An alternative interpretation of the North East Coast (NEC) deep seismic reflection profile is presented. Seismic reflectors in the lower crust of the UK sector of the North Sea, which were formerly attributed to the complex effects of Caledonian continental collision, are shown to be consistent with the inferred processes of Carboniferous igneous intrusion and magmatic underplating in the Iapetus convergence zone. The nature and distribution of Carboniferous magmatism in southern Scotland and northern England are briefly described. ENE-trending intrusions and chains of volcanic centres are widely recognized. Similarly trending aeromagnetic anomalies are used to identify where the offshore continuation of this igneous province intersects the NEC line. As a result, an area of crustal thickening in the northern part of the line is directly related to the presence of Carboniferous igneous intrusions beneath southern Scotland. In the same way, the single deep reflector on the NEC line which was previously identified as the Iapetus suture, is reinterpreted as a lower crustal intrusion associated with the emplacement of the end-Carboniferous Whin sill. The new seismic interpretation is incorporated in a speculative, regional crustal model of Carboniferous magmatism.