ALBERT

Description: Receiver-function observations in the oceanic upper mantle can test causal mechanisms for the depth, sharpness and age-dependence of the seismic wavespeed decrease thought to mark the lithosphere-asthenosphere boundary (LAB). We use a combination of frequency-dependent harmonic decomposition of receiver functions and synthetic forward-modeling to provide new seismological constraints on this “seismic LAB” from 17 ocean-bottom stations and 2 borehole stations in the Philippine Sea and northwest Pacific Ocean. Underneath young oceanic crust, the seismic LAB depth follows the ∼1300 K isotherm but a lower isotherm (∼1000 K) is suggested in the Daito ridge, the Izu-Bonin-Mariana trench and the northern Shikoku basin. Underneath old oceanic crust, the seismic LAB lies at a constant depth ∼70 km. The age-dependence of the seismic LAB depth is consistent with either a transition to partial-melt conditions or a sub-solidus rheological change as the causative factor. The age-dependence of interface sharpness provides critical information to distinguish these two models. Underneath young oceanic crust, the velocity gradient is gradational, while for old oceanic crust a sharper velocity gradient is suggested by the receiver functions. This behavior is consistent with the prediction of the sub-solidus model invoking anelastic relaxation mediated by temperature and water-content, but is not readily explained by a partial-melt model. The Ps conversions display negligible two-lobed or four-lobed back-azimuth dependence in harmonic stacks, suggesting that a sharp change in azimuthal anisotropy with depth is not responsible for them. We conclude that these ocean-bottom observations indicate a sub-solidus elastically-accommodated grain-boundary sliding (EAGBS) model for the seismic LAB. Because EAGBS does not facilitate long-term ductile deformation, the Seismic LAB may not coincide with the conventional transition from lithosphere to asthenosphere. This article is protected by copyright. All rights reserved.