Abstract

Christopher Talbot writes: Holdsworth et al. (1999a) are persuasive in their argument that the Loch Loyal syenites are different parts of the same syenite pluton exposed at different structural levels. However they also argue that emplacement of this pluton was relatively passive and that its boundaries assumed the architecture of pre-existing structures and fabrics. Thus the smooth NW contact parallels the Ben Blandy shear zone which has the NE-SW strike typical of the F1+F2 Caledonian grain to the west, while sheet-like apophyses in the roof strike NW-SE, parallel to axial surfaces of major F3 crossfolds in the overlying nappes to the east. I would like to learn their arguments against a simpler alternative model, which has never (to my knowledge) been suggested before, but which is equally relevant to late orogenic plutons elsewhere. At its simplest, why are those major apophyses in the roof of the syenite pluton not simply F3 cross-mullions due to thinskinned NE-SW shortening of a weaker syenite sheet and its more competent roof rocks above a decollement along the Ben Blandy shear zone?

Firstly, I use the maps and account by Holdsworth et al. to modify their summary diagram (Fig. la), which shows the smooth NW boundary of the syenite pluton as a sideboundary, to Fig. lb which reinterprets the NW boundary as the smooth floor to a plutonic sheet dipping generally SE. Just beneath this smooth floor, the Ben Blandy shear zone acted as a basal decollement to overlying F3 cross-folds in both the interpretation

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