THE SIGMUNDUR COMPLEX, A QUESTIONABLE-TERTIARY IGNEOUS CENTER IN THE NORTHERN ROCKALL TROUGH

Seismic reflection data have revealed the presence of a large dome-lik e volcanic structure in the northern Rockall Trough, known as the Sigm undur Complex. The complex comprises a single, nearly circular platfor m marked by scarp edges, with some evidence for intra-basalt sequence reflectors, representing the original depositional attitude of former lava flows, dipping away from an eroded central volcano. The complex i s mostly buried beneath Lower Eocene-Recent sediments although the cul mination of the structure does crop out at sea bed. A free-air gravity anomaly over the complex reaches a peak value of +600 gu. Gravity mod elling suggests the presence of a large mafic plutonic mass immediatel y underlying the volcanic pile. The intrusion possesses an asymmetric shape with a sloping top and base at depths of 2.4-3.0 km and 24.5-25. 4 km respectively. The diameter increases downwards from 3.6-7.2 km at the top to 14.5-21.1 km at the base.