ИСТИНА

We estimate temperature and compositional structure of the Australian upper mantle by applying an iterative technique, which jointly interprets seismic tomography and gravity data constrained by mineral physics. This technique consists in removing the effect of the crust from the observed gravity field and topography. In the second step, the residual mantle gravity field and residual topography are inverted to obtain a 3-D density model of the upper mantle. The thermal contribution to the density structure is estimated by inverting the seismic model AusREM, assuming a laterally and vertically uniform ‘‘fertile’’ mantle composition. After removing this effect from the total mantle anomalies, the residual ‘‘compositional’’ fields are obtained. They are used to predict compositional structure of the Australian cratons, which, in turn, used to correct the thermal model. In this way, we improved iteratively the initial thermal and compositional models. The obtained temperature distribution shows distinct differences between the warmer eastern margin of the continent and the cooler zones beneath the Archean and Proterozoic domains and strong lateral variations within the cratons. The West Craton is significantly iron depleted and the depletion extends also beneath the lithosphere, which suggests some component of lithospheric erosion due to the rapid northward motion of the Australian Plate. Beneath the Proterozoic terranes, depletion is much reduced below 150 km depth. The Palaeozoic domains display a generally more fertile composition. We used the thermo-compositional structure of the upper mantle together with the most recent crustal model of Australia (AusREM) to construct two alternative models of strength and efective elastic thickness (EET) of the lithosphere. The first model (Model I) assumes a constant value of 10− 15 s−1 for the strain rates. In the second model (Model II), we used the strain rates obtained from a global mantle flow model. In both models we assumed a stiff rheology, on account of the mafic composition of the Australian crust. The results of Model II show larger variability of the rigidity of the plate within the cratonic areas, reflecting the long tectonic history of the Australian plate. On the other hand, the younger eastern terranes are uniformly weak, due to the higher temperatures.