A Global Lithosphere Asthenosphere Boundary

We use scattered waves recorded at permanent seismic stations from 1990-2004 to map a global interface that may be associated with the lithosphere-asthenosphere boundary. We consider stacked P-to-S (Ps) data at stations where crustal structure is relatively simple, the Moho is well-defined, and the data distribution enables easy identification of crustal phases in nine epicentral distance bins at frequencies up to 2 Hz. These restrictions allow us to identify crustal phases and search for conversions that could be associated with the lithosphere-asthenosphere boundary. Data from stations that fit these criteria suggest the presence of sharp negative discontinuities (velocity decreases with depth) in the depth range of 40 to 140 km. S-to-P results also generally confirm the presence of these discontinuities. So far, we see no consistent feature at these depths in back-azimuthally binned SH-component data from corresponding stations, indicating that anisotropy is not the dominant mechanism causing these apparent velocity changes. The persistence of this feature in single-station SV-component data suggests a global discontinuity. Indeed, globally the depth to the boundary correlates with tectonic environment varying from an average of 110 km beneath cratons, thinning to ~80 km at continental margins and to ~50 km at some oceanic island stations. Beneath noncratonic regions the boundary depth is in general agreement with the sharpest part of the velocity gradient in global velocity models. Beneath cratons the boundary is significantly shallower than the base of the seismically fast keels in global surface-wave models. One explanation is that beneath cratons a compositional boundary in depletion and/or hydration exists at shallow depths (80 to 140 km), while a thermally cold root extends to greater depths. The high frequencies inherent to the Ps converted phases image the potentially sharp shallow compositional boundary, but longer-period surface waves image a deeper, thermally-defined craton. In this model, the compositional and thermal boundaries would be coincident beneath noncratonic areas. Alternative explanations are that the discontinuities illuminated by this study beneath continents represent fossil slabs that were originally stacked during the formation of the continents, or the lids of thin melt reservoirs.