Structural Control of the Deposition and Subsequent Deformation of the Killary Habour-Joyce Country Silurian Succession

Abstract

The Killary Harbour–Joyce Country succession of Silurian rocks forms the remnant of one of three successor basins that developed after the Grampian Orogeny and which are preserved in western Ireland. Its outcrop everywhere obscures the contact between the Dalradian rocks of Connemara and the Ordovician rocks of the South Mayo Trough. The outcrop is bisected by the prominent north-west trending Maam Valley Fault Zone that is shown to have a component of synsedimentary movement down-throwing to the southwest. The original basin margins lay well outside the current outcrop area. North-south shortening and inversion of the basin was accomplished by reactivation and inversion of earlier fault structures and was exclusively in the brittle zone. Folding of the Silurian rocks with weak to moderate cleavage and deformation of the basement rocks of Connemara was accomplished by largely vertical movements along numerous strike parallel and oblique fractures. Break-back inversion structures were developed along the southern margin of the South Mayo Trough, which include thrusting associated with footwall shortcut faults and clockwise rotation. Although the timing of deformation overlaps that of diorite sill emplacement, it remains poorly constrained in absolute terms.