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M. G. Norton, W. J. Wilks & S. J. Cuthbert write: In their recent paper Torsvik et al. 1988 presented the results of a survey of palaeomagnetic and magnetic fabric data from the Hornelen Devonian in western Norway.
We wish to comment on the following aspects:
(1) The nature of the eastern boundary of the Hornelen Devonian;
(2) The evidence presented against the accepted interpretation of near coincidence of present tectonic boundaries with boundaries tectonically active during sedimentation.
In addition there are a number of points of fact which we would like to challenge which are dealt with at the end of this discussion.
(1) Torsvik et al. 1988 refer to the eastern boundary of the Hornelen Devonian as an unconformity complete with 'well-preserved weathering profiles'. They give no details and refer only to a published abstract (Ramsay et al. 1987) which itself does not mention the unconformity, although this observation overturns previous interpretations of the contact as a fault (Bryhni 1963, 1978; Steel et al. 1985). We have recently examined this boundary at a number of localities (Fig. 1). The structure of the boundary was found to be very similar at all localities (Fig. 2). The basal c. 800 m of the section below the Devonian consists of overall gently westward-dipping mylonitic gneisses. A strong west-plunging mineral elongation lineation is developed which, together with secondary shear structures and rotated feldspar porphyroclasts, indicate an overall 'top down to the west' sense of movement. Some 150 m beneath the boundary the
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  S. J. Cuthbert Evolution of the Devonian Hornelen Basin, west Norway: new constraints from petrological studies of metamorphic clasts Geological Society, London, Special Publications, 1991; 57: 343 - 360. [Abstract] [PDF]
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