CRUSTAL OBSERVATIONS BENEATH THE SOUTHERN NORTH-SEA AND THEIR TECTONIC AND GEOLOGICAL IMPLICATIONS

In 1991, a deep seismic reflection line, MPNI-9101, was acquired in th e southern North Sea from the Mesozoic Broad Fourteens Basin, across t he West Netherlands Basin onto the London-Brabant Massif (LBM). The re sultant section shows a strongly reflective lower crust beneath the ar ea of Mesozoic basin development. This lower crustal reflectivity cont inues to be strong beneath the LBM. The travel time to the base of the reflective zone increases from approximately 11.0 s beneath the Mesoz oic basins to 12.5 s beneath the LBM, suggesting a southward thickenin g of the crust (Rijkers et al., 1993). Based on these travel times and information from deep wells and refraction surveys, Moho depth is est imated to increase from about 31 km beneath the Mesozoic basins to abo ut 38 km beneath the LBM. This difference in depth to the Moho can par tly be explained by coaxial stretching of the crust beneath the Mesozo ic basins. In comparison with the Mesozoic basins, the crust beneath t he LBM was thickened during the Caledonian and Variscan orogenies.