Relative positions of Africa and Antarctica in the Upper Cretaceous: evidence for non-stationary behaviour of fracture zones

New bathymetric and magnetic data have been collected in the Enderby Basin, southern Indian Ocean, during cruise MD34 of the N/O "Marion-Dufresne" (January-February, 1983). One can identify in this basin oceanic crust created along the same portion of the Southwest Indian Ridge as the crust of the Mozambique Basin. Two sets of magnetic lineations with different amplitude, split by a large offset fracture zone, are localized in each basin and enable us to obtain accurate reconstructions of the Africa-Antarctica plate boundary at the time of anomalies 34, 33 and 32; the data west of Prince Edward fracture zone are taken into account to adjust the parameters of rotation. These reconstructions of isochrons allow us to discuss the evolution of the geometry of accretionary plate boundaries and to emphasize the unstable behaviour of fractures zones. In particular we give evidence for an important change in the spreading direction along Prince Edward fracture zone which demonstrates that stability of a pole of rotation is not guarantueed by major offset fracture zones.