Tertiary dykes in northwest Ireland: field occurrence, emplacement mechanisms, host rock deformation, and post-emplacement strain and denudation

Abstract

This work provides insight into the Tertiary tectonic and stress history of northwest Ireland. It focuses on igneous dykes, as these are the only rocks of Tertiary age in the region. The work also looks at the fundamental processes of dyke emplacement and host rock deformation. Three groups of dykes, the West Connacht systems, and the Killala and Donegal-Kingscourt swarms, are studied. By analysing the forms of the dykes and their relationships with the host rock structure, it is shown that the dykes largely exploited pre-existing structures in the oldest host rocks, but propagated hydraulic fractures in segmented en echelon arrays in the relatively young Carboniferous host rock. It is shown that the West Connacht systems were emplaced under NW-SE extension, the Killala swarm under N-S extension, and the Donegal-Kingscourt swarm under NE-SW extension. It is envisaged that the E-W trend of the Killala swarm is representative of a swing in the crustal stresses, influenced by the proximity of the Brendan centre and its apparent large-scale radial dyke arrangement. Large intra-swarm range in dyke orientation, and the tendency for dykes to open orthogonal to strike, implies that the difference in magnitude between the horizontal principal stresses was not great during the Palaeocene at the time of emplacement.