THE OCEAN STUDY AREA - TECTONIC HISTORY FROM MAGNETIC ANOMALY DATA AND SEISMIC REFLECTIVITY

The OCEAN experiment is a detailed geophysical study of a region of th e Cape Verde basin. A dense network of new magnetic and gravity profil es has enabled us to constrain the spreading rate history of the regio n and the location of fracture zones. The main features on the gravity profiles are lineated perpendicular to the seafloor spreading magneti c anomaly lineations. Significant along-axis variability in spreading history suggests that the Mid-Atlantic Ridge behaved as a series of lo osely coupled segments within which spreading was fundamentally asymme tric. Such variability is associated with a minor jump in the ridge ax is which changes the offset and expression of one of the fracture zone s. Deep seismic reflection and refraction lines were oriented parallel and perpendicular to the magnetic lineations; seismic reflections occ ur at all levels within the crust, decreasing in amplitude and coheren ce below the level of the Moho. Analysis of the subbasement reflectivi ty provides compelling evidence that at least two major sets of dippin g structure are present and are imaged separately on the two perpendic ular 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 th e paleoridge axis. The majority of reflections that may be identified as faults dip toward the west, and although basement topography sugges ts that east dipping faults are also present, no reflections may be in terpreted unambiguously as such, East dipping reflections observed onl y in the middle to lower crust have a more obscure origin, Dipping ref lections seen on isochron (''strike'') profiles show clear contrasts i n strength, lateral coherence, depth, and dip population; a number of these strike parallel to flow lines. Comparing reflection and refracti on 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 t hey may represent important structural, as well as seismological, boun daries within the oceanic lithosphere.