Oceanic lithosphere-asthenosphere boundary from surface wave dispersion data

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.

Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

“…Importantly, the depth to which ψ is well aligned with the plate motion direction appears to depend on plate age, roughly in accord with standard thermal models for oceanic upper mantle. Based on these motivating observations, we test the previously suggested hypothesis (13,15,17,18) that the well-aligned region represents a relatively low-viscosity asthenosphere, whose upper boundary is thermally controlled.Second, recent seismological studies (13, 15) suggest a transition in the magnitude of radial anisotropy (ξ, the ratio between the velocity of horizontally polarized S waves and vertically polarized S waves) as a function of depth. Although these surface-wave studies have not included the high-frequency data required to fully resolve anisotropy at the shallowest depths, a transition from low values of ξ at shallow depths to higher values of ξ at greater depths appears to occur near 80 km.…”

“…Importantly, the depth to which ψ is well aligned with the plate motion direction appears to depend on plate age, roughly in accord with standard thermal models for oceanic upper mantle. Based on these motivating observations, we test the previously suggested hypothesis (13,15,17,18) that the well-aligned region represents a relatively low-viscosity asthenosphere, whose upper boundary is thermally controlled.…”

“…Extrapolations from laboratory experiments (9) suggest that a dehydration front established by melting near the ridge axis can produce a water-poor layer whose thickness would not depend on plate age (8). In both interpretations, the sharp drop in velocity is often taken to indicate a change in viscosity structure and therefore the base of the lithosphere.Instead of compositional variations, recent investigations of seismic anisotropy (13,15,16) provide evidence for a lithospheric base that is thermally controlled and measurably deeper than the sharp seismic discontinuities. Beghein et al (13) highlighted two observations.…”

“…Instead of compositional variations, recent investigations of seismic anisotropy (13,15,16) provide evidence for a lithospheric base that is thermally controlled and measurably deeper than the sharp seismic discontinuities. Beghein et al (13) highlighted two observations.…”

“…Second, recent seismological studies (13,15) suggest a transition in the magnitude of radial anisotropy (ξ, the ratio between the velocity of horizontally polarized S waves and vertically polarized S waves) as a function of depth. Although these surface-wave studies have not included the high-frequency data required to fully resolve anisotropy at the shallowest depths, a transition from low values of ξ at shallow depths to higher values of ξ at greater depths appears to occur near 80 km.…”