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

Abstract

D r J. C. G uezou writes: I should like to congratulate the author for the first important attempt to quantify tectonic finite strain in the Scandinavian Caledonides. The first question which requires comment is whether the strain resulted from 'pure shear' or a 'simple shear'. This has an essential bearing on his conclusions, and on the bulk finite strain analysis. The validity of the strain analysis presented may be questioned in different ways. In a previous account on the Bygdin–Beito region, the author ( Hossack 1976 ) described the Caledonian deformation phases to which he referred in the present paper. He defined XY trajectories within his phase 2 slaty cleavage, and X trajectories were traced parallel to the stretching lineation, which is a phase 2 structure (i.e., pebble elongations, mineral grains lineation). This phase 2 seems more or less contemporaneous with the thrusting period of the Valdres unit ( Hossack 1976 , pp. 17–18). In this paper it was stated that the structures associated with thrusting (phase 2 structures?) have been formed by a simple shear mechanism. My own investigations in the Trondheim region and some adjacent areas led to similar conclusions, both in terms of successive phases and strain analysis ( Guezou 1978 ). Mesoscale and microscale structures associated with the main Caledonian thrust zones (i.e. stretching lineations close to the X axis of the finite strain, rotation of fold axes in shear zones towards parallelism with the X axis, sub-parallelism of XY plane and schistosity in shear zones) point to a simple shear