Structural style and evolution of the Caledonian foreland, northeast Tyrifjorden, Oslo Region.
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The Palaeozoic sediments present in the Oslo area reflect the outward growth of the Caledonian Orogen, making it a classical area for the study of frontal thrust systems. Bruton et al. (2010) proposed a revised structural model for the tectonic development of this basin, but detailed structural field data to support the model is still lacking. Analogue mechanical laboratory models were performed to better understand the structural inhomogeneities affiliated with the proposed structural model. The present study covers two profiles; an 8 km N-S profile along the eastern shore of Tyrifjorden just south of Sundvollen and a 1.5 km profile along the western shore of Tyrifjorden just south of the first profile. The section along the shore south of Sundvollen shows open folding on the scale of hundred of meters and severe strain localized in the Utstranda pop-up generating an anticlinorium. Deformation in this section is strongly affected by back thrusting, which is localized below the mechanically strong Ringerike sandstone. Similar strain localization and strain transfer are observed in the analogue models. The analogue models and field observations agree in terms of variation in deformation styles and presence of high and low strain regimes. The field observations also match the model from Bruton et al. (2010) and indicate a local tectonic transport direction towards the NW and a homogeneous fold axis of 060/10 throughout the area.