Characterising the role of basin margin structure on finite strain patterns across a cleavage front from the Variscides of southern Ireland

The northern margin of the Munster Basin has been traditionally placed at the Dingle Dungarvan Line (DDL), a line which runs through Dingle Bay in the west and coincides with the Kilarney Mallow Fault (KMF) further to the east, on the Iveragh Peninsula, southwest Ireland. This region lies at the northern boundary of the Rhenohercynian Zone of the European Variscides. Deformation of 7 km+ Upper Devonian continental clastic sequence and overlying Carboniferous marine carbonate/clastic sequence at the end of the Carboniferous is thought to have consisted of an initial phase of layer parallel shortening, followed by folding, ongoing cleavage development and late stage accommodation thrusting. Previous models of inversion at the northern margin consist of an initial extensional bounding fault subsequently reactivated as a reverse fault during Variscan compression, with resulting higher strain to the south in the Munster Basin and lower strain to the north in the Foreland Zone. These previous basin margin models do not account for the nature of deformation observed in the western vicinity of the basin margin. The results from the Anisotropy of Magnetic Susceptibility, Strain Analysis and structural field work carried out on Upper Devonian ORS from the MB and northern peripheral areas show that the Coomnacronia Fault, which lies to the south of the DDL, marks a significant tectonic boundary. The Coomnacronia Fault marks a divide between a higher strain zone: ‘the Intrabasinal Zone’ with axial planar cleavage to the south and a lower strain zone: ‘the Transition Zone’ with weak rotated cleavage/early LPS to the north. These proposed tectonic zones are intrinsically related to the geometry of the Caledonian/Acadian basement and the inversion mechanisms of Coomnacronia Fault. The Coomnacronia Fault is thus considered to be the northern margin of the Munster Basin and marks a significant northern boundary of the Variscan front