Fanning of fracture zones and a three-dimensional model of the Mid-Atlantic Ridge

Abstract

Present-day spreading directions as represented by the active (transform fault) sections of fracture zones in the central North Atlantic do not fit a small circle pattern about a unique pole^1–5, as recently confirmed by a ‘Gloria’ survey. Contrary to this pattern, which is geometrically required if ocean floor spreading is to occur⁶, the active transform fault directions show a convergence towards the east³. Because of the fanning of the present directions, the African and American plates are caught in what might be likened to a dovetail construction. To allow motion, the African plate must shrink about 2% in a north–south direction. Such a shrinkage might be accomplished by thermal contraction. Tension due to the horizontal component of this contraction has been proposed as the primary cause of fracture zones^4,7. However, horizontal thermal contraction alone cannot explain the recorded pattern of spreading segments and transform faults, because it does not explain why transform faults converge to the east and diverge to the west. For this, an active driving mechanism is needed. We show here that the gravitational drive (ridge push) caused by thermal contraction in the lithosphere formed along a sinusoidal mid-ocean ridge delivers such a mechanism.