| Summary: | The aim of this thesis is to improve the understanding of the brittle tectonic evolution in Fennoscandia, which is known to be complex and contain multiple stages of fault nucleation and reactivation. Building on prior work, this work provides new structural descriptions and temporal constraints of fractures and faults from the Paleoproterozoic bedrock of southwestern Finland. Integration of structural and isotopic data from Olkiluoto, the site of planned nuclear waste disposal, allowed constraining the geometric and temporal relationships of specific sets of brittle faults within the fault network at local scales. However, the Olkiluoto dataset leaves a substantial period of uncertainty in crustal evolution at 1.6-1.3 Ga, and this thesis aims to close this gap through examining novel regional structural datasets from the 1.58 Ga Åland rapakivi batholith for paleostress analysis. Moreover, this work displays the results of multiscale lineaments mapped from southern Finland, providing information on the scalability of brittle structures, localization of regional deformation, and insights into the evolution of the brittle crust under various stress conditions. According to the results of this study, the tectonic development of the brittle crust within southwestern Finland includes at least six major tectonic stages: 1) brittle deformation initiated under NW–SE to NNW–SSE compression at the brittle-ductile transition at around 1.75 Ga. Two successive stages of regional extension followed: 2) N–S extension at around 1.64 Ga during the emplacement of the pre-1.6 Ga rapakivi granites, and 3) E–W to NW–SE extension during the intrusion of the 1.58 Ga Åland rapakivi, diabase dykes, and the onset of the development of a sedimentary basin where the Bothnian Sea is now located. This extensional period was succeeded by strike-slip tectonics including 4) WNW–ESE to NNW–SSE compression between 1.55–1.4 Ga and 5) NE–SW compression at around 1.3–1.2 Ga. 6) NE–SW to ENE–WSW compression during the early phases of the 1.1 Ga ...
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