Abstract

Professor W. S. Pitcher remarked: Dr Hutton has enthusiastically addressed himself to the main problem highlighted by the previous work: the reason for the wide variety of emplacement mechanisms within coeval and contiguous plutons at the same level in the crust, central to which is the relationship in time between the Main Donegal Granite and the localized but intense deformation.

This new structural analysis clearly establishes the presence of an important sinistral shear component, and my reservations concern only the timing and the magnitude of the calculated displacements. Concerning the latter, the evidence is that where the ductile shear zone leaves the Main Donegal Granite pluton it seems not to displace the contact zone, and where it runs off SSW it barely displaces an earlier dislocation known as the Knockateen slide. I am equally in doubt of the extension of the postulated shear zone far into northern Fanad, unless it has been conveniently cut out by movement on the post-shear Cashelmore Fault. Within the granite outcrop itself, where the shear zone runs along the apparent boundary between the Trawenagh Bay lobe and the Main Granite, to the S of Meenderryherk, a train of xenoliths straddles the zone of deformation, by which it is distorted but not dislocated. I conclude that whilst sinistral rotational strain has been proved, the displacements suggested seem excessive.

I am much attracted to the proposed model whereby magma was emplaced into space created by a curvature in an active shear zone, particularly so as the early

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