A vertical distribution of temperatures in the lithosphere is used to generate a simple gravimetric model which closely matches the observed free-air anomaly across athe Atlantic equatorial fractures zones. Topographic features associated with fracture zones are not compensated in the active transform part but become progressively compensated in the fossil one. At distances corresponding to a time period of 10--20 m.y., compensation is fully achieved. As the fossil fracture zone complex is often as much as 50 km wide, the position of plate boundary cannot be determined on morphological grounds only. Based on correspondance between the foot of the sharp wall of the fracture zone and the inflexion point of the free-air anomaly curve, a criterion is proposed to define the position of the plate boundary which corresponds to the contact at depth between the lithospheric blocks. Thus using this simple criterion, the fracture zone pattern of an ocean can be better defined, giving more severe constraints on the kinematic evolution of this ocean.