Analysis and tectonic interpretation of gravity data over the Variscides of southwest Ireland

The gravity data set for southwest Ireland comprising 5500 stations with a spacing of about 1-3 km has been filtered using standard wavenumber domain techniques. A band pass filtered map, emphasizing gravity anomalies originating to a maximum depth of 3-4 km, correlates well with surface geology. It shows major faults and density contrasts among six lithostratigraphical units, the most important being between the Old Red Sandstone and underlying basement. A low pass filtered map, emphasizing gravity anomalies from a maximum depth of 10-12 km shows major negative gravity anomalies at Killarney and Fermoy and positive anomalies beneath the Shannon estuary, on the Dingle peninsula and along the south coast. These data suggest that Caledonian basement was involved in Variscan deformation and any detachment lies at mid-crustal levels. Two north-south gravity profiles are modelled along Eastings 100 and 200, constrained by surface geology and published seismic refraction data. On Line 100 the source of the negative anomaly at Killarney is modelled as either a granite within basement or a deformed Devonian basin. The source of the Shannon positive anomaly is modelled as a large dense body within the crust whose southern limit coincides with the Iapetus Suture Zone. On Line 200, the Fermoy low is modelled as a laccolithic granite within basement. The positive anomaly along the south coast is a consequence of Mesozoic crustal thinning.