The bedrock at the Olkiluoto site has been subjected to extensive hydrothermal alteration, which has taken place at reasonably low temperature conditions, the estimated temperature interval being from slightly over 300 deg C to less than 100 deg C. Two types of alteration can be observed: (1) pervasive (disseminated) alteration and (2) fracture-controlled (veinlet) alteration. Kaolinisation and sulphidisation are the most prominent alteration events in the site area. Sulphides are located in the uppermost part of the model volume following roughly the lithological trend (slightly dipping to the SE). Kaolinite is also located in the uppermost part, but the orientation is opposite to the main lithological trend (slightly dipping to the N). The third main alteration event, illitisation, consists of two distinct volumes, which lie one on the other and converge in the northwest, and are spatially associated with site-scale thrust faults. The fault zones at Olkiluoto are mainly SE-dipping thrust faults formed during contraction in the last stages of the Fennian orogeny, approximately at 1800 Ma ago and were reactivated in several deformation phases, as indicated by fault-slip data and K-Ar age determinations. In addition, NE-SW striking strike-slip faults are also common. Fault zone intersections from drillholes, the ONKALO access tunnel and outcrops have been correlated by the application of slickensides orientations, mise-O-la-masse measurements, eletctromagnetic soundings, 3D seismics and VSP-reflectors, resulting in six site-scale and 84 local-scale fault zones. (orig.)