A deep seismic reflection transect across the Irish Caledonides

Abstract

New reflection profiles recorded to 18s ( c .60km) along a 600 km north-south transect West of IREland (WIRE lines) provide a cross-section of the Irish Caledonides from Lewisian basement in the north to the Variscan fold-and-thrust belt in the south, and constrain the offshore extrapolations of major Caledonide structures. We interpret the profiles as showing the Great Glen fault and associated, NE-trending strike-slip faults as near-vertical structures which cut the whole crust and the uppermost mantle. Beneath the surface trace of the Fair Head-Clew Bay line we image a north-dipping zone which soles into horizontal layers just above the reflection Moho. The north-dipping zone separates south-dipping reflections above and to the north from north-dipping reflections below and to the south. From the Fair Head-Clew Bay line as far south as the Iapetus suture the lower crust contains north-dipping packages of reflectors possibly related to Acadian accretionary underplating. The Iapetus suture zone can be identified as a zone, more than 50 km wide, dipping north at 30° from 5 km depth offshore Dingle peninsula to sole into the Moho at c . 30 km depth west of the Shannon Estuary. In the upper crust the shallow part of the Iapetus suture zone appears to be transported to the north by late Caledonian retrocharriage or by Variscan thrusting. The Iapetus suture does not penetrate through the present-day reflection Moho, but there are scattered reflectors in the mantle not continuous with crustal reflectors, perhaps relics of a subducted slab. We recognize ‘lower-crustal’ reflectivity and the reflection Moho along almost all the WIRE profiles. Regionally the travel-time to the reflection Moho varies from 9 s ( c . 28 km?) offshore Donegal (northwest of Ireland) to 12 S ( c . 35 km?) offshore Kerry and Cork (southwest of Ireland). Apparent Moho offsets (rapid lateral changes in the travel-time to the reflection Moho) north of Donegal and north of Mayo are probably due to the Great Glen fault and associated faults. The ‘lower-crustal’ reflectivity varies considerably in thickness and prevalent dip direction along the WIRE profiles. In Donegal Bay this reflectivity is observed at depths less than 5 km ( c . 1.5s) whereas west of Kerry prominent reflectivity is only observed beneath 6s ( c . 17 km). Though the age of crustal reflectors cannot in general be determined, beneath central Ireland from Donegal to Dingle the lower-crustal reflectors mimic in dip direction the structural vergence of the exposed Dalradian and early Palaeozoic rocks, so that the lower-crustal reflectivity patterns, and probably much of the crust, must be Caledonian in age.