Seismology, dT/d and Deep Mantle Convection

Abstract

Summary dT/dA and azimuth measurements at the Yellowknife Seismic Array (YKA) for P and pP phases that have penetrated to depths greater than 1850 km show systematic bias relative to those values calculated from spherically symmetric Earth models. Moreover, the corresponding arrival vectors for the phases PcP, ScP, PnKP and SnKP (n > 2) approaching YKA from within the same narrow azimuth ranges as the P andpP phases show related systematic errors. From an analysis of combinations of all these phases, we show that most of the observed bias for P and pP is produced along the upgoing portion of the ray path, and that any heterogeneities and velocity anomalies near the base of the mantle cannot be clearly elucidated by a dT/dA study of the P phase alone. P wave data must be supplemented by other phases that traverse similar paths within the upper mantle beneath arrays. Evidence for regional variations in structure at depths greater than 1850 km is presented. An abrupt drop in dT/dA between distances of 87 and 90 is reported for two different regions of the Earth and possibly a third; this drop represents a fairly rapid increase in velocity gradients at about 2700 km depth that may be a world-wide phenomenon. There are also strong lateral variations in the upper mantle to the north-east and east of YKA. Array observations of precursors to PP, 1'' P' and PKP indicate small-scale lateral variations in the crust and upper mantle and in the lowest 200 km of the mantle that give rise to extensive scattered P wave energy. We initially try to relate our data on lateral variations to regional variations in travel times and mantle convection. Our main concern is the problem of motion within the lower mantle, and how dT/dA measurements in combination with theoretical studies of the properties of the scattering regions might indicate flow. The combined P wave data on small-scale and large-scale lateral variations suggest that the mantle is most homogeneous between depths of 800 km and 2500 km; consequently, we propose that mantle convection is confined to the upper mantle and the lowest 400 km of the mantle, and that convection plumes, if they exist, do not originate at depths greater than 800 km.