Marine geophysical investigation of the Hatton bank volcanic passive continental margin

Abstract

The Durham/Cambridge/Birmingham Universities two-ship marine geophysical cruise to the Hatton Bank continental margin took place in May/June 1985, during which single- and two-ship seismic reflection/refraction data, together with under way gravity and magnetic anomaly, and bathymetric data were collected within a 200 km by 150 km area straddling the continent-ocean transition. The processing, modelling and interpretation of four two-ship synthetic aperture profiles (SAP) and the gravity and magnetic anomaly data is presented. Gravity models show that a density model based on the crustal velocity structure defined by synthetic seismogram modelling of the two-ship expanding spread profiles is insufficient to reproduce the observed gravity profile across the margin. This requires additional contributions in the form of density gradients in the underlying sub-crustal part of the lithosphere and asthenosphere, and this is investigated by thermal modelling. Analysis of the magnetic anomaly data shows that oceanic magnetic anomalies 21 and 22 are developed in the north-west of the 1985 survey area. Anomalies 23 and 24 cannot be recognised due to post-rift igneous activity and/or subaerial seafloor spreading. The positions of anomalies 23 and 24B are reconstructed within the survey area, and the theoretical anomaly 24B position is used to determine the position of the continent-ocean boundary. Analysis of the anomalies recorded on the upper continental slope shows that the acoustic basement in this area is volcanic. Interpretation of the SAP profiles shows that the margin can be described in terms of three distinct volcanic sequences. The continental sequence is composed of lavas extruded onto continental crust during a period of continental volcanism which occurred before the onset of seafloor spreading just prior to anomaly 24B.The sequence thickens to the north-west, to form a set of seaward-dipping reflectors. The oceanic sequence comprises oceanic crust within which structurally different seaward-dipping dipping reflectors are developed. This sequence is associated with the reconstructed positions of anomalies 23 and 24B, and is interpreted as having formed during a period of subaerial seafloor spreading. The late sequence separates, and in part overlies the continental and oceanic sequences. Magnetic anomalies associated with the late sequence are arcuate, and have high amplitudes, implying an origin other than simple seafloor spreading. The late sequence is interpreted as originating from post-rift igneous activity in the Eocene.