On the origin of recent intraplate volcanism in Australia

The widespread occurrence of intraplate volcanism is difficult to explain using the primary features of plate tectonics, such as subduction at convergent boundaries or upwelling at rifted margins, which are responsible for most volcanism on Earth. Instead, other mechanisms are invoked, including mantle plumes, edge driven convection (EDC), shear driven upwelling (SDU) of the asthenosphere, lithospheric cracking, ductile removal of lithosphere via gravitational instability, glacial rebound and slab tear. Recently, EDC has been postulated as the trigger for Quaternary intraplate volcanism in Australia due to the proximity of a craton edge. However, the Precambrian shield region of the Australian continent has a boundary at least 10,000 km long, yet the Newer Volcanics Province (NVP) is contained within a 400 x 100 km region. This brings into question EDC as a causal mechanism unless an explanation can be found as to how it can nucleate at a single location. Here, we use a combination of seismic tomography and geodynamic modelling to show, for the first time, that (i) the source of the NVP is restricted to the upper mantle, and (ii) mantle upwelling induced by EDC is localised and intensified beneath the NVP as a result of 3-D variations in lithospheric thickness and plate motion induced SDU. This study helps to solve the global puzzle of why step changes in lithospheric thickness, which occur along craton edges and at passive margins, only produces volcanism at isolated locations.