Full waveform tomography for radially anisotropic structure: New insights into present and past states of the Australasian upper mantle

Abstract

We combine spectral-element simulations and adjoint techniques in a non-linear full seismic waveform tomography for the radially anisotropic structure of the Australasian upper mantle. Our method allows us to explain 30 s waveforms in detail, and it yields tomographic images with locally 2{ring operator} lateral resolution. In the course of 19 conjugate-gradient iterations the total number of exploitable waveforms increased from 2200 to nearly 3000. The final model, AMSAN.19, thus explains data that were not initially included in the inversion. This is strong evidence for the effectiveness of the inversion scheme and the physical consistency of the tomographic model. AMSAN.19 confirms long-wavelength heterogeneities found in previous studies, and it allows us to draw the following inferences concerning the past and present states of the Australian upper mantle and the formation of seismic anisotropy: (1) Small-scale neutral to low-velocity patches beneath central Australia are likely to be related to localised Paleozoic intraplate deformation. (2) Increasing seismic velocities between the Moho and 150 km depth are found beneath parts of Proterozoic Australia, suggesting thermochemical variations related to the formation and fragmentation of a Centralian Superbasin. (3) Radial anisotropy above 150 km depth reveals a clear ocean-continent dichotomy: We find strong vsh > vsv beneath the Coral and Tasman Seas. The anisotropy is strongest at the top of the inferred asthenospheric flow channel, where strain is expected to be largest. Radial anisotropy beneath the continent is weaker but more variable. Localised patches with vsh < vsv appear, in accord with small-scale intraplate deformation. (4) The ocean-continent dichotomy disappears gradually between 150 and 250 km depth, where the continental lithospheric mantle and the oceanic asthenosphere pass into the underlying convecting mantle. (5) Significant anisotropy exists below 250 km depth. Its character can be explained by sublithospheric small-scale convection and a change in the dominant glide system of olivine.