CRUSTAL STRUCTURE OF THE LONDON-BRABANT MASSIF, SOUTHERN NORTH-SEA

In 1991 a deep seismic line, MPNI-9101, was acquired in the southern N orth Sea. The line runs from the Mesozoic Broad Fourteens Basin in the north, across the West Netherlands Basin, onto the London-Brabant Mas sif in the south. The London-Brabant Massif is a WNW-ESE trending stab le structure located beneath southeastern England, the southern North Sea and Belgium. The London-Brabant Massif represents the most easterl y part of the Anglo-Brabant Massif. At the northern margin of the Lond on-Brabant Massif, Devonian and Carboniferous siliciclastic and carbon ate rocks onlap the massif. Farther south, shallow parts of the seismi c line in the vicinity of the axial zone of the London-Brabant Massif are almost completely devoid of primary reflections. This zone is comp osed of strongly folded Lower Palaeozoic sedimentary units which have been mapped in the onshore part of Belgium. Numerous seismic reflectio n multiples from the base of the Cretaceous are observed on this part of the section. The southern limit of the zone is very abrupt and may correspond to a fault belt delimiting an area of magmatic rocks known in the onshore part of Belgium. Unusually the deeper parts of the seis mic line show a strongly reflective lower crust beneath the London-Bra bant, a phenomenon which has not been observed on other deep seismic s ections across the massif. Two-way travel times to the base of the low er crustal reflective zone (corresponding to the Moho), increase from 10 seconds beneath the West Netherlands Basin in the north to 12 secon ds beneath the London-Brabant Massif, suggesting a thickening of the c rust.