Quaternary Hot-spot Activity within the Australian Continent Fueled by Edge-driven Convection: Combined Evidence from Seismic Tomography and 3D Geodynamic Modelling

Although the Australian continent is often regarded as tectonically quiescent owing to its intra-plate setting, it is not entirely free from large-scale processes, which result in lithospheric-scale changes in composition and structure. A clear example of this is the Newer Volcanics Province (NVP), located in the state of Victoria, southeast Australia, which exhibits the youngest evidence (~5ka) of basaltic intra-plate volcanism within Australia. Over the last few decades there has been much conjecture as to whether the source of volcanism originates deep in the mantle - for example, a rising plume - or is somehow localized in the uppermost mantle. In order to directly address this question, we use teleseismic arrival time residuals recorded by the WOMBAT transportable array in eastern Australia - the largest of its type in the southern hemisphere - to image 3-D variations in P-wavespeed in the upper mantle beneath the NVP. Our results unequivocally show the presence of a low velocity anomaly beneath the NVP which terminates at around 250 km depth. Assuming that this distinct anomaly is associated with increased temperatures and melting, then it appears likely that the NVP is a shallow, rather than deeply rooted feature. It is not possible to completely rule out the presence of a plume, because a narrow plume that spreads out on contact with the base of the lithosphere may yield a similar signature; however, our seismic results, combined with complementary evidence, including: (i) the lack of a hotspot chain; (ii) modest topographic response; and (iii) elongation of the eruption centers in a direction perpendicular to contemporary plate motion, all support the same conclusion. An idea that has been invoked to explain the NVP is so-called edge driven convection, where a change in lithospheric thickness drives a localized convection cell, which can transport deeper and hotter material to the surface. It has been inferred, from regional surface wave models, that the edge of the Australian shield has a roughly E-W orientation around 500-600 km north of the NVP, and that there may be a step change from thicker lithosphere in the north to thinner lithosphere in the south. We use the recently developed computational framework, Fluidity, to perform several idealized 2-D numerical experiments, involving a rigid lithosphere and a viscous (composite Newtonian and Non-Newtonian rheology) mantle, which demonstrate that such a step change would induce localized upwelling. However, given that there are many regions in Australia where step changes in lithospheric thickness occur with no associated volcanism, this cannot be the complete explanation. Therefore, to unravel the controls on the geographic location of the NVP, we construct a detailed map of the lithosphere-asthenosphere boundary (LAB) in this region, using constraints from both surface and body waves. This map is subsequently used to define a rigid lithosphere for a 3-D numerical model. Such models demonstrate that the particular geometry of the LAB, south of the main step in lithospheric thickness, significantly intensifies mantle upwelling in the vicinity of the NVP.