The East Australian, Tasmantid, and Lord Howe Volcanic Chains: Possible mechanisms behind a trio of hotspot trails

Abstract:

The east Australian and Tasman Sea region is home to a unique example of intraplate volcanism: three long-lived, sub-parallel volcanic chains spaced only about 500\,km apart. Here we present new $^{40}$Ar/$^{39}$Ar results from the centre chain, the Tasmantid Seamounts, and show that the chain is strongly age-progressive, with an excellent correspondence to the age of the continental East Australian Volcanic Chain to the west and to the more limited ages available for the Lord Howe Seamount Chain to the east. Results from the Louisiade Plateau at the northern end of the Tasmantid chain suggest that it is composed of basalts of the correct age to be a large igneous province formed by the impact of the Tasmantid plume head reaching the lithosphere. This record of relative movement between the plate and the magma source over the last 55\,Ma shows two clear deflections from the overall linear trend, one at 26--23\,Ma, also observed in the continental chain and linked with the Ontong-Java Plateau jamming the South Melanesian subduction zone, and another at 50--43\,Ma, beyond the end of the continental record and contemporaneous with the Hawaiian-Emperor bend.

How does such a unique trio of volcanic chains form? The clear age progression, long lifespan, and tie to the Louisiade Plateau are classic indicators of deep-seated plumes, but it is difficult to explain how three separate plumes could remain stable for over 30\,Ma when separated by little more than the radii of the plume conduits. Here we examine alternative possible explanations for this volcanic pattern, including small plumes rising from a single deep-seated plume pooling at the 660\,km discontinuity, a single plume splitting around a subducting slab fragment, and small-scale convection triggered by topography on the lithosphere-asthenosphere boundary.