A new regional aeromagnetic anomaly map covering the southern Weddell Sea and surrounding areas was produced by combining Russian and British magnetic anomaly data acquired between 1973 and 1989. The map is divided into four zones on the basis of characteristic anomaly wavelength: Antarctic Peninsula, Weddell Sea embayment, Haag Nunataks and the western coast of East Antarctica. Over the southern Weddell Sea and the Ronne and Filchner ice shelves, the magnetic field is relatively smooth indicating a thick sedimentary cover although there are some distinctive anomalies. A broad positive anomaly is clearly visible running W-E across the north of the area, which is interpreted as representing the continent — ocean boundary. The most prominent magnetic feature is the elongate Explora Anomaly which runs parallel to the coast of East Antarctica and continues over Berkner Island terminating at the Dufek Massif. We interpret the source of this anomaly as rift-related volcanic rocks probably emplaced around the same time as the Dufek intrusion. The rift itself is interpreted as running south between the Andenes escarpment and East Antarctica and terminating south of Berkner Island. Russian gravity profiles show that Berkner Island does not have an associated Bouguer anomaly, although a large positive anomaly (approx 80 mGal) is seen to the east of the island over the Filchner Ice Shelf, suggesting the presence of a second rift structure. Gravity modelling across these features shows thinned crust below the Ronne and Filchner ice shelves, indicating that the whole area has undergone significant extension. A gravity model across the flanks of Berkner Island is used to estimate the ratio of unstretched to stretched crustal thickness. The resulting value of approximately 2.7 indicates that enough extension has taken place to induce melting, supporting the idea that the source of the magnetic anomaly over Berkner Island is rift-related volcanic rocks.