A crustal model for a northeast-southwest profile through Ireland

Abstract The Irish Caledonian Suture Seismic Project (ICSSP) consisting of a 250 km seismic refraction line was carried out in conjunction with, and as an extension of, the Caledonian Suture Seismic Project (CSSP) of the University of Durham. 51 seismic recording sites at 5 km intervals were deployed on the northwest flank of the Suture Zone in Ireland. Recordings were made from three shot points at the southwest of the line and from 31 shot points of the Durham programme in the North Irish Sea. A crustal model is presented which is based exclusively on a travel-time interpretation. The P g velocities recorded in eastern Ireland rise quite rapidly with depth to 6.3–6.5 km/s but in the western part of the profile lower velocities between 6.0 and 6.3 km/s were recorded. Prominent energy in later arrivals can be correlated by two reflected phases from a mid-crustal boundary and the Moho. A low-velocity zone is present at the northeastern end of the line and a mid-crustal layer with an average velocity of about 6.4 km/s is separated from the lower crust, mean velocity 6.8 km/s, by a well established mid-crustal boundary zone of 1–2 km thickness at about 20 km depth. The Moho, around 32 km at its deepest, shallows towards the sea at both ends of the line. It is a sharp discontinuity under western Ireland but a transition zone of 3–4 km thickness in the east. An interpretation is given which takes into account 1. (a) the geologic and tectonic history of Ireland and 2. (b) data derived from the gravity and magnetic fields.