The structure and origin of the Azores--Biscay Rise, North-east Atlantic Ocean

Abstract

Summary. The Azores-Biscay Rise is a roughly linear north-east-south-west trending feature rising 1500-3000 m above its surroundings, which extends from about 45'N, 1530'W towards the Azores. Its south-western termination is near 4030'N, 2130'W. About halfway along its length the Rise intersects the WNW-trending King's Trough. In 1978 a set of bathymetric, magnetic, gravity, GLORIA and seismic reflection and refraction data were obtained in the vicinity of the Rise. Together with earlier data these observations suggest that: (1) there has been no substantial post-emplacement tectonic activity, with the possible exception of the construction of some volcanic seamounts at the south-western end of the Rise, and (2) the Rise is underlain by a low-velocity (low-density) lower crust and is in isostatic equilibrium . The Rise can be convincingly shown to be the eastern half of. a pair of ridges formed by abnormal crustal generation at the Mid-Atlantic Ridge crest between the times of anomalies 33 and 24 (76-56Ma ago). The western counterpart of the Rise includes Gauss and Milne seamounts in the Newfoundland Basin. Magnetic anomaly 3 1 passes uninterruptedly across the Rise and therefore hypotheses that the northern part of the Rise was the site of a Cenozoic transform fault or subduction zone are not supported by our data. It is speculated that King's Trough was linked to the North Spanish Trough by an early Cenozoic east-west transform fault across the northern Iberia Abyssal Plain. This plate boundary became inactive about the middle of the Oligocene epoch.