The Australian continental upper mantle structure and deformation inferred from surface waves

Abstract

We present a new three-dimensional model for the SV wave heterogeneities and azimuthal anisotropy in the upper mantle of the Australasian region. The model is constrained by the waveforms of 2194 Rayleigh waves seismograms with a dense ray coverage that ensure a lateral resolution of the order of few hundred of kilometers. The use of higher modes allows the resolution of the structure down to depths of at least 400 km. In the upper 200 km of the model, seismic velocities are lower on the eastern Phanerozoic margin of the continent compared to the Precambrian central and western cratons, in agreement with previous results for Australia. The boundary between Phanerozoic and Precambrian Australia is not clear, especially in the south, where a broad positive seismic anomaly underlays the Lachlan Fold Belt. The high-velocity lid beneath the continent shows significant variations in thickness. Locally, it may extend down to a depth of 300 km in the mantle, but for most of Precambrian Australia the lithospheric thickness oscillates around 200 km, while it is thinner on the eastern Phanerozoic margin. We found significant SV wave azimuthal anisotropy in the upper 250 km of the mantle, with a drastic change in the organization of anisotropy between the upper 150 km of the model and the deeper part, as revealed in a preliminary inversion. In the upper 150 km of the mantle, azimuthal anisotropy appears more likely to be related to past deformation frozen in the lithosphere, and in central Australia we found clear evidence that deformation is preserved since the Alice Springs orogeny. Below 150 km, a smoother pattern of anisotropy is observed, more likely to be related to present-day deformation due to the northward motion of the Australian plate. Our current data set allows constraint of the anisotropic directions at different depths with an unprecedented lateral resolution. The observation of significant changes of anisotropic directions with depth in the Australian continental mantle suggests that care should be taken in the interpretation of anisotropy from SKS observations. Show more