The crustal structure beneath northern England and adjacent sea areas

Abstract

In the summer of 1982 the Department of Geological Sciences, University of Durham carried out a seismic survey across the north Irish Sea, northern England and the Mid North Sea High. Recording stations were set up at 2 km intervals across northern England , with other stations offline. Sea-bottom sesimometers were deployed at sea. Explosions were fired at 4 km intervals in the Irish and North Seas and in two boreholes on land. Airgun shots were also fired at sea. The data has been digitised and interpreted using the plus-minus method, time-term analysis, and modelling techniques. Results from other geophysical surveys have been used to complement the interpretation. An Upper Palaeozoic and Mesozoic sequence is underlain by Lower Palaeozoic rocks (5.5 to 5.7 km/s) at a depth of 0.5 to 3.0 km. A seismic basement (Pg) with a velocity of 6.15 km/s can be recognised at a depth of 4 km, except in the north-east where it appears to be absent, and is identified as Precambrian crystalline basement from south of the lapetus Suture. Beneath the whole line the crust appears to have an average crustal velocity of 6.3-6.4 km/s and a crustal thickness of about 30 km. Wide-angle reflections from a mid-crustal discontinuity are observed at a depth of 18-20 km beneath the Northumberland Trough and Mid North Sea High but appear to be absent beneath the Solway and Carlisle Basins. Crustal phases are best developed for shots in deeper water which have energy in the 2-5 Hz band. Energy above 6 Hz from shots in shallower water is believed to be scattered by inhomogeneities within the crust.