Abstract

Dr Borradaile thanked Dr Tarling for allowing him to read a pre-print of the paper and asked what would be the effects on the palaeolatitude results if some correction were made for tectonic distortion of the pre-deformation magnetic fabrics. Such a correction would seem necessary since Spencer's memoir illustrates many features indicative of tectonic deformation and the author had noted that the rocks were cleaved. Furthermore, some finite strain analysis results had just been made available for dolomitic lithologies in a very similar position within the regional structure on the Island of Islay where Borradaile & Johnson (I973, Tectonophysics 18, 249–259) had there determined strains associated with the formation of the first cleavage. The minimum strains they found (33% shortening normal to cleavage; 22% and 25% principal extensions within the cleavage) might be used as a conservative guide to the degree of deformation in the Garvel-lachs.

Dr A. M. Spencer congratulated Dr Tarling on a most interesting piece of research and offered two comments.

Firstly, the deformation which affected these rocks during the Caledonian orogeny produced a cleavage dipping southeast at 65°. The shortening normal to this cleavage is from 17% to 33%, as assessed using deformed oolite pebbles in the mixtite beds. Dr Tarling has corrected his data for the tectonic tilt of 31° towards 185°. This is probably not the exact correction applicable in order to subtract the effects of the tilting and shortening. The resulting error is, however, probably small (of the order of 4-10° ± -20°)

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