The lithosphere - asthenosphere system seen by surface waves: New insights from radial anisotropy

We propose a conceptual model of the lithosphere-asthenosphere system that reconciles a number of surface wave observations. It comes in the context of the development of a new SV wave velocity, V s v , and radial anisotropy, ξ , global model based on multi-mode Rayleigh and Love wave data. The Rayleigh dataset has also been used in previous studies to constrain the azimuthal anisotropy, the quality factor Q and the melt content. All these models are based on self-consistent datasets and use the same waveform modeling and tomographic approaches, with similar regularization and smoothing. The first important finding of this study is the fact that a 1-D ξ model close to PREM but including a 3D crust structure explains our dataset almost everywhere, except in oldest parts of the continents (mainly Archean and Proterozoic terrains) and youngest parts of the Pacific ridge (mainly ages ≤ 50 Ma) where 3-D variations of ξ are required. Based on this, we build an hybrid model, Hybrid 1D/3D, where ξ is fixed to a 1-D profile when sufficient to fit the data, and inverted for when the dataset requires lateral variations. Our radially anisotropic model shows that there is no age dependance of the radial anisotropy ξ in the oceanic lithosphere while age is the main parameter controlling V s v , melt content and azimuthal anisotropy, particularly under fast plates. We also find that positive radial anisotropy ( ξ > 1 ) extends deeper beneath oceans (down to 350 km depth) than beneath continents. We propose that the thicker radially anisotropic layer under the oceans compared to the continent indicates a change in origin from frozen-only anisotropy under the continent to a more complex situation under the oceans. Cratons appear fast, melt-free and anisotropic, down to 200-250 km depth, and anomalies with ξ ≤ 1 are present in the deeper part of the cratonic roots (around 250 km) that could reflect past delamination or compression of the deep lithospheric roots. • Lateral variations of radial anisotropy ξ are only required under cratons and under the Pacific ridge. • There is no age dependance of radial anisotropy in the oceanic lithosphere. • Lithospheric age is the main parameter controlling V s v , melt content and azimuthal anisotropy, particularly under fast plates. • Cratons are fast, melt-free and anisotropic, down to 250 km depth, with ξ ≤ 1 below. • The ξ ≤ 1 anomalies in the cratonic root may reflect past delamination or compression.