THE DARWIN COMPLEX, A TERTIARY IGNEOUS CENTER IN THE NORTHERN ROCKALL TROUGH

Seismic reflection data and well information in the North Rockall Trough reveal a large dome of Palaeocene-Eocene basalts and Upper Cretaceous dacites known as the Darwin Igneous Complex. The extrusives dip radially away from the centre of the dome where a volcanic vent with a diameter of c. 1625 m and a depth of 100 m or more is present. The complex comprises five major volcanic sequences which can be mapped as a series of arcuate scarps. Modelling of the large positive free-air gravity anomaly over the complex suggests the presence of a large mafic pluton with a two-cylinder configuration. The upper cylinder has its top at 3.5-4.0 km beneath mean sea-level and its base at 8.0 km with a 17.0 km diameter while the lower cylinder has its top and base at 8.0 km and 15.0 km respectively with a diameter of 20-30 km. The model suggests that 4-4.5 km of pre-Palaeocene sediments exist between the base of the volcanic pile and the top of the lower pluton. It is possible that some of these sediments are pre-Cretaceous in age.