A geoelectric section across Ireland from magnetotelluric soundings

A magnetotelluric survey across Ireland has been completed. The 40 sites were positioned on a line normal to the predominant Caledonian geological trend (i.e. NE-SW) of the country. The average site separation was 5 km and frequencies from 640 to 0.01 Hz were sampled. Bostick-Niblett and Occam inversions of the data suggest that the Carboniferous limestone of the main part of the profile is underlain by Ordovician volcanic rocks in a block and trough structure with a wavelength of ∼ 15 km. This supports previous gravity and magnetic interpretations. At greater depths which vary significantly along the traverse, an anomalous mid-lower crustal conductor is apparent. This conductor is probably a complex manifestation of the Caledonian tectonic zone known as the Iapetus suture. In this respect, it is significant that comparable conducting zones of similar geometry have previously been found in the suture region in Southern Scotland and Northern England.