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British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK
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 (83 Ma). 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. 100 km. 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.
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