Structure of the Mid Atlantic Ridge province between 12° and 18°N

Abstract

An analysis is given of air-gun profiler and magnetic data obtained in the central North Atlantic between 12° and 18°N. Eight fracture zones were crossed, one of which (the 15°20′N fracture zone) was traced over a distance of 1500 km. The mode of adjustment of fracture zones to a change in direction of spreading is discussed. It is shown that also if this new direction would lead to an opening of the fracture zone, and adjustment fracture can originate and actually does so in several instances. The about E-W fracture zones dominate the structure of the Ridge province entirely, both with regard to the topography and to the magnetics. A magnetic model is proposed accounting for the different types of anomalies found over fracture zones. No intrusive bodies are needed to explain these anomalies. The origin of fracture zones is related to thermal contraction of a cooling lithosphere while moving from the ridge. Thermal contraction may also explain how the American and the African plates are freed from the grip they are caught in by the fanning of the fracture zones in the central North Atlantic. The fanning of fracture zones has consequences for the determination of the pole of spreading. This pole can only be found as a best fit from a synthesis of the total plate boundary, i.e. from the Azores to Bouvet Island. ‘Local’ poles have only restricted value, since deviations up to 5 deg occur from a small circle pattern based on existing data. Several huge structures, viz. Researcher Ridge and Royal Trough, are found in the area which seem to parallel the flow lines of the fracture zone system. No adequate explanation exists for these structures.