The BIRPS deep seismic reflection data of WIRE (West of IREland) 1/1B from Galway Bay to the Celtic Sea are integrated with geological and other geophysical data to produce a synthesis of the fundamental tectonic character of the crust in and around southern Ireland. The Iapetus Suture forms a zone of north-dipping reflectors which projects up between the Shannon estuary and the Dingle peninsula and is overlain by a late Caledonian reflector with a shallow, south dip. Across the Iapetus Suture zone the crust thickens southward and the lower crust becomes less reflective. It is proposed that the Lower Palaeozoic volcano-sedimentary rocks which form the Leinster massif extend southwestward beneath the Variscan province and at least 30 km offshore to the WIRE lines. We argue that the weak south-dipping reflectors in the upper and middle crust south of the Iapetus Suture represent an early-Caledonian fault system linked to a mid-crustal detachment which formed during the south-dipping subduction event recorded onshore by the Ordovician volcano-sedimentary successions. These faults were selectively reactivated causing the predominant ENE trend of younger regional tectonic features in and around southern Ireland. The three onshore Upper Palaeozoic basins close east of the WIRE lines.
We point out that the ‘Variscan Front’ is only a map line across the British Isles with no consistent tectonic character. Thus the south-dipping reflector below the north margin of the North Celtic Sea Basin on SWAT 4, generally referred to as the ‘Variscan Front’, is correlated by us with the northern boundary of the Rosslare complex, thus making it the northern tectonic boundary of the Caledonian basement block known as the Rosslare-Monian Terrane. Furthermore the north-dipping South Irish Sea Lineament on WINCH 4 can be correlated with the Menai Straits Lineament and could thus be the southern boundary of this terrane. The south-dipping reflector on SWAT 2, also previously called the ‘Variscan Front’ is correlated with the Johnston-Benton fault system in southwest Wales.