Problem of High Heat Flow and Volcanism associated with Zones of Descending Mantle Convective Flow

Abstract

EVER since the discovery by Vening Meinesz¹ of the large negative gravity anomalies associated with the deep oceanic trenches found in the south-west Pacific and elsewhere, it has frequently been proposed that trenches are the loci of some form of descending mantle flow. With the development of concepts of sea-floor spreading, oceanic trenches have been interpreted as areas where the spreading ocean floor turns downwards into the upper mantle and is destroyed. Zones of earthquake activity extending from close to the bottom of the trench down to depths of 700 km suggest that the shearing associated with downward motion of the oceanic crust and upper mantle occurs parallel to a plane which dips downwards in the direction of motion at an angle of about 45° to the surface² (Fig. 1). Fault plane solutions for the earthquakes along these zones, and apparently anomalous distributions of Q (anelasticity) with depth³, tend to support this interpretation, although the presence of undeformed sedimentary filling in many trenches must be explained.