Thermal and Compositional Anomalies of the Australian Upper Mantle From Seismic and Gravity Data

To discern temperature and compositional variations of the Australian upper mantle, we apply an integrative technique, which jointly interprets seismic tomography and gravity data. The final thermal model, obtained by changing the upper mantle composition according to the density variations, shows temperatures higher by 100–150°C in the Archean and Proterozoic upper mantle, with respect to the initial model based on a uniform “fertile” composition. In the North and West Australian cratons, the upper mantle is cold, with composition depleted in heavy constituents. This suggests the presence of an Archean lithosphere, which remained relatively undisturbed through the Proterozoic. Central Australia is predominantly characterized by a thick, low-temperature lithosphere having a more fertile composition. Its shallow part is characterized by a thin layer of low-velocity mantle, which is interpreted by our results as a thermal anomaly. However, this high-temperature anomaly is hard to reconcile with the tectonic history of the region. A low-density mineral phase, such as amphibole, may reduce the density relative to our assumed composition. Furthermore, we observe larger iron depletion in the Western Australian Craton than in the Proterozoic terranes. At the depths larger than 150 km, the depletion becomes negligible beneath the Proterozoic regions, while it also persists in the Western Australian Craton at the depths larger than 200 km.