The Jan Mayen Ridge and its southern extension were studied by means of multichannel reflection seismic profiles gathered during the Cepan 1 (Cercle Polaire Atlantique Nord) 1975 survey. These data have been interpreted with reference to the evolution of the Norwegian-Greenland Sea, which may be chronologically summarized as follows : 1) opening of the Norwegian basin from the now-extinct Aegir axis 60 myrs ago ( magnetic anomalies 23 to 20) ; 2) westward jumping of the spreading axis : this new axis (Iceland Plateau Extinct Axis) lasted between 5 D and 6 A anomaly times (24 to 18 myrs) ; 3) second westward ridge jump leading to the setting of the currently active Kolbeinsey ridge. [NOT CONTROLLED OCR]