The age of the Tasman Sea basement can be roughly estimated from the 50 m.y. time constant associated with subsidence of sea floor generated by the mid-oceanic ridge. Present basement depths suggest Cretaceous age, as does sediment thickness. It is further argued that the Tasmantid Guyots, whose tops deepen systematically northward, were formed during Tertiary times by northward movement of the Australia plate over a fixed magma source in the mantle. As Antarctica was also approximately fixed with respect to the mantle, sea-floor spreading between the two continents implies that the guyots increase in age at a rate of 5.6 yr/cm from south to north. Their northward deepening then yields an average subsidence rate of 18 m per m.y. for the Tasman basement on which the seamounts were extruded. This rate again yields a Cretaceous age for the basement.

Great circles constructed normal to the Tasmantid Seamount chain, and other probable volcanic lineaments reflecting movement of the Australia plate over mantle magma sources, pass near Ethiopia, not far from the Antarctica-Australia spreading pole. This result supports the hypothesis that the mantle sources are approximately fixed with respect to Antarctica.

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