The correction of stratigraphic sections for tectonic finite strain in the Bygdin area, Norway

Abstract

The deformed sediments beneath the Jotun Nappe were folded and then shortened vertically to produce the main cleavage. All the measurements from the Bygdin Conglomerate ( Hossack 1968a ) have been recalculated using harmonic means ( Lisle 1977 ) to get a better estimate of the conglomerate strain. The symmetry of this strain varies locally, but the regional strain can be estimated by integrating the extensions along finite strain trajectories. The minimum value of the integrated conglomerate strain consisted of 65 per cent vertical shortening, a transverse NW elongation of 160 per cent, and a NE elongation of 11 per cent along the Caledonian strike. The vertical shortening is the most constant strain, whereas the NW elongation varies between 390 per cent and 115 per cent. The early folding is estimated to have consisted of 26 per cent shortening along the bedding with 35 per cent extension normal to the bedding. The folding and the integrated strains have been combined in the simplest manner and it is estimated that the present stratigraphic thicknesses have to be multiplied by 2.15 to calculate their original thicknesses. The positions of the nappes before thrusting have also been estimated using the folding and the integrated strains. The Jotun Nappe has a minimum displacement of 290 km and the lower nappes have been thrust for distance between 100 km and 270 km.