Erroneous Phase Velocities from Frequency-Wavenumber Spectral Sections

Summary Two-dimensional cross-sections of finite frequency-wavenumber spectra can easily be misinterpreted, since leakage of energy occurs along lines of constant wavenumber. In particular, the signal phase velocity determined from measurements on cross-sections normal to the frequency axis can be incorrect. We discuss an algorithm which corrects this situation. Examples using real and synthetic data are given.     

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

Summary. The Azores—Biscay Rise is a roughly linear north-east—south-west trending feature rising 1500–3000m above its surroundings, which extends from about 4°N, 1°30'W towards the Azores. Its south-western termination is near 40°30'N, 21°30'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 (76–56 Ma ago). The western counterpart of the Rise includes Gauss and Milne seamounts in the Newfoundland Basin.
Magnetic anomaly 31 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.