Constraints on the Nature of the Lithosphere-Asthenosphere Boundary: Comparison of Observed Textural Evolution to Measured Seismic Anisotropy
Abstract:The nature of the lithosphere-asthenosphere boundary (LAB) determines the mechanical and compositional coupling between rigid plates and underlying convecting mantle. Seismological studies reveal distinct reflectors in the uppermost oceanic mantle that are sometimes interpreted as the LAB. These reflectors roughly correlate with the location of vertical gradients in radial seismic anisotropy. However, these proxies for the LAB 1) do not exhibit the depth-age relationship predicted by thermal models and 2) are not detected consistently throughout the major ocean basins. In contrast, gradients in azimuthal anisotropy in oceanic upper mantle do appear to coincide with the predicted thermal structure. This overall seismic signature is suggested to arise both from melt-related processes at mid-ocean ridges and from “freezing in” anisotropy during changes in the direction of plate motion.
We test recent interpretations of this upper-mantle seismic structure with measurements of crystallographic textures in experimentally and naturally deformed peridotites. Key observations: 1) Experimental deformation of nominally melt-free olivine aggregates reveals a protracted increase in texture strength and, therefore, in magnitude of elastic anisotropy with progressive deformation, 2) melt-rich systems attain a steady state at very low strain (<~1) with fast seismic directions normal to those in the melt-free case, and 3) textures measured in samples from peridotite massifs exhibit a cm- to m-scale compositional heterogeneity associated with melt production and extraction that could be a significant source of radial anisotropy.
In conjunction with constraints on finite strain and melting extent from geodynamic simulations, we use textural observations to predict seismic structure in an oceanic setting. Comparison of our predictions to observed seismic anisotropy provide insight into the role of composition, melting, and rheology in defining the LAB.