The OCEAN study area: Tectonic history from magnetic anomaly data and seismic reflectivity

Abstract

The OCEAN experiment is a detailed geophysical study of a region of the Cape Verde basin. A dense network of new magnetic and gravity profiles has enabled us to constrain the spreading rate history of the region and the location of fracture zones. The main features on the gravity profiles are lineated perpendicular to the seafloor spreading magnetic anomaly lineations. Significant along‐axis variability in spreading history suggests that the Mid‐Atlantic Ridge behaved as a series of loosely coupled segments within which spreading was fundamentally asymmetric. Such variability is associated with a minor jump in the ridge axis which changes the offset and expression of one of the fracture zones. Deep seismic reflection and refraction lines were oriented parallel and perpendicular to the magnetic lineations; seismic reflections occur at all levels within the crust, decreasing in amplitude and coherence below the level of the Moho. Analysis of the subbasement reflectivity provides compelling evidence that at least two major sets of dipping structure are present and are imaged separately on the two perpendicular sets of seismic profiles. Dipping reflections on flow line (“dip”) profiles, which are interpreted as faults due to their association with offsets in the basement surface, appear to strike parallel to the paleoridge axis. The majority of reflections that may be identified as faults dip toward the west, and although basement topography suggests that east dipping faults are also present, no reflections may be interpreted unambiguously as such. East dipping reflections observed only in the middle to lower crust have a more obscure origin. Dipping reflections seen on isochron (“strike”) profiles show clear contrasts in strength, lateral coherence, depth, and dip population; a number of these strike parallel to flow lines. Comparing reflection and refraction data shows that both the layer 2/layer 3 boundary and the Moho are marked by a change in the character of reflections and suggests that they may represent important structural, as well as seismological, boundaries within the oceanic lithosphere.