The Lithosphere-Asthenosphere Boundary: Clues From Joint Interpretation of Global Seismic Attenuation and Velocity Models

The zone of low viscosity beneath oceanic plates appears to also be characterized by anomalous seismic properties (low velocity, high attenuation, strong radial anisotropy) and high electrical conductivity. In recent years, the oceanic seismic low-velocity zone (LVZ) has been attributed by various authors to solid-state thermal mechanisms, to partial melt, and to elevated water content. Considering seismic attenuation (1/Q) in addition to velocity may help in determining the origin of the LVZ, since attenuation and velocity have different sensitivities to temperature, composition, partial melt, and water. We have developed a new global model of shear attenuation in the upper mantle, QRFSI12. The model is derived from large data sets of Rayleigh wave amplitudes in the period range 50--250 s, and we account for source, instrument, and focusing effects on the data in order to isolate the signal of attenuation. QRFSI12 is strongly anti-correlated with global velocity models throughout the upper mantle, and individual tectonic regions are each characterized by a distinct range of attenuation and velocity values at 100-km depth. Comparison with laboratory measurements of the temperature sensitivity of attenuation and velocity (Faul and Jackson, 2005) shows good first-order agreement between the seismological and experimental values, suggesting that temperature variations can explain much, but not all, of the observed global variability in velocity and attenuation. Specifically, the seismological velocity-attenuation relationship for oceanic regions agrees with the experimental values at depths > 100 km, but the seismic properties of cratonic regions deviate from the experimental values for depths < 250 km, suggesting that temperature alone is not a sufficient explanation for the observations in the shallow cratonic mantle. Globally, seismic properties shift into better agreement with the experimental (thermal) trends in the depth range 100--150 km and 200--250 km for oceans and cratons, respectively, which may indicate the base of a chemical boundary layer and the depth of the lithosphere-asthenosphere boundary in these regions.