A re-evaluation of Weddell Sea magnetic anomaly data, in the light of satellite gravity maps, provides important constraints on plate kinematics prior to chron C34 (83Ma). Although M-series anomalies are difficult to identify, the evidence favours younger ages than previously suggested, based on the assumption that these anomalies formed at the South America-Antarctica plate boundary. Geosat-derived free-air gravity maps show a distinct anomaly, 'Anomaly-T', associated with the southern termination of a herring-bone pattern of flow lines. The associated magnetic anomaly has been identified previously as anomaly M20, on the basis of a model of very slow spreading, but is interpreted here as an edge-effect, associated with an abrupt decline in spreading rate which occurred between M4 (126 Ma) and M0 (118 Ma), and which induced the fine segmentation pattern to the north. Although anomalies are present to the south of Anomaly-T, they are of reduced amplitude, and are difficult to correlate over distances of more than c. 100km. Nevertheless, they appear to strike approximately W-E, sub-parallel to the younger anomalies to the north, and show a resemblance to anomalies M4-M12, modelled at a rate of 15 km Ma -1.Flow-line modelling, based on published rotations for the Southwest Indian Ridge and southern Mid-Atlantic Ridge, gives a reasonable fit to the observed trends of Weddell Sea gravity ridges. Two alternative models for the M-series reconstruction of Africa and Antarctica are employed, corresponding to the two M2 fits suggested by Bergh. Although it is not possible to reject either of the models, the tighter fit of the two gives slightly better agreement with Weddell Sea trends. In both models, a drop in rates occurs at M0 time.Anomalies farther south correspond with previously identified features, such as the Orion, Explora and Andenes anomalies, and the Polarstern Bank, all characterized by magnetic anomaly highs, attributed to excess volcanism during early break-up.
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 indu6e melting, supporting the idea that the source of the magnetic anomaly over Berkner Island is rift-related volcanic rocks.
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