Age progressive volcanism in the Tasmantid Seamounts

Abstract

The Tasmantid Seamounts comprise a northerly trending linear chain of submarine volcanoes that extend over more than 1300 km in the middle of the Tasman Basin, located to the east of the Australian continental margin. The volcanoes are situated upon deep oceanic crust of Late Cretaceous/Early Cenozoic age. Several of the volcanoes were built from sea floor depths of more than 4000 m to above sea level, and were then eroded to flat-topped mountains which have subsided to depths as great as 400 m. Basalt samples dredged from Gascoyne, Taupo, Derwent Hunter, Britannia and Queensland Seamounts have been dated by the K-Ar and 40Ar/ 39Ar methods, yielding results in the range 24 to 6.4 Ma, Early to Late Miocene. A progressive younging of the volcanism southward along the seamount chain at an average rate of 67 ± 5mm/year is indicated. The predicted present position of the volcanic focus is at 40.4°S latitude, and between 155° and 156°E longitude, virtually coincident with the epicentre of a recent large earthquake. These results provide strong evidence that the Tasmantid Seamounts represent a hotspot track, effectively recording motion of the Australian plate across the sublithospheric mantle source region for the volcanism. Comparison with results from hotspot traces on the same plate and on the African plate further demonstrate that these hotspots provide a useful frame of reference for plate motions, and that relative movement between individual hotspots must be less than about 5 mm/year.