A possible local anomaly in the 670-km discontinuity beneath Kerguelen

Abstract

The mapping of the 670-km discontinuity in regions of ascending or descending flows is of major interest for a better understanding of the nature of this discontinuity. In this paper, an attempt is made to investigate the properties of deep mantle discontinuities beneath the Kerguelen hot spot. Anomalous P to S converted waves are observed for some earthquakes recorded at the Kerguelen long-period GEOSCOPE station; the characteristics of these phases are specified owing to a three-dimensional polarization analysis of the signals. A tilted discontinuity is able to explain the data; this discontinuity dips toward the northeast with at 20°. The depth at which the conversion takes place is ∼ 880 km, assuming standard velocities in the upper mantle beneath Kerguelen, or 810 km if a hot upper mantle with a Poisson's ratio of 0.30–0.31 is present. However, the constraints given by the data are not strong enough to specify unambiguously this depth, a conversion at 600 km (or 550 km assuming a hot upper mantle) may alternatively be proposed. A strong velocity increase at the discontinuity is in each case necessary to explain the amplitude of the converted waves. Geophysical arguments show that this tilted discontinuity must be a local feature; possible other mechanisms, such as wave focusing due to undulations of the discontinuity, may also be proposed.