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The Swedish Nuclear Fuel and Waste Management Company (SKB) is undertaking site investigations at Forsmark and Laxemar-Simpevarp in south-eastern Sweden, with the aim of identifying a suitable crystalline bedrock for the disposal of highly radioactive nuclear waste. In this context, a comprehensive understanding of the bedrock geology, including geological evolution, character and modelling in 3D space, is of vital importance for both sites. This paper presents the results from the Laxemar-Simpevarp site.
The bedrock at Laxemar-Simpevarp is composed of c. 1.8 Ga intrusive rocks that belong to the 1.86-1.65 Ga Transscandinavian Igneous Belt (TIB). 40Ar/39Ar amphibole age data indicate that the bedrock had cooled below 500°C at 1.76 Ga, while there remains more uncertainty concerning the cooling history after 1.76 Ga, in particular concerning the time at which the bedrock passed through the brittle-ductile transition in the crust. However, 40Ar/39Ar biotite ages indicate that the bedrock had entered the realm of brittle deformation at c. 1.5 Ga, and possibly prior to c.1.65 Ga. Local reheating at c. 1.45 Ga in conjunction with the emplacement of two younger granite intrusions, north and south of the investigated area, have been documented by resetting of the 40Ar/39Ar biotite system. Far-field effects of the Sveconorwegian and Caledonian orogenies are also present.
The TIB rocks in the area show variable composition (granite to quartz monzodiorite to diorite-gabbro), grain size and texture. They have been affected by magma-mingling and magma-mixing processes, and a close genetic relationship between the rocks is inferred. The latter has been confirmed by U-Pb zircon dating. Although there is a non-uniformly developed faint to weak ductile fabric in the rocks, discrete, low-temperature, brittle-ductile to ductile shear zones form the most prominent ductile structures in the area. The latter formed in response to an approximately NNW-SSE shortening during the waning stages of the Svecokarelian orogeny. As with other Precambrian shield areas, complex networks of brittle deformation zones (faults) transect the bedrock in the Laxemar-Simpevarp area.
Rock domains, which comprise predominantly different rock types, are defined both at the surface (2D) and at depth along cored boreholes (1D). Deformation zones are identified at the surface by the interpretation of detailed geophysical and topographical data, and at depth by the interpretation of geological and geophysical data from cored boreholes. By an integration of the detailed surface and subsurface data, it has been possible to construct deterministic 3D geological models for rock domains and deformation zones. The resulting rock domain and deformation zone models constitute the geometric basis for subsequent geoscientific modelling based on thermal, rock mechanical, hydrogeological, and solute transport properties.
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