Nature of the Lithosphere-Asthenosphere Boundary Beneath Normal Oceans from High-Resolution Anisotropic Receiver Functions

Abstract:

The oceanic lithosphere-asthenosphere boundary (LAB) has a seismological expression that is characterized by a sharp velocity reduction located at relatively shallow depths (40-100 km). Observations suggest that the LAB deepens with age for relatively young oceans, and may be coincident with anisotropic gradients. Models to explain these features include partial melting, sub-solidus physical dispersion caused by grain-boundary sliding, and a change in anisotropic rock texture. To assess these models, we provide new seismological constraints using frequency-dependent harmonic decomposition of receiver functions. We analyze data from 17 ocean-bottom stations and 2 borehole stations in the Philippine Sea and Northwest Pacific ocean. Underneath young oceans, we observe that the depth of the LAB follows the ~1300°K thermal contour. For old oceans the LAB lies at a constant depth ~70 km. These results are consistent with the predictions of the grain-boundary sliding model. Preliminary analysis of the width of the velocity gradient, shows that at some young oceans, the velocity reduction is gradational, while at old oceans a sharper velocity reduction is required to fit the receiver function data. This behavior is also consistent with the prediction of the grain-boundary sliding model, where thermal relaxation in the young oceans leads to a gradational velocity reduction, while the water-dependent relaxation leads to a sharp velocity reduction at old oceans. Additionally, harmonic decomposition of the receiver functions suggests that strong anisotropic gradients do not coincide with the velocity reduction. At undisturbed ocean lithosphere, therefore, we argue that anisotropy is not a causal mechanism for the LAB. We also fail to observe oppositely-polarized Ps conversions from the top and bottom boundary of a putative melt-layer, except underneath stations above the subducting Pacific-plate slab.