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Smøla island, situated within the mid-Norwegian passive margin, contains crystalline-basement-hosted intricate fracture and fault arrays formed during a polyphase brittle tectonic evolution. Its detailed study may strengthen correlation attempts between the well-exposed onshore domain and the inaccessible offshore domain, further the understanding of the passive margin evolution, and provide useful constraints on petrophysical properties of fractured basement blocks. A combination of geophysical and remote sensing lineament analysis, field mapping, high-resolution drill hole logging, 3D modelling, petrographic and microstructural studies, and fault gouge K–Ar geochronology made it possible to define five deformation episodes (D1 to D5). These episodes occurred between the post-Caledonian evolution of the regional-scale Møre–Trøndelag Fault Complex (MTFC) and the Late Cretaceous and younger crustal extension preceding the final stages of Greenland–Norway break-up. Each reconstructed deformation stage is associated with different structural features, fault and fracture geometries, and kinematic patterns. Synkinematic mineralisations evolved progressively from epidote–prehnite, sericite–chlorite–calcite, chlorite–hematite, hematite–zeolite–calcite, to quartz–calcite. K–Ar geochronology constrains brittle deformation to discrete localisation events spanning from the Carboniferous to the Late Cretaceous. Multiscalar geometrical modelling at scales of 100, 10, and 1 m helps constrain the extent and size of the deformation zones of each deformation episode, with D2 structures exhibiting the greatest strike continuity and D1 features the most localised. Overall, the approach highlighted here is of great utility for unravelling complex brittle tectonic histories within basement volumes. It is also a prerequisite to constrain the dynamic evolution of the petrophysical properties of basement blocks.

[1] The Swedish Nuclear Fuel and Waste Management Company (SKB) is undertaking site investigation at two locations in Sweden, Forsmark and Laxemar-Simpevarp, with the aim of identifying a suitable area for the construction of a deep repository for the disposal of highly radioactive nuclear waste. Fault slip data from outcrops and oriented drill cores were used to compute paleostress states and to unravel the sites' brittle deformation history. Results from the Laxemar-Simpevarp area show that its suggested brittle history results from multiple reactivation of fracture and fault sets caused by the many orogenic episodes that affected the region during at least 1.5 Ga of geological evolution in the brittle deformational regime. Two compressional, approximately NW/ NNW-SE/SSE and NNE-SSW oriented shortening events generated sets of conjugate, steep strike-slip fractures. These sets formed during the late stages of the Svecokarelian and possibly also of the Gothian orogeny, soon after the region entered the brittle deformation domain. The Mesoproterozoic Sveconorwegian orogeny generated fractures and faults that are assigned to a third set of conjugate strike-slip faults, which constrain an approximately E-W [[sigma].sub.1]. The Caledonian shortening, oriented approximately NW-SE to E-W, reactivated the latter but also formed a new, similarly oriented set of subvertical strike-slip fractures. Permian transtension was oriented NE-SW and caused a prominent set of moderately dipping NW-SE trending normal faults in the Precambrian basement of the study area.* Two other approximately NE-SW and NW-SE oriented shortening events are recorded in Ordovician limestones and can be tentatively linked to the far-field effects of the Laramide and Alpine orogenies. * Citation: Viola, G., G. Venvik Ganerod, and C.-H. Wahlgren (2009), Unraveling 1.5 Ga of brittle deformation history in the Laxemar-Simpevarp area, southeast Sweden: A contribution to the Swedish site investigation study for the disposal of highly radioactive nuclear waste, Tectonics, 28, TC5007, doi: 10.1029/2009TC002461.