Oceanic lithosphere-asthenosphere boundary from surface wave dispersion data
Résumé
Abstract According to different types of observations, the nature of lithosphere-asthenosphere
boundary (LAB) is controversial. Using a massive data set of surface wave dispersions in a broad period
range (15–300 s), we have developed a three-dimensional upper mantle tomographic model (first-order
perturbation theory) at the global scale. This is used to derive maps of the LAB from the resolved elastic
parameters. The key effects of shallow layers and anisotropy are taken into account in the inversion process.
We investigate LAB distribution primarily below the oceans, according to different kinds of proxies that
correspond to the base of the lithosphere from the shear velocity variation at depth, the amplitude
radial anisotropy, and the changes in azimuthal anisotropy G orientation. The estimations of the LAB depth
based on the shear velocity increase from a thin lithosphere (∼20 km) in the ridges, to a thick old-ocean
lithosphere (∼120–130 km). The radial anisotropy proxy shows a very fast increase in the LAB depth from
the ridges, from ∼50 km to the older ocean where it reaches a remarkable monotonic subhorizontal profile
(∼70–80 km). The LAB depths inferred from the azimuthal anisotropy proxy show deeper values for the
increasing oceanic lithosphere (∼130–135 km). The difference between the evolution of the LAB depth with
the age of the oceanic lithosphere computed from the shear velocity and azimuthal anisotropy proxies and
from the radial anisotropy proxy raises questions about the nature of the LAB in the oceanic regions and of
the formation of the oceanic plates
Domaines
Planète et Univers [physics]
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