Detailed investigation of multi-scale fracture networks in glacially abraded crystalline bedrock at Åland Islands, Finland

Abstract

Abstract. Using multiple scales of observation in studying the fractures of the bedrock increases the reliability and representativeness of the respective studies. This is because the discontinuities, i.e. the fractures, in the bedrock lack any characteristic length and instead occur within a large range of scales of approximately 10 orders of magnitude. Consequently, fracture models need to be constructed based on representative multi-scale datasets. In this paper, we combine a detailed bedrock fracture study from an extensive bedrock outcrop area with lineament interpretation using light detection and ranging (lidar) and geophysical data. Our study offers lineament data in an intermediary length range (100–500 m) missing from discrete fracture network modelling conducted at Olkiluoto, a nuclear spent-fuel facility in Finland. Our analysis provides insights into multi-scale length distributions of lineaments and fractures and into the effect of glaciations on lineament and fracture data. A common power-law model was fit to the lineament and fracture lengths with an exponent of −1.13. However, the fractures and lineaments might follow distinct power laws or other statistical distributions rather than a common one. When categorising data by orientation, we can highlight differences in length distributions possibly related to glaciations. Our analysis further includes the topological, scale-independent fracture network characteristics. For example, we noticed a trend of decreasing apparent connectivity of fracture networks as the scale of observation increases.

Mesoproterozoic Strike-Slip Faulting within the Åland Rapakivi Batholith, Southwestern Finland

“…However, without actual confirmatory studies, they should be considered as unspecified zones of discontinuities in the bedrock that, in Finland, resulted from selective glacial erosion associated with intense bedrock fracturing (Krabbendam and Bradwell, 2014;Skyttä et al, 2015Skyttä et al, , 2023. We used the major lineaments from the 1:200,000 integrated lineament interpretation by Ovaskainen et al (2023), which we further extended to cover all the onshore areas of the Åland main islands (Figure 3A: Åland, Föglö). The integrated interpretation is based on airborne topographic LiDAR-data (Light Detection and Ranging; Abdullah et al, 2013;Soliman and Han, 2019) produced by the National Land Survey of Finland (2019, with 0.5 points/m 2 and a mean altitude error of 0.3 m) and the airborne magnetic (Mag) and electromagnetic (EM) datasets of the Geological Survey of Finland (collected from an altitude of 30 m, with a line spacing of 200 m; Hauta-niemi et al, 2005;Geological Survey of Finland, 2007a,b).…”

“…BGSZ = Björkö-Gullkrona Shear Zone, HSZ = Hämeenlinnna Shear Zone, JSZ = Jyly Shear Zone, KSZ = Kisko Shear Zone, KoSZ = Kolinummi Shear Zone, KySZ = Kynsikangas Shear Zone, SSZ = Somero Shear Zone. Rapakivi intrusions: Fs = Fjällskär, Kö = Kökar(Suominen, 1991; Geological Survey of Finland, 2001;Väisänen and Skyttä, 2007;Torvela et al, 2008;Kraatz, 2013;Pitkälä, 2019;Ovaskainen et al, 2023).…”