A north-south seismic profile across the Caledonian Suture zone in Ireland

Abstract

COOLE I (Caledonian Onshore-Offshore Lithospheric Experiment—Line I) is a 284 km seismic refraction profile extending from Bundoran (N) to Ardmore (S) in Ireland. Fifty-nine recording stations were deployed along this profile and it was served by a total of six shotpoints. The profile crosses the predominant NE-SW (Caledonian) strike at a steep angle. Using both one-dimensional and two-dimensional seismic inversion techniques, a travel-time and relative amplitude study of the observed P-wave data was undertaken. Gravity data along the profile was modelled to test the plausability of the derived velocity-depth section. The upper crustal structure along the profile is highly variable but some features stand out. A large, low-velocity body towards the south of the traverse correlates well with a Bouguer anomaly low and is interpreted as the southwest extension of the buried Leinster batholith. The mid-crust is approximately 10 km thick and is bound at its top and bottom by two laterally continuous reflecting horizons. Both reflectors shallow northwards by about 5 km over a horizontal distance of some 60 km. This northwards shallowing is a feature also displayed by a number of isolines in the upper crust and it marks a broad region of crustal change along the traverse in the zone of proposed Caledonian suturing. Crustal thickness beneath COOLE I is approximately 30 km and the Moho is a transition zone about 2 km thick which shallows slightly towards the coasts.