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Bundesanstalt für Geowissenschaften und Rohstoffe, Postfach 510151, D-30631, Hannover, Germany
The southern Astrid Ridge and the Gunnerus Ridge together with the Kainan Maru Seamount are continental fragments which delimit the oceanic crust of the Riiser-Larsen Sea off Dronning Maud Land. Geophysical investigations in the Riiser-Larsen Sea show the magnetic anomalies M0 to M24 with half spreading rates between 1.4 and 2.7 cm a–1. In the western part of the Riiser-Larsen Sea, spreading started approximately 15 Ma years before chron M22, that is about 165 Ma years ago. At chron M10, India and Sri Lanka separated from Antarctica, the Gunnerus Ridge and Madagascar remained attached to Antarctica. In the eastern part of the Riiser-Larsen Sea, seafloor spreading started at chron M0, when Madagascar and the Madagascar Ridge became attached to the African plate. Break-up did not take the shortest possible line. This may depend on variations of the strength of the lithosphere. Previous work has demonstrated that rifting prefers to dissect continental crust. The Gunnerus Ridge, the Madagascar Ridge and other similar ridges of continental crust may owe their existence to this circumstance. However, in contrast to examples presented previously, the continental fragments discussed here are detached partly by strike-slip motion. Furthermore, it is required that the stability of the crust varies widely.
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