GLOBAL DE-CORRELATION OF THE TOPOGRAPHY OF TRANSITION ZONE DISCONTINUITIES

Abstract

Analysis of long-period shear-wave underside reflections from upper mantle discontinuities shows the shape of the 410 km discontinuity to be neither globally nor locally correlated with that of the 660-km discontinuity. The long wavelength pattern of 410 topography is dominated by degree-l spherical harmonics and has a reasonable correlation with the seismic velocity perturbations in the mantle above it. The global topography of 660, dominated by degree-2 spherical harmonics, is strongly correlated with shear-wave velocity variations in the transition zone. The poor correlation between the observed topography of the 410-and 660-km discontinuities is inconsistent with a simple model of large scale, whole mantle flow, thus implying a more complex pattern, which includes at least temporary accumulation of subducted material in the transition zone [Ringwood and Irifune, Nature 33 1 (1988) I3 l-1 36; Tackley et al.. Nature 361 (1993) 699-7041. The modest seismic discontinuity at 520 km is well observed when the reflections are under mid-age oceans. but is not observed when they are under continental shields. Interpretation of this observation in terms of olivine depletion in the deep continental mantle indicates the possible existence of continent-ocean chemical heterogeneity at depths exceeding 500 km.