The upper mantle velocity distribution

Abstract

In plate tectonic theory one thinks of rigid blocks of lithosphere moving relative to one another over a weak asthenosphere. In the oceans the base of the lithosphere is associated with the top of the low velocity layer at a depth of 70–100 km. Current models of the upper mantle velocity distribution beneath continental platforms on shield regions give little indication of the depth of the base of the lithosphere in these regions. Gerver and Markusevitch (1966, 1967) have shown that, providing travel times from earthquakes covering a range of depths are available in addition to surface focus travel times, it is possible to determine the upper mantle velocity distribution even if low velocity layers are present. This paper summarizes information on surface focus travel times from controlled sources in platform or shield regions and describes a complex model of the upper mantle velocity distribution based on observations of the travel times of seismic waves from Banda Sea events covering a wide range of depth. It is pointed out that surface focus travel times in continental platform or shield regions differ significantly from the Jeffreys-Bullen travel times. The deviations from the Jeffreys-Bullen times are much larger than the differences between the deviations for different continental platform or shield regions. One of the features of the model based on the travel times from the Banda Sea events is a velocity discontinuity at a depth of about 200 km. It is difficult to account for this discontinuity in terms of a phase transformation. It is pointed out that there is supporting evidence for the existence of relatively sharp velocity discontinuities at 200, 400 and 650 km from studies of converted phases by Vinnik and Jordan and Frazer. The principal uncertainty in the upper mantle velocity distribution arises from uncertainties in the depths of foci of the earthquakes. It is in fact possible that there may be a low velocity zone in the upper mantle beneath continental platform or shield regions between depths of 150 and 200 km.